Question Re: 2 FET follower use

[amdx]

I have a FET follower RF amp. It is a high input impedance ~330kΩ and a
100 Ω output impedance. I have it on an antenna that has a 330 Ω output
impedance. The antenna works fine NOT being terminated with 330 ohm,
i.e. seeing a 330k Ω load of the amp input.
The amp does two things, it doesn't load the antenna so I get about 2
times the voltage output. Second, it does an impedance conversion, in
this case a 1.8 voltage gain over the step down of a matching
transformer. That all conspires to be ~ 9.8db of gain.
My question with the caveat, that over driving the input of an amp will
cause unwanted problems.(but how far can I push it)
Now, say I put a step up transformer on the antenna to feed the amp
input, say 1 to 3, i.e. 330 Ω to 2,970 Ω, a 3 X voltage step up.
Now I have 3 times the voltage on the input of the amp. and another
~10db of gain. That seems to come at no cost, unless I start to over
drive the amp.
My question: The antenna impedance is 330 ohms, I wind a transformer
that is 330 Ω to 2,970, that would be driving a 330k Ω circuit.
That's seems to be a problem, but maybe not, the antenna would just
see the transformer as a much higher impedance, just as it originally
saw the amp, when it was connected directly to the antenna.
I would think the antenna sees a inductive reactance load instead of a
resistive load. The obvious solution would be to put a 2,970 Ω resistor
on the amp input.
Do I calculate the inductance of the primary any different than usual
since the secondary is unloaded? Should I just add the resistor.
Here's a schematic of the amp.
> https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0

Your suggestions please.

Mikek

[amdx]

Hard question? I ask it wrong? Nobody cares? ;-)

Mikek

[Jeroen Belleman]

I think there are so many misunderstandings and loose ends
that setting everything straight is just too much work.

Jeroen Belleman

[Anthony Stewart]

No details specs, an answer. BW ATI, NF, Cable type , etc etc

[Jeff Liebermann]

On Wed, 20 May 2020 22:28:53 +0200, Jeroen Belleman
<jer...@nospam.please> wrote:


>>>> https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0

>> Hard question? I ask it wrong? Nobody cares? ;-)
>>
>> Mikek

>I think there are so many misunderstandings and loose ends
>that setting everything straight is just too much work.
>
>Jeroen Belleman

Yup. That's about what I was thinking. Before I could start, I had
to redraw the schematic and get a few things clarified. I cleaned up
the schematic so I could read it:
<http://www.learnbydestroying.com/jeffl/crud/2%20FET%20BCB%20amp.jpg>
and started on a list of dumb question:
1. Is the "1" capacitor in farads, millifarads, nanofarads,
microfarads, or picofarads?
2. The bifilar winding instructions in the upper right are for T3.
There is no T3, but there is a T2 that's missing the instructions.
3. How did you manage to get -6dBm loss? It's a source follower,
which doesn't have any voltage gain, but should have some power gain.
4. All the voltages in red are wrong. For example, if you power the
amp from 12V, the J271 gate voltage should be 1/2 the 12V power supply
voltage or 6V, not 5.2V. If you measured 5.2V on the gate, then the
J271 gate is drawing current which suggests it might be fried.
5. Why a pot for adjusting the drain current on the J310, but no pot
for doing the same for J271?

Only then did I re-read the question and discover that there are too
many questions and oddities. I don't have the time.

Since this thing works only at 1MHz, it should be easy enough to build
an LTspice model in order to see what the input and output impedances
might be, and what effect input loading might have.



--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Phil Hobbs]

I don't believe that a pair of mismatched discrete FETs running at 10 mA
has an IP2 of +88 dBm, or an IP3 of +41 dBm. +10 dBm / 0 dBm would be
more like it.

Where in the world did you get that circuit?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

[amdx]

On 5/20/2020 4:35 PM, Jeff Liebermann wrote:
> On Wed, 20 May 2020 22:28:53 +0200, Jeroen Belleman
> <jer...@nospam.please> wrote:
>
>
>
>>> Hard question? I ask it wrong? Nobody cares? ;-)
>>>
>>> Mikek
>
>> I think there are so many misunderstandings and loose ends
>> that setting everything straight is just too much work.
>>
>> Jeroen Belleman
>
> Yup. That's about what I was thinking. Before I could start, I had
> to redraw the schematic and get a few things clarified. I cleaned up
> the schematic so I could read it:
> <http://www.learnbydestroying.com/jeffl/crud/2%20FET%20BCB%20amp.jpg>
> and started on a list of dumb question:
> 1. Is the "1" capacitor in farads, millifarads, nanofarads,
> microfarads, or picofarads?

1 uf.

> 2. The bifilar winding instructions in the upper right are for T3.
> There is no T3, but there is a T2 that's missing the instructions.

T1 and T2 are the same. T3 is not relevant to this circuit.

> 3. How did you manage to get -6dBm loss? It's a source follower,
> which doesn't have any voltage gain, but should have some power gain.

I don't see where I said -6dbm. what I said was in the circuit I have
the amp adds "9.8db of gain" I did use ~ maybe you thought it was a
negative sign.
Below, I clarify, Gain over the use of a transformer.

> 4. All the voltages in red are wrong. For example, if you power the
> amp from 12V, the J271 gate voltage should be 1/2 the 12V power supply
> voltage or 6V, not 5.2V. If you measured 5.2V on the gate, then the
> J271 gate is drawing current which suggests it might be fried.

Ya, sorry about that, I recorded those voltages when I was
troubleshooting the circuit.

> 5. Why a pot for adjusting the drain current on the J310, but no pot
> for doing the same for J271?

The original design did have two pots, the author had the equipment to
adjust for maximum IPs. I think he simplified it as most people can't
set IPs any way.

>
> Only then did I re-read the question and discover that there are too
> many questions and oddities. I don't have the time.

The question is about the transformer,

Do I calculate the inductance of the primary any different than usual

since the secondary is unloaded? Should I just add the resistor on the
secondary?

>
> Since this thing works only at 1MHz, it should be easy enough to build
> an LTspice model in order to see what the input and output impedances
> might be, and what effect input loading might have.
>
>
>

I already know and said, the input is about 330kΩ and the output os
about 100Ω.
Loading the input with 330Ω works fine and has a measured 9.8db gain,
over the use of a matching transformer.
I just realized I added that "over the use of a matching transformer",
that should have been added previously, may have helped.
To clarify, if I'm using the matching transformer on the antenna and
have 0.1v output and then I swap the transformer for the amp, I then
have 0.312V output. Thus my 9.8db gain.
Thanks Guys, Mikek

[amdx]

Hi Phil,
I can't vouch for any of the numbers, the designer is Dallas Lankford.
He was well aware of of problems caused by intercepts, especially in the
AMBCB, because there is a signal every 10kHz to cause havoc. I think he
started building and experimenting with Norton noiseless feedback amps,
in 93' or 94'. I have only started testing the antenna system, So far
the amp acted as expected. In the little use I gave it, I didn't notice
any intermod. I have other bugs I'm working on, Mainly feed line
ingress. I'm making all the mistakes, so learning a lot.

Dallas was well respected in the AM DXing community and had dozens of MW
antenna designs. The BCB is difficult because you have many stations on
the same frequency and because of the wavelength, it is more difficult
to make a directional antenna. He had a few designs with 4 antennas
phased together. He had most of them on a yahoo group and then 6 or 8
years ago, he said, if you want anything on the site get now, I'm
shutting down the site, he did and and I haven't heard anything about
him since. I suspect the amp is better than you think, he was pretty
meticulous about everything he did. Also a bit cantankerous.
btw, here's the two trimpot version of the schematic.
> https://www.dropbox.com/s/a6a9vchg1ukk6qp/Dallas%20FET%20follower.jpg?dl=0

Mikek

[Anthony Stewart]

On Wednesday, 20 May 2020 09:09:16 UTC-4, amdx wrote:

Where are your design specs and tolerances for the input/output?

Haven't you ever used a CD4000 series non-buffered gate as a linear amplifier? Self-heating is highly dependent on Vdd so start low until it feels warm. Then it will self-regulate without runaway. It is obvious dependant on supplier and if it unbuffered or not. GBW also increases with Vdd until it self-heats then it slows down as RdsOn decreases.

I've never tried it on 74HC series but that might work at 3.3V but too hot at 5V.


Link to SIM http://tinyurl.com/y9eur754 with gm=20m Vt=3V
This is just a conceptual simulation. Gain depends on Vdd around 10, BW= 1MHz Miller capacitance added.
Here's is a simulation.

[Jeff Liebermann]

On Wed, 20 May 2020 17:51:29 -0500, amdx <noj...@knology.net> wrote:


>> 3. How did you manage to get -6dBm loss? It's a source follower,
>> which doesn't have any voltage gain, but should have some power gain.
>
> I don't see where I said -6dbm. what I said was in the circuit I have
>the amp adds "9.8db of gain" I did use ~ maybe you thought it was a
>negative sign.

Your original schematic at:
<https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0>
shows "-6 dBm preamp gain" in the box at the lower right.

>> 5. Why a pot for adjusting the drain current on the J310, but no pot
>> for doing the same for J271?
>
> The original design did have two pots, the author had the equipment to
>adjust for maximum IPs. I think he simplified it as most people can't
>set IPs any way.

You can blow up a JFET with a gate voltage pot that goes the full
range from +12V to 0V. Did you actually build the circuit per the
schematic?

>The question is about the transformer,
>Do I calculate the inductance of the primary any different than usual
>since the secondary is unloaded? Should I just add the resistor on the
>secondary?

I still don't understand the question (or what problem you're trying
to solve). Too much is unknown or ambiguous. What do you mean by
"usual way"? Add what resistor on the secondary? Series or parallel?
For what purpose?

> To clarify, if I'm using the matching transformer on the antenna and
>have 0.1v output and then I swap the transformer for the amp, I then
>have 0.312V output. Thus my 9.8db gain.

9.8dB voltage or power gain? The input and output are two radically
different impedances, neither of which is 50 ohms. Did you measure
this with an oscilloscope or spectrum analyzer? If so, how did you
arrive at the 9.8dB gain?

Source followers just love to oscillate. Did you look for
oscillations when measuring the output? The J310 is a UHF device that
works nicely up to about 1GHz. If you build your BCB 1MHz amplifier
as if it were a broadband amplifier (i.e. no 1MHz tuning), it's going
to oscillate at some frequency because of the inductances of the long
lead lengths. The ferrite beads might help, but in my never humble
opinion, if you're going to use UHF devices at 1MHz, you'll need to
bypass everything from 1MHz to 1GHz and probably do something to limit
the gain above about a dozen MHz.

[Lasse Langwadt Christensen]

https://assets.nexperia.com/documents/data-sheet/74HCU04.pdf section 13

[Phil Hobbs]

I don't know anything about him, for or against, but that IP3 spec is a
good seven orders of magnitude off from credibility. Even with ideal
parts, you can't set the balance pot any where near that accurately.

[jla...@highlandsniptechnology.com]

On Wed, 20 May 2020 08:09:08 -0500, amdx <noj...@knology.net> wrote:

>I have a FET follower RF amp. It is a high input impedance ~330k? and a
>100 ? output impedance. I have it on an antenna that has a 330 ? output

>impedance. The antenna works fine NOT being terminated with 330 ohm,

>i.e. seeing a 330k ? load of the amp input.

> The amp does two things, it doesn't load the antenna so I get about 2
>times the voltage output. Second, it does an impedance conversion, in
>this case a 1.8 voltage gain over the step down of a matching
>transformer. That all conspires to be ~ 9.8db of gain.
> My question with the caveat, that over driving the input of an amp will
>cause unwanted problems.(but how far can I push it)
> Now, say I put a step up transformer on the antenna to feed the amp

>input, say 1 to 3, i.e. 330 ? to 2,970 ?, a 3 X voltage step up.

> Now I have 3 times the voltage on the input of the amp. and another
>~10db of gain. That seems to come at no cost, unless I start to over
>drive the amp.
> My question: The antenna impedance is 330 ohms, I wind a transformer

>that is 330 ? to 2,970, that would be driving a 330k ? circuit.

> That's seems to be a problem, but maybe not, the antenna would just
>see the transformer as a much higher impedance, just as it originally
>saw the amp, when it was connected directly to the antenna.
> I would think the antenna sees a inductive reactance load instead of a

>resistive load. The obvious solution would be to put a 2,970 ? resistor

>on the amp input.
> Do I calculate the inductance of the primary any different than usual
>since the secondary is unloaded? Should I just add the resistor.
>Here's a schematic of the amp.
>> https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0
>
> Your suggestions please.
>
> Mikek

The ~1M wideband input impedance will have dc-to-daylight range, so
anything from 60 Hz up will cross-modulate a small RF signal.

The usual way to match an antenna to a fet is a tuned network. That
limits bandwidth and optimizes the impedance match.

I'd use one fet, with real gain.

If you add a transformer, the secondary *is* loaded... by the fets.





--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

[jla...@highlandsniptechnology.com]

On Wed, 20 May 2020 17:27:56 -0700 (PDT), Anthony Stewart
<tony.s...@gmail.com> wrote:

>On Wednesday, 20 May 2020 09:09:16 UTC-4, amdx wrote:

>> I have a FET follower RF amp. It is a high input impedance ~330k? and a

>> 100 ? output impedance. I have it on an antenna that has a 330 ? output

>> impedance. The antenna works fine NOT being terminated with 330 ohm,

>> i.e. seeing a 330k ? load of the amp input.

>> The amp does two things, it doesn't load the antenna so I get about 2
>> times the voltage output. Second, it does an impedance conversion, in
>> this case a 1.8 voltage gain over the step down of a matching
>> transformer. That all conspires to be ~ 9.8db of gain.
>> My question with the caveat, that over driving the input of an amp will
>> cause unwanted problems.(but how far can I push it)
>> Now, say I put a step up transformer on the antenna to feed the amp

>> input, say 1 to 3, i.e. 330 ? to 2,970 ?, a 3 X voltage step up.

>> Now I have 3 times the voltage on the input of the amp. and another
>> ~10db of gain. That seems to come at no cost, unless I start to over
>> drive the amp.
>> My question: The antenna impedance is 330 ohms, I wind a transformer

>> that is 330 ? to 2,970, that would be driving a 330k ? circuit.

>> That's seems to be a problem, but maybe not, the antenna would just
>> see the transformer as a much higher impedance, just as it originally
>> saw the amp, when it was connected directly to the antenna.
>> I would think the antenna sees a inductive reactance load instead of a

>> resistive load. The obvious solution would be to put a 2,970 ? resistor

>> on the amp input.
>> Do I calculate the inductance of the primary any different than usual
>> since the secondary is unloaded? Should I just add the resistor.
>> Here's a schematic of the amp.
>> > https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0
>>
>> Your suggestions please.
>>
>> Mikek
>
>Where are your design specs and tolerances for the input/output?
>
>Haven't you ever used a CD4000 series non-buffered gate as a linear amplifier? Self-heating is highly dependent on Vdd so start low until it feels warm. Then it will self-regulate without runaway. It is obvious dependant on supplier and if it unbuffered or not. GBW also increases with Vdd until it self-heats then it slows down as RdsOn decreases.
>
>I've never tried it on 74HC series but that might work at 3.3V but too hot at 5V.
>
>
>Link to SIM http://tinyurl.com/y9eur754 with gm=20m Vt=3V
>This is just a conceptual simulation. Gain depends on Vdd around 10, BW= 1MHz Miller capacitance added.
>Here's is a simulation.

The 1K resistor makes about 4 nV/rootHz noise. But a CMOS gate ain't
very quiet itself.

[bitrex]

BF862

[Ricketty C]

Maybe I'm showing my ignorance here, but the question I have is what is the purpose of this amplifier and the transformers you may or may not use?

Usually the front end of radios are designed for low noise and are pretty adequate unless there is a problem of noise pickup in the feed. The battle is typically S/N ratio rather than voltage. The first stage of amplification needs to add as little noise as possible which means a design optimized for that, not gain. Even just 10dB of gain means noise added by any subsequent stages will be minute in comparison.

So is this intended to be a low noise preamp? Is it needed because of some problem with the antenna system?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

[bitrex]

If you don't need a good match on the output side, just input matching
and gain, why not use an amp with feedback?

<https://www.dropbox.com/s/doa3w4wk1z4g2nk/IMG_20200520_223432126.jpg?dl=0>

[amdx]

On 5/20/2020 5:51 PM, amdx wrote:
> On 5/20/2020 4:35 PM, Jeff Liebermann wrote:

Sorry Jeff,
On my reader your post doesn't display, went to google groups
for a copy and paste.

Your original schematic at:
<https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0>
shows "-6 dBm preamp gain" in the box at the lower right.

>> 5. Why a pot for adjusting the drain current on the J310, but no pot
>> for doing the same for J271?
>
> The original design did have two pots, the author had the equipment to
>adjust for maximum IPs. I think he simplified it as most people can't
>set IPs any way.

<You can blow up a JFET with a gate voltage pot that goes the full
>range from +12V to 0V. Did you actually build the circuit per the
>schematic?

*******************************************************
I built it per the schematic.
********************************************************

>The question is about the transformer,
>Do I calculate the inductance of the primary any different than usual
>since the secondary is unloaded? Should I just add the resistor on the
>secondary?

>I still don't understand the question (or what problem you're trying
>to solve). Too much is unknown or ambiguous. What do you mean by
>"usual way"? Add what resistor on the secondary? Series or parallel?
>"For what purpose?

********************************************************************
I have a transformer that is seeing a 330kΩ load, I can't put enough
turns on to match that. So my primary will not see any load reflected
back. That's my conundrum.

I just wound a transformer, it has a 4.3 to 1 voltage step up, 10 turns
to 44 turns, it would match 330 Ω to 6,250 Ω. But I'm connecting it to a
330kΩ amp input.
A detail: I have the amp and transformers setup with relays so I can
switch from one to the other.
I ran a test, first with only a 330Ω to 100Ω transformer and a 100Ω
to 50Ω transformer to match the RF voltmeter. (there's a good reason for
two transformers)
I adjusted the signal generator to have a 330Ω output impedance, (to
simulate my antenna). I set the voltage on the signal generator so the
output of the 100Ω to 50Ω transformer had an output of 0.01V at 1MHz.
Now I switch to the amp, the voltage increases to 0.058V. That is a
voltage ratio of 5.8. I plug that into this calculator and it says I
have a 15.2db gain. (Remember this is the ratio the output voltage using
the transformers vs the amp)
> http://www.sengpielaudio.com/calculator-gainloss.htm

I have not played it on the radio at this gain, that's for another day,
I don't think it has any problems, but it might.
************************************************************************

> To clarify, if I'm using the matching transformer on the antenna and
>have 0.1v output and then I swap the transformer for the amp, I then
>have 0.312V output. Thus my 9.8db gain.

>9.8dB voltage or power gain? The input and output are two radically
>different impedances, neither of which is 50 ohms. Did you measure
>this with an oscilloscope or spectrum analyzer? If so, how did you
>arrive at the 9.8dB gain?

******************************************************************
As above, I measured the voltage ratio and used the online converter.
Voltage ratio was 3.2.

********************************************************************

Source followers just love to oscillate. Did you look for
oscillations when measuring the output? The J310 is a UHF device that
works nicely up to about 1GHz. If you build your BCB 1MHz amplifier
as if it were a broadband amplifier (i.e. no 1MHz tuning), it's going
to oscillate at some frequency because of the inductances of the long
lead lengths. The ferrite beads might help, but in my never humble
opinion, if you're going to use UHF devices at 1MHz, you'll need to
bypass everything from 1MHz to 1GHz and probably do something to limit
the gain above about a dozen MHz.

*******************************************************************
Noted: I'll be looking for that.
With the amp energized, when I disconnect the signal generator from
the input, the RF voltmeter drops to 0V, even on the 1 mV scale. That's
a hopeful sign, but the meter is only good to 10MHz.

Thanks for engaging with me on this, this project has taken a long time,
I finally got signal from the antenna to the radio 3 or 4 days ago.
My first RF was mostly from the feed line, then I added the 100Ω to 50Ω
transformer at the radio. In my haste to hear it play, I just connected
the CAT6 wires to the connector on the radio. That let all the common
made signal through. I still have some feed line pick up even with the
isolation transformer, I have some ideas to reduce it further.
Here's a schematic drawing of the switching box. I added the voltmeter,
might help understanding.

> https://www.dropbox.com/s/f5cxm8cawmgcx4i/Bog%20Switching%20Box%20two%20relays%205-20%20revision.jpg?dl=0

And a picture of the layout.

> https://www.dropbox.com/s/li51nmg9q9a9596/bog%20antenna%20switching%20box.jpg?dl=0



Thank, Mikek

[Ricketty C]

On Wednesday, May 20, 2020 at 11:18:40 PM UTC-4, amdx wrote:
>
> > 9.8dB voltage or power gain? The input and output are two radically
> >different impedances, neither of which is 50 ohms. Did you measure
> > this with an oscilloscope or spectrum analyzer? If so, how did you
> >arrive at the 9.8dB gain?

> ******************************************************************
> As above, I measured the voltage ratio and used the online converter.
> Voltage ratio was 3.2.
>
> ********************************************************************

There's a problem with that. You said you got a 1.8 voltage gain from matching a high impedance with the antenna low impedance. The voltage follower has 0 dB of gain. How could you get 3.2 voltage gain from using this amp???

Are you familiar with the theory of simple transistor amps? Did I misunderstand what you are doing? There is some sort of disconnect somewhere.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

[piglet]

Yes you can place a 2970R resistor across the hi-z input so it presents
a load for your matching transformer to reflect 330R at the primary. For
a hint remember those RIAA phono preamps from the 19702-80s that had a
hi-Z amp with a 47k shunt resistor?

I very much doubt the 330k input impedance of that follower, at 1MHz
that would require stray capacitance to be below 0.5pF

piglet

[piglet]

On 20/05/2020 22:35, Jeff Liebermann wrote:
> Yup. That's about what I was thinking. Before I could start, I had
> to redraw the schematic and get a few things clarified. I cleaned up
> the schematic so I could read it:
> <http://www.learnbydestroying.com/jeffl/crud/2%20FET%20BCB%20amp.jpg>
> and started on a list of dumb question:

Are those phasing dots right?

> 4. All the voltages in red are wrong. For example, if you power the
> amp from 12V, the J271 gate voltage should be 1/2 the 12V power supply
> voltage or 6V, not 5.2V. If you measured 5.2V on the gate, then the
> J271 gate is drawing current which suggests it might be fried.

The 5.2V reading could be influenced by the voltmeter the OP used?

piglet

[Phil Allison]

amdx has not one tiny clue wrote:

=========================================

> I have a FET follower RF amp. It is a high input impedance ~330kΩ

** Really ?

So the total input capacitance is under 0.5 pF ?


.... Phil

[Tauno Voipio]

300 kohm is a ridiculously large impedance on a RF frequency.

Mike, how much capacitance corresponds to 300 kohm at 1 MHz?

It is practically impossible to get less than around 10 pF
or more of stray capacitances.

--

-TV

[amdx]

On 5/20/2020 10:30 PM, Ricketty C wrote:
> On Wednesday, May 20, 2020 at 11:18:40 PM UTC-4, amdx wrote:
>>
>>> 9.8dB voltage or power gain? The input and output are two radically
>>> different impedances, neither of which is 50 ohms. Did you measure
>>> this with an oscilloscope or spectrum analyzer? If so, how did you
>>> arrive at the 9.8dB gain?
>
>> ******************************************************************
>> As above, I measured the voltage ratio and used the online converter.
>> Voltage ratio was 3.2.
>>
>> ********************************************************************
>
> There's a problem with that. You said you got a 1.8 voltage gain from matching a high impedance with the antenna low impedance. The voltage follower has 0 dB of gain. How could you get 3.2 voltage gain from using this amp???
>
> Are you familiar with the theory of simple transistor amps? Did I misunderstand what you are doing? There is some sort of disconnect somewhere.
>

The antenna is a voltage source with a 330 ohm internal impedance, if
I match that with a 330Ω to 50Ω transformer and connect it to a 50 ohm
radio, I loss 1/2 of my voltage across the internal impedance, vs
feeding the antenna into the high input impedance amp.
Second, a 330Ω to 100Ω impedance transformer is a voltage step down
of 1.8 to 1. The amp does the impedance conversion without any voltage
step down.
The amp gain is relative to the transformer step down.

Mikek

[amdx]

On 5/21/2020 3:25 AM, Tauno Voipio wrote:
> 300 kohm is a ridiculously large impedance on a RF frequency.


>
> Mike, how much capacitance corresponds to 300 kohm at 1 MHz?

About 0.5pf.

>
> It is practically impossible to get less than around 10 pF
> or more of stray capacitances.
>

I don't know what that havoc conspires to do.
Resonance, but...

Mikek

[amdx]

I recall calculating the meter as a 10MΩ resistor in the circuit.
But please, I posted that schematic with those numbers when I was
trouble shooting the amp. I should have removed those voltages
before I posted it.
Mikek

[amdx]

Ok, and I suspect it is mostly a capacitive load.
My feeble attempt at measuring the input resistance was to insert a
series resistor with the input, when that resistor was 330kΩ the output
voltage dropped by 1/2. That's where I got my 330kΩ from.

Mikek

[amdx]

The usual practice is to match an antenna to the impedance of the
radio, in my case I'm matching the antenna to the 100Ω feed line and
then I match the feed line to the radio with a 100Ω to 50Ω transformer.
The amp does the impedance transformation without a voltage step down.
Half of this project is to see how this amp works. The other half is
getting an antenna up. I lost all my antennas in the hurricane 1-1/2
years ago.

>
> Usually the front end of radios are designed for low noise and are pretty adequate unless there is a problem of noise pickup in the feed. The battle is typically S/N ratio rather than voltage. The first stage of amplification needs to add as little noise as possible which means a design optimized for that, not gain. Even just 10dB of gain means noise added by any subsequent stages will be minute in comparison.
>
> So is this intended to be a low noise preamp? Is it needed because of some problem with the antenna system?
>

S/N is why I put down the Beverage on the Ground, (BOG).
I don't know the noise figure, I do trust that the designer is well
aware that the first amp sets the noise figure and his design was
with that in mind. I built 6 amps, if someone is interested in doing
measurements, I would send them one.

I found a commercial version that is based on the Dallas Lankfords
design. Has schematics, voltage readings and IP measurement method and
specs.
> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf

Mikek

[amdx]

On 5/20/2020 5:10 PM, Phil Hobbs wrote:

Phil,

I found a commercial version that is based on Dallas Lankfords design.
It has schematics, voltage readings and IP measurement method and specs.
> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf

Mikek

[amdx]

I'm aware of dozens of antenna preamps I can build. I'm working with
this one for now. I went to a lot of trouble building it into a relay
switchable configuration, just so I can easily comparison testing.
Mikek

[amdx]

I'll be trying it with and without the resistor.

Mikek

[Tauno Voipio]

On 21.5.20 13:47, amdx wrote:

It spoils your impedance level by a factor of at least a decade.

--

-TV

[upsid...@downunder.com]

On Wed, 20 May 2020 08:09:08 -0500, amdx <noj...@knology.net> wrote:

>I have a FET follower RF amp. It is a high input impedance ~330k? and a

>100 ? output impedance. I have it on an antenna that has a 330 ? output
>impedance.

How dis you determined that the antenna was 330 ohms ? Was it really
the impedance or just the resistive part of the impedance ?

What is your antenna like ? Is it a resonant antenna with pure
resistive Z=330 ohms (R=330 ohm, X=0). For instance a full size 1/2
wave folded dipole would produce 330 ohms (with slightly tweaking tube
diameters) ? A transformer should work OK.

Or is it some electrically small highly reactive antenna with Z=330
(e.g. R=5, X=-329) ohms ? For instance a short whip on HF

Even worse, has the antenna resistance 330 ohms and the capacitive
reactance tens of kilo-ohms and the complex impedance also in tens of
kilo-ohms.

These kinds of short antennas are usually interfaced with a very high
impedance but low capacitance preamplifier.

To make sense of your questions, one has to first know the antenna
impedance (both resistance and reactance).

>The antenna works fine NOT being terminated with 330 ohm,

>i.e. seeing a 330k ? load of the amp input.

That suggests that the antenna is small relative to the wavelength and
hence highly capacitive.

> The amp does two things, it doesn't load the antenna so I get about 2
>times the voltage output.

If the difference is only 6 dB this would indicate a power mach vs.
voltage match situation.

>Second, it does an impedance conversion, in
>this case a 1.8 voltage gain over the step down of a matching
>transformer. That all conspires to be ~ 9.8db of gain.

??

> My question with the caveat, that over driving the input of an amp will
>cause unwanted problems.(but how far can I push it)
> Now, say I put a step up transformer on the antenna to feed the amp

>input, say 1 to 3, i.e. 330 ? to 2,970 ?, a 3 X voltage step up.

If the antenna has a highly capacitively reactive impedance, any stray
capacitance to ground will form a capacitive voltage divider,
attenuating the signal. A step up transformer will have some extra
stray capacitance, so the gain obtained by the turns ratio might be
lost due to the stray capacitance.

> Now I have 3 times the voltage on the input of the amp. and another
>~10db of gain. That seems to come at no cost, unless I start to over
>drive the amp.

If you really need more gain, put a voltage amplifier stage _after_
the impedance matching source follower. Do not mess up with the input
side.

,

[piglet]

On 21/05/2020 11:55 am, amdx wrote:
>  Ok, and I suspect it is mostly a capacitive load.
> My feeble attempt at measuring the input resistance was to insert a
> series resistor with the input, when that resistor was 330kΩ the output
> voltage dropped by 1/2. That's where I got my 330kΩ from.
>
>                                  Mikek
>

That is a valid method at low frequencies, did you measure this at 1MHz
or somewhere lower?

A garden variety 0.25W through hole resistor will have approx 0.5pF end
to end capacitance without even thinking about wiring strays so that
could have confounded your measurement. Hi-Z and RF do not mix easily.

piglet

[piglet]

On 20/05/2020 10:35 pm, Jeff Liebermann wrote:

> 5. Why a pot for adjusting the drain current on the J310, but no pot
> for doing the same for J271?
>

Perhaps there is no need since one pot will set the Id for both FETs?

piglet

[amdx]

On 5/21/2020 8:32 AM, piglet wrote:
> On 21/05/2020 11:55 am, amdx wrote:
>>   Ok, and I suspect it is mostly a capacitive load.
>> My feeble attempt at measuring the input resistance was to insert a
>> series resistor with the input, when that resistor was 330kΩ the
>> output voltage dropped by 1/2. That's where I got my 330kΩ from.
>>
>>                                   Mikek
>>
>
> That is a valid method at low frequencies, did you measure this at 1MHz

Yes, measured at 1 MHz.

[amdx]

On 5/21/2020 8:03 AM, upsid...@downunder.com wrote:
> On Wed, 20 May 2020 08:09:08 -0500, amdx <noj...@knology.net> wrote:
>
>> I have a FET follower RF amp. It is a high input impedance ~330k? and a
>> 100 ? output impedance. I have it on an antenna that has a 330 ? output
>> impedance.
>
> How dis you determined that the antenna was 330 ohms ? Was it really
> the impedance or just the resistive part of the impedance ?

Here's the measurement I made, from 200kHz to 4MHz.
> https://www.dropbox.com/s/1s24sg2awuxyngf/bog%20r%20%26%20x%20graph%20341%20ohm%20termination.jpg?dl=0

>
> What is your antenna like ? Is it a resonant antenna with pure
> resistive Z=330 ohms (R=330 ohm, X=0). For instance a full size 1/2
> wave folded dipole would produce 330 ohms (with slightly tweaking tube
> diameters) ? A transformer should work OK.

No, see graph.

>
> Or is it some electrically small highly reactive antenna with Z=330
> (e.g. R=5, X=-329) ohms ? For instance a short whip on HF

It is a 260ft wire on the ground with a 340 ohm termination resistor.
The Value of the termination resistor was chosen because it gave the
flatest impedance curve as posted above. I'm not sure that's the optimum
termination, there my be other important characteristics that may be
affected by the termination resistance, like pattern, F/B.

Part of the project is to see how this amp works. I'm aware there are
dozens
of antenna preamp designs on line. I'm working with this one, until I'm not.

Mikek

[Ricketty C]

So why do you think you need to worry about the voltage from the impedance match? Have you tried it and found it lacking? As I mentioned, typically the enemy is S/N, not voltage.

Why not try a simple matching transformer and see what it does. Then if the reception is not good you can return and consider low noise amps.

I just don't understand the reason for trying to fix a problem you don't know you have. It was some time ago I realized the battle at the antenna is not as much about signal strength as it is about S/N.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209

[amdx]

Right. I have a matching transformer. The part sequence is 330Ω to
100Ω transformer and then a 100Ω to 50Ω transformer, with a 50 ohm
termination resistor. I scanned 600kHz to 1600kHz.
At 600kHz, R=360 ohms, X= +165 ohms, at 1600kHz, R=430 ohms X= +23
ohms. Could use some adjustment, but, I'm not sure how to correct the
reactance. Do I need more turns with just a slightly adjustment to the
turns ratio?

Now the surprise to me.
I have the 330Ω to 6,250Ω ohm transformer driving the input to the FET
amp.
At 600kHz R=105 ohms X=-380 ohms, at 1600kHz R=14 ohms and X=-122ohms.

Now with a 6,250 ohm resistor across the amp input making the
transformer match.
At 600kHz R=201 X=-235 ohms at 1600kHz R=36 ohms X= -133 ohms.

What do I do to tame the input to the the amp?

Mikek

[bitrex]

As you like. But don't marry circuits because you designed them or built
them.

John Larkin is right sometimes good designs appropriate for the job
shouldn't fight you too hard. If they do it may be indicative one needs
to come at the problem another way.

[jla...@highlandsniptechnology.com]

>series resistor with the input, when that resistor was 330k? the output
>voltage dropped by 1/2. That's where I got my 330k? from.
>
> Mikek

One trick is to add a big resistor and see how it affects the
frequency rolloff, and calculate capacitance from that.





--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

[amdx]

I don't know why I can't have just a few days to experiment with this
amp.
BTW, here is the manual. >
https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf

Page 3 has some specs, one is the input R and C.
At 500kHz it's 122kΩ with a parallel 37pf capacitor.
At 1700kHz it's 52kΩ with a parallel 37pf Capacitor.

Typical input intercepts are: IIP2 +87 to +93 dBm; IIP3 +40dBm to +45 dBm.

3db frequency range is 50 kHz to 300 MHz.

Not shown is the noise figure.

I think the first time I applied power to it was 4 days ago. I haven't
listened to it more than 60 seconds.
The original question was not even about the amp, it was about
designing a transformer to feed a high impedance load.

Seems I now have built a transformer and have a question about it.

I have a 10 turn to 40 turns transformer on a #43 material, binocular
core. I realize the transformer is a rather high impedance for RF, but
I'm here to learn what the complications are.
If I put a 12,100Ω resistor on the 40 turn winding and scan the
Primary at 1MHz, I get R=336Ω and X= -292Ω. The R is right on, the
Reactance is capacitive and to high. Is there anything I can do to make
this less reactive?
A wider range sweep has a downward slope to the R but the Reactance is
fairly constant. Sweep shown here.
> https://www.dropbox.com/s/s26u8jxwpei0rpp/10%20to%2040%20turn%20transformer.jpg?dl=0

Mikek

[Jeff Liebermann]

On Thu, 21 May 2020 08:12:54 +0100, piglet <erichp...@hotmail.com>
wrote:

>On 20/05/2020 22:35, Jeff Liebermann wrote:
>> Yup. That's about what I was thinking. Before I could start, I had
>> to redraw the schematic and get a few things clarified. I cleaned up
>> the schematic so I could read it:
>> <http://www.learnbydestroying.com/jeffl/crud/2%20FET%20BCB%20amp.jpg>
>> and started on a list of dumb question:
>
>Are those phasing dots right?

Oops. You're right. The phasing dots should be on the same end of
the transformer.
<https://www.ato.com/Content/Images/uploaded/power-line-filter-circuit-diagram.jpg>

>> 4. All the voltages in red are wrong. For example, if you power the
>> amp from 12V, the J271 gate voltage should be 1/2 the 12V power supply
>> voltage or 6V, not 5.2V. If you measured 5.2V on the gate, then the
>> J271 gate is drawing current which suggests it might be fried.

>The 5.2V reading could be influenced by the voltmeter the OP used?
>piglet

Oops 2.0. You're right again.

The voltage is set by a resistor divider made of two 1M resistors. In
order to produce 5.2v instead of the theoretical 6.0V, the voltmeter
would need to have an input resistance of:
5.2/12 = 0.433
To get this ratio, the grounded 1M resistor should be:
0.433 * 2M = 0.866M
To obtain 0.866M, a meter resistance in parallel with 1M would be:
0.866M = (R * 1M) / (R + 1M)
Therefore R (meter resistance) = 6.5M

I couldn't find any ohms/volt specs for my Extech/RS 22-816
multimeter. So, I measured it. I put a 10V power supply and a pile
of high value resistors in series with the DVM input. When the meter
read 5.0V, the meter resistance is equal to the resistor value. I
strung 10ea 1M 1/4w 5% carbon film resistors in series and ended up
with 4.95V. Therefore the meter resistance is about 10M. It's not my
predicted 6.5M but with all the probable errors involved, is close
enough.

Since I had the 1M resistors handy, I put two in series to form a
divider similar to the RF amp circuit. With the DVM across the
grounded resistor and 12V applied, I got 5.48V, which is close enough
to the measured 5.2V.

So, you're right. It was meter loading that caused the voltage error
and not a blown J271.

My background is in marine radio design, where equipment is expected
to operate normally when somewhat wet. Under some conditions, G10/FR4
is a can simulate a sponge. Therefore, low impedance and resistance
design is the norm. Every time we used resistors higher than about
10K, we got into trouble with board leakage and water related
problems. I think 100K was deemed the largest value considered
acceptable, although we did use larger values when desperate. I would
never have used 1M resistors in anything. With low resistances, meter
loading isn't even a consideration (although capacitive loading on RF
circuits is a consideration). My apologies, but after years of living
in a low impedance world, I just didn't think about the possibility
that the meter would affect the measurement.

In this design, the antenna impedance is about 400 ohms. 10 times
that is considered sufficient to prevent the JFET bias resistors from
having any influence on the antenna loading. Therefore, the resistors
could have been as low as 80K and not had an effect. Optimum NF
(noise figure) is not an issue in the BCB (broadcast band) where the
atmospheric noise is so much higher than front end receiver noise[1].



[1]Except if one is designing a MiniWhip RF amp, where the antenna
impedance is so high that 1M to 10M bias resistors are common:
<https://www.google.com/search?q=pa0rdt+mini+whip+amplifier&tbm=isch>
<http://www.pa3fwm.nl/technotes/tn07.html>
<http://dl1dbc.net/SAQ/miniwhip.html>




--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Jeff Liebermann]

On Thu, 21 May 2020 14:33:51 +0100, piglet <erichp...@hotmail.com>
wrote:

You might be right. I didn't notice that.

The source voltages for both JFET's on the original schematic
<https://www.dropbox.com/s/x6z2vpbseri7ysu/2%20FET%20Troubleshoot.jpg?dl=0>
show 7.62V and 5.35V, which is roughly what I would expect for maximum
output swing from the source followers. The average is:
(7.62 + 5.35) / 2 = 6.49V
which is close enough to 6.0V to work properly. It would be tempting
to split the 220 ohm resistor in two with two 100 or 120 ohm
resistors, and adjust the potentiometer for 6.0V at the junction of
the two resistors. However, that assumes that the two JFET's are
reasonably well matched. If that's not the case, then a 2nd
potentiometer might be a useful addition.

[upsid...@downunder.com]

On Thu, 21 May 2020 10:19:44 -0500, amdx <noj...@knology.net> wrote:

>On 5/21/2020 8:03 AM, upsid...@downunder.com wrote:
>> On Wed, 20 May 2020 08:09:08 -0500, amdx <noj...@knology.net> wrote:
>>
>>> I have a FET follower RF amp. It is a high input impedance ~330k? and a
>>> 100 ? output impedance. I have it on an antenna that has a 330 ? output
>>> impedance.
>>
>> How dis you determined that the antenna was 330 ohms ? Was it really
>> the impedance or just the resistive part of the impedance ?
>
> Here's the measurement I made, from 200kHz to 4MHz.
>> https://www.dropbox.com/s/1s24sg2awuxyngf/bog%20r%20%26%20x%20graph%20341%20ohm%20termination.jpg?dl=0

So this is some kind of Beverage with a 341 ohm termination resistor
at the far end, which explains the comparatively flat resistance and
reactance curves. If the interest is mainly reception in the MF
broadcast band, you could even tune out the capacitive reactance.

But the real question is, why do you need the source follower
amplifier at all ?. Why not just put a 9:1 impedance transmission line
transformer between the antenna and coaxial cable (and 50 ohm receiver
input) ? Alternatively use 4:1 transmission line and 100 ohm balanced
feedline. No need for some amplifiers,

On LF/MF the band noise is so strong that with a half decent antenna,
it will mask the receiver input noise.

Unfortunately Beverages have a gain much worse than 0 dBd. Only
ferrite rods are worse with gains of -40 to -60 dBd, but still usable
thanks to the high band noise.

One alternative would be to put just a matching transformer between
antenna and feedline and if the receiver has a bad sensitivity at
LF/MF, use a manually tuned indoor preselector followed by 10-20 dB
gain. The preselector will reduce the risk of overloading the
receiver.



The source follower amplifier with 330 kohm input impedance is
intended for some random wire antenna, which is much shorter than 1/4
wavelength and hence gas a highly capacitive reactance.

[amdx]

Yes, I have that, with a little complication, I match the antenna 330Ω
to a 100Ω feedline, then at the end of the feed line, I have a 100Ω to
50Ω matching transformer to the radio.

You can follow the signal path through the relays and transformers here.
> https://www.dropbox.com/s/f5cxm8cawmgcx4i/Bog%20Switching%20Box%20two%20relays%205-20%20revision.jpg?dl=0

Note: if I don't power the circuit the amp is not used. This makes for a
very simple A/B test of amp vs transformer.

and if the receiver has a bad sensitivity at
> LF/MF, use a manually tuned indoor preselector followed by 10-20 dB
> gain. The preselector will reduce the risk of overloading the
> receiver.
>
>
>
> The source follower amplifier with 330 kohm input impedance is
> intended for some random wire antenna, which is much shorter than 1/4
> wavelength and hence gas a highly capacitive reactance.

On page 15 and 16 of the manual it list some antennas that this was
designed to work with.
> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf
If you need articles on those antenna, I may be able to find them.

Mikek

[John Miles, KE5FX]

On Wednesday, May 20, 2020 at 6:31:25 PM UTC-7, Phil Hobbs wrote:
> I don't know anything about him, for or against, but that IP3 spec is a
> good seven orders of magnitude off from credibility. Even with ideal
> parts, you can't set the balance pot any where near that accurately.

It's pretty reasonable for preamps in this class. Remember that the IP3 is
the theoretical IMD intercept, not a valid operating point.

Lankford and Jack Smith at Clifton Labs both had a lot of experience with
this sort of thing.

-- john, KE5FX

[Phil Hobbs]

On 2020-05-21 18:31, John Miles, KE5FX wrote:
> On Wednesday, May 20, 2020 at 6:31:25 PM UTC-7, Phil Hobbs wrote:
>> I don't know anything about him, for or against, but that IP3 spec is a
>> good seven orders of magnitude off from credibility. Even with ideal
>> parts, you can't set the balance pot any where near that accurately.
>
> It's pretty reasonable for preamps in this class. Remember that the IP3 is
> the theoretical IMD intercept, not a valid operating point.

I'm quite aware of that. However, IP3 is commonly within 20 dB of the
1-dB compression point even for a very linear amplifier, which for 10 mA
of bias current is not +28 dBm, by a lot.

The claimed IP2 is the really ludicrous one. It requires a level of
matching between the N- and P-FETs that beggars belief.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

[amdx]

On 5/21/2020 6:00 PM, Phil Hobbs wrote:
> On 2020-05-21 18:31, John Miles, KE5FX wrote:
>> On Wednesday, May 20, 2020 at 6:31:25 PM UTC-7, Phil Hobbs wrote:
>>> I don't know anything about him, for or against, but that IP3 spec is a
>>> good seven orders of magnitude off from credibility.  Even with ideal
>>> parts, you can't set the balance pot any where near that accurately.
>>
>> It's pretty reasonable for preamps in this class.  Remember that the
>> IP3 is
>> the theoretical IMD intercept, not a valid operating point.
>
> I'm quite aware of that.  However, IP3 is commonly within 20 dB of the
> 1-dB compression point even for a very linear amplifier, which for 10 mA
> of bias current is not +28 dBm, by a lot.
>
> The claimed IP2 is the really ludicrous one.  It requires a level of
> matching between the N- and P-FETs that beggars belief.
>
> Cheers
>
> Phil Hobbs
>

I don't know if you saw my post of the manual, It has some specs on
page 3 and says,

> Intermodulation: Measured with 611 and 703 KHz test tones at 0 dBm per tone,

typical input intercepts are: IIP2 +87 to +93 dBm; IIP3 +40dBm to +45 dBm

You can believe it or not, I believe the numbers will be high, that's is
one of the main criteria for a BCB preamp. There are so many signals you
do everything to avoid mixing them.

> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf

2nd and 3rd order intermodulation measurements on page 11/12.

Mikek

[Phil Hobbs]

So they claim. With input signals at 0 dBm, an amp with +93 dBm IP2
would generate second order products at -93 dBm as well. That requires
matching of the discrete FETs' transconductance vs I_D to a few parts in
10**5 at signal levels near the amp's P_1dB.

Maybe in the SPICE spherical-cow universe.

[Gerhard Hoffmann]

Am 22.05.20 um 01:38 schrieb Phil Hobbs:

> On 2020-05-21 19:13, amdx wrote:

>>   I don't know if you saw my post of the manual, It has some specs on
>> page 3 and says,
>>
>>> Intermodulation: Measured with 611 and 703 KHz test tones at 0 dBm
>>> per tone,
>>
>> typical input intercepts are: IIP2 +87 to +93 dBm; IIP3 +40dBm to +45 dBm
>>
>> You can believe it or not, I believe the numbers will be high, that's
>> is one of the main criteria for a BCB preamp. There are so many
>> signals you do everything to avoid mixing them.
>>
>>> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf
>>
>> 2nd and 3rd order intermodulation measurements on page 11/12.
>>
>
> So they claim.  With input signals at 0 dBm, an amp with +93 dBm IP2
> would generate second order products at -93 dBm as well.  That requires
> matching of the discrete FETs' transconductance vs I_D to a few parts in
> 10**5 at signal levels near the amp's P_1dB.

Around 0 dBm is way too much input power for measuring IP2/IP3.

3rd order intermodulation products rise by 30 dB for an input power
increase of 10 dB. This is simply impossible for an amplifier
approaching saturation.

Therefore one gets results that are _much_ too good.

IP must be measured at levels where the intermodulation products
just come out of the noise.

cheers, Gerhard

[upsid...@downunder.com]

On Fri, 22 May 2020 02:26:11 +0200, Gerhard Hoffmann <dk...@arcor.de>
wrote:

>Am 22.05.20 um 01:38 schrieb Phil Hobbs:
>> On 2020-05-21 19:13, amdx wrote:
>
>>>   I don't know if you saw my post of the manual, It has some specs on
>>> page 3 and says,
>>>
>>>> Intermodulation: Measured with 611 and 703 KHz test tones at 0 dBm
>>>> per tone,

0 dBm into 50 ohms is about 220 mVrms and 300 mVpk. The worst sum
voltage is 600 mVpk or 1.2 Vpp, which is the required linear voltage
swing.

>>>
>>> typical input intercepts are: IIP2 +87 to +93 dBm; IIP3 +40dBm to +45 dBm
>>>
>>> You can believe it or not, I believe the numbers will be high, that's
>>> is one of the main criteria for a BCB preamp. There are so many
>>> signals you do everything to avoid mixing them.
>>>
>>>> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf
>>>
>>> 2nd and 3rd order intermodulation measurements on page 11/12.
>>>
>>
>> So they claim.  With input signals at 0 dBm, an amp with +93 dBm IP2
>> would generate second order products at -93 dBm as well.  That requires
>> matching of the discrete FETs' transconductance vs I_D to a few parts in
>> 10**5 at signal levels near the amp's P_1dB.

At +12 Vdc, the available swing might be 9 Vpp or 3 Vrms or about +20
dBm, so the P_1dB is slightly larger than this.

>
>Around 0 dBm is way too much input power for measuring IP2/IP3.
>
>3rd order intermodulation products rise by 30 dB for an input power
>increase of 10 dB. This is simply impossible for an amplifier
>approaching saturation.
>
>Therefore one gets results that are _much_ too good.

After all, this is a voltage follower. At least for bipolar common
collector circuits the voltage gain is often quoted as 0.99.

The distortion is created due to the curvature of the transfer
function. How much curvature does a voltage follower have well inside
the supply voltage limits ?

Does a JFET common drain vollower behave differently from a bipolar
common collector voltage follower ?

[amdx]

I'm still looking for what I can change to get a less reactive
transformer. I have ordered a higher permeability core material.

[Phil Hobbs]

On 2020-05-22 03:16, upsid...@downunder.com wrote:
> On Fri, 22 May 2020 02:26:11 +0200, Gerhard Hoffmann <dk...@arcor.de>
> wrote:
>
>> Am 22.05.20 um 01:38 schrieb Phil Hobbs:
>>> On 2020-05-21 19:13, amdx wrote:
>>
>>>>   I don't know if you saw my post of the manual, It has some specs on
>>>> page 3 and says,
>>>>
>>>>> Intermodulation: Measured with 611 and 703 KHz test tones at 0 dBm
>>>>> per tone,
>
> 0 dBm into 50 ohms is about 220 mVrms and 300 mVpk. The worst sum
> voltage is 600 mVpk or 1.2 Vpp, which is the required linear voltage
> swing.
>
>>>>
>>>> typical input intercepts are: IIP2 +87 to +93 dBm; IIP3 +40dBm to +45 dBm
>>>>
>>>> You can believe it or not, I believe the numbers will be high, that's
>>>> is one of the main criteria for a BCB preamp. There are so many
>>>> signals you do everything to avoid mixing them.
>>>>
>>>>> https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf
>>>>
>>>> 2nd and 3rd order intermodulation measurements on page 11/12.
>>>>
>>>
>>> So they claim.  With input signals at 0 dBm, an amp with +93 dBm IP2
>>> would generate second order products at -93 dBm as well.  That requires
>>> matching of the discrete FETs' transconductance vs I_D to a few parts in
>>> 10**5 at signal levels near the amp's P_1dB.
>
>
> At +12 Vdc, the available swing might be 9 Vpp or 3 Vrms or about +20
> dBm, so the P_1dB is slightly larger than this.

You aren't getting 3V RMS with 10 mA of bias current. Remember, dB is a
power ratio, not voltage.

>> Around 0 dBm is way too much input power for measuring IP2/IP3.
>>
>> 3rd order intermodulation products rise by 30 dB for an input power
>> increase of 10 dB. This is simply impossible for an amplifier
>> approaching saturation.
>>
>> Therefore one gets results that are _much_ too good.
>
> After all, this is a voltage follower. At least for bipolar common
> collector circuits the voltage gain is often quoted as 0.99.

-93 dBc is 0.002% THD. You ain't getting that with an unbalanced
complementary follower in large signal conditions with no feedback.

>
> The distortion is created due to the curvature of the transfer
> function. How much curvature does a voltage follower have well inside
> the supply voltage limits ?

Depends on the load, but with a JFET with no drain bootstrap, it has
lots. The drain impedance of JFETs is the pits, and yours are
mismatched. Second-order distortion comes from the two half-cycles
being amplified differently. At any given frequency and one choice of
signal levels, there might be a pot setting that nulls out the second
harmonic, but that's a cheat--anyplace much away from that, the
performance will go back into the tank.

Intercept point is not measured at a single input level--you start from
a low level, and plot the spur amplitude vs. input level. At low
levels, an Nth order product will go as the Nth power of the input
levels. You draw the straight lines on a log-log plot, and the
intercept point is where the extrapolated lines intercept. Saying that
the intercept point is measured 'at 0 dBm' is complete bollocks, and
strongly suggests chicanery, as explained above.

> Does a JFET common drainollower behave differently from a bipolar

> common collector voltage follower ?

Yes, it's worse because of the low drain impedance.

With a bootstrap and some local feedback to keep the JFETs' bias
conditions constant, you can do much better.

>>
>> IP must be measured at levels where the intermodulation products
>> just come out of the noise.
>>
>> cheers, Gerhard
>>
>