NOISE FIGURE OF A BIPOLAR TRANSISTOR

[RealInfo]

Hi all

In an article about low noise PU preamp for magnetic stylus
it was written that the bipolar transistors were chosen for their low noise figure .My question is how exactly a noise figure of a single bip[olar transistor is defined measured .

Thanks
Elico

[Jeroen Belleman]

Buy or borrow a copy of 'Art of Electronics' by Horowitz & Hill
and read the relevant chapter. That should teach you more than
any Usenet discussion.

Jeroen Belleman

[upsid...@downunder.com]

I assume by the context that by PU you are actually referring to some
audio pick-up preamplifier.

If your intension is to achieve best power match (source impedance =
load impedance) then some grounded base amplifier is the best choice
(a big grounded base 2N3055 or a half doxen grounded base transistors
in parallel).

However, typically magnetic pick-ups are designed for much greater
load impedance to give a flat (after RIAA correction) frequency
response.

So what do you exactlly want to do ?

[Jeroen Belleman]

You're off on a tangent. He's asking about noise, not about
power transfer. Moreover, power matching and noise matching
aren't the same.

Jeroen Belleman

[Phil Allison]

"RealInfo"

>
> In an article about low noise PU preamp for magnetic stylus
> it was written that the bipolar transistors were chosen for
> their low noise figure .My question is how exactly a noise

> figure of a single bipolar transistor is defined measured .

** Noise figures for transistors are not numbers, but graphs.

http://www.cytium.net/hobby/bc107.pdf

They show the measured results for a typical example of a BC109 for varying
bandwidths, Ic and input resistance for a fixed Vce of 5 volts.

The definition of "noise figure" is when the measured noise is so many dB
*above* the calculated value for the particular source resistance and
frequency/bandwidth.

A noise figure of 0dB implies that the device ( under some specified
condition) adds NO noise to that inherent in the source, a figure of 1dB
implies that the noise level is 1dB above the theoretical limit.

FYI:

All resistive sources have " thermal noise " which follows the formula:

" Nv = sq.rt. 4.K.T.B.R " where

Nv = rms noise voltage

K = Boltzman's constant ( 1.38 exp-23)

T = absolute temperature in degrees K

B = effective test bandwidth

R = resistance value

Eg:

For a 200ohm resistor and a 20kHz bandwidth at room temp, the calculated
value is 0.255uV rms.

For a 20kohm resistor, the result is 2.55uV rms.


... Phil

[radam...@gmail.com]

For a bipolar transistor you have both input voltage noise and input current noise, so the impedance of the cartridge plays a role. If you increase the bias of the input devices, the voltage noise goes down by the square root of the current and the input current noise goes up by the square root of current, so there is an optimum bias point for a given impedance. This rule holds until you reach the thermal noise of the bulk base resistance, after which the voltage noise does not continue to fall with increasing bias. For all these reasons, a good low-noise transistor will have low bulk base resistance and high Beta, which is tricky because these parameters tend to go in opposite directions.
You can get JFETS with very low voltage noise and these are often preferred over bipolars because they have 0 input noise current. This makes it easier to get a good noise figure for a given cartridge impedance.

Whenever I read threads about phono preamps I feel like I've been transported in time back to 1970, when these topics were all the rage. Ten years from now this topic will be covered by people giving invited talks in retirement homes :)


Bob

[Jeroen Belleman]

On 2013-02-12 12:08, Phil Allison wrote:
> "RealInfo"
>>
>> In an article about low noise PU preamp for magnetic stylus
>> it was written that the bipolar transistors were chosen for
>> their low noise figure .My question is how exactly a noise
>> figure of a single bipolar transistor is defined measured .
>
> ** Noise figures for transistors are not numbers, but graphs.
>
> http://www.cytium.net/hobby/bc107.pdf

>[...]

I wonder what to make of the two plots on the 2nd row of page 5.
Same scales, same measurement conditions, different curves.
Comparing with the Philips datasheet, I gather the measurement
frequency was probably different.

Anyway, say we were to use a BC109 at Ic=10uA and a source
resistance of 20kOhm, the 1.5dB noise figure works out to
a tad under 12nV/rtHz, if I got my arithmetic right.

Almost any JFET can beat that with ease!

Jeroen Belleman

[RealInfo]

Thanks
Elico

[Tim Wescott]

IIRC the best noise match impedance of a bipolar transistor amplifier
does not change when you go from common-emitter to common base. The
power match does, but not the noise match.

If you have a data sheet for a transistor that's designed as a low-noise
amplifier it should have various curves showing noise figure vs. various
things. IIRC Motorola would plot noise figure vs. collector current and
source impedance.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

[Mark]

>
> IIRC the best noise match impedance of a bipolar transistor amplifier
> does not change when you go from common-emitter to common base.  The
> power match does, but not the noise match.
>
>

speaking of noise figure...
something thats troubled me...

it seems to me the any LNA ***that provides a good input match****
(talking about RF amplifiers in a 50 Ohm system) and is physically at
room temperature cannot also have a noise figure better then 3 dB.

To look at it another way, can you create an active (or otherwise)
50 Ohm load that creates less noise than a 50 Ohm resistor creates?

Mark

[Phil Hobbs]

Grab yourself a cold beer from the fridge, and think about how it could
be at 40F in a 70F ambient. Same basic answer.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

[Phil Hobbs]

On 02/12/2013 12:59 PM, Phil Hobbs wrote:
> On 02/12/2013 12:44 PM, Mark wrote:
>>
>>>
>>> IIRC the best noise match impedance of a bipolar transistor amplifier
>>> does not change when you go from common-emitter to common base. The
>>> power match does, but not the noise match.
>>>
>>>
>> speaking of noise figure...
>> something thats troubled me...
>>
>> it seems to me the any LNA ***that provides a good input match****
>> (talking about RF amplifiers in a 50 Ohm system) and is physically at
>> room temperature cannot also have a noise figure better then 3 dB.
>>
>> To look at it another way, can you create an active (or otherwise)
>> 50 Ohm load that creates less noise than a 50 Ohm resistor creates?
>>
>> Mark
>>
>
> Grab yourself a cold beer from the fridge, and think about how it could
> be at 40F in a 70F ambient. Same basic answer.
>

Well, I suppose _maybe_ I could be a bit less of a smartass about it. ;)

The small-signal emitter resistance of a bipolar transistor is

r_E = kT/(eI_C),

i.e. about 26 mV/I_C at room temperature. If you multiply that by the
shot noise of the emitter current, which is

i_N = sqrt(2*e*I_C),

and do two lines of algebra, you get

v_N = sqrt(2*k*T*r_E).

Comparing this with the usual Johnson noise formula, you find that the
noise temperature of a forward-biased emitter is T/2, i.e. 150K at room
temperature.

(At thermal equilibrium, you can't have a bias voltage or a net emitter
current, so the forward and reverse diffusion currents are equal. They
each contribute half of the fluctuations, so the factor of 2 is
restored. You need the full Ebers-Moll expression to show this, but I'm
too lazy to type it out.)

The beta of a BJT is the really low noise thing in electronics. The
intrinsic base (i.e. neglecting the actual resistance of the silicon)
has an impedance r_B = beta * r_E, but has exactly the same noise as the
emitter. (It has to, because there are only two wires involved.)

Thus the noise temperature of the input resistance of an ideal BJT CE
amplifier ought to be right around T_J/(2*beta).

It's never quite that good, of course, because the base current has shot
noise and there are real physical resistances that have noise of their own.

[Robert Baer]

Start by biasing the transistor the same way as in final circuit,
most especially the collector current and base input resistance.
Use as a common emitter amplifier & pick off signal at collector with
low noise amplifier.
Calibrate the gain, use bandwidth filters.
May calibrate by inserting white noise at input of base (without
changing the base source impedance) and cranking up to double reading;
note value, convert to dBm.
May then use that to calibrate meters at those filters.
Refine instrumentation and sell it!!!

[Robert Baer]

Check on that last point..good low noise transistors have low base
spreading resistance which always makes things worse than the theory
mentioned.
What is interesting is that noise measured in the audio region
correlates very well with RF NF, as long as that base spreading
resistance is low.
..and that can be approximated from spot noise measurements at
nominal currents and very low (collector) currents.

[Tim Wescott]

I've seen suggestions (in, for example, "Radio Frequency Design" by
Hayward) that you can get simultaneous power match and noise match in an
amplifier using transformer feedback (he called it "advanced feedback
methods"). I'm not sure if this gets you below that magic T/2 noise
temperature, though -- I haven't even done the math on the things, much
less built them and tried them out.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

[bloggs.fred...@gmail.com]

That's not what noise figure is. It is defined as the multiple of the system resistance noise. The most advanced low noise amplifiers extant are probably the RF amps for satellite receiver front ends, typically just a few tenths of db NF last time I checked.

[Phil Hobbs]

In an amplifier, it's the output power you care about, not the power
delivered to the input stage. Even at low frequency, the "noise match"
isn't the same as maximum power transfer on the input.

The output voltage is proportional to the input voltage, which for a
constant available input power goes as sqrt(Zs)*Zin/(Zs+Zin).

That peaks at Zs = Zin, as expected, but since the noise temperatures
of the two resistances are different, the noise voltage at the input
continues to change. That shifts the SNR peak away from maximum power
transfer.

[John Larkin]

On Tue, 12 Feb 2013 09:44:28 -0800 (PST), Mark <mako...@yahoo.com>
wrote:

Microwave LNAs have noise temps in the 60K range, fraction of a dB
noise figure. The input impedance of an amplifier can look like a very
cold 50 ohm resistor.

If you connect a 50 ohm resistor, at room temp, to such an amplifier,
it will cool the resistor. A little.


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

[John Larkin]

I've tested a lot of InGaAs (? ERA-type stuff) MMICs and almost all
seem to have Zin well below 50 ohms, closer to 30 usually. I've
assumed that is to optimize NF. Sirenza makes one SiGe part that is
really 50 ohms, and can be tuned to exactly 50 by fiddling with the
device current.

The RF guys seem to think that an SWR of 2:1 is good enough.

[Phil Hobbs]

RF amplifiers are always about 2:1 in my experience too. If you don't
like it, put in a pad. ;)

[Robert Macy]

ok I'll bite.

The noise at the junction is actually based on sqrt(25ohms) because
the two 50 ohms are in parallel, the supply Z and 50 ohm load Z are in
parallel.

Wait. you say that the 50 ohm resistor makes more noise? Yes, but by
an additional sqrt(2) then that noise is divided by two to the same
junction and then is added as the square root of the sum of the
squares because of the lack of coherence and you're right back to the
same noise as from a 25 ohm resistor. So that means *if* you compare
the input noise to that caused by a 50 ohm resistor, anything above
that becomes the NF.

[radam...@gmail.com]

In theory it's possible to synthesize a 50 ohm resistor using the Miller effect, and end up with a resistor that has less than 4KTR noise. Assuming the amplifier you use for the Miller effect is ultra-low-noise. This could buy you 3db in the limit, assuming you used this resistor as a termination. Don't know how practical this really is.

Back to the OP's topic, generally real products use JFETS. You can get them with less than 2 nv/root-hz, I think. Some of those parts have probably gone obsolete.

Bob

[Tim Williams]

"Phil Hobbs" <pcdhSpamM...@electrooptical.net> wrote in message
news:UNqdnf4z7q-lJofM...@supernews.com...

>> The RF guys seem to think that an SWR of 2:1 is good enough.
>>
> RF amplifiers are always about 2:1 in my experience too. If you don't
> like it, put in a pad. ;)

A pad? As in, resistors?

/Imagines Phil dragging his fingernails across a chalkboard ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

[Phil Hobbs]

On 2/12/2013 7:08 PM, Tim Williams wrote:
> "Phil Hobbs" <pcdhSpamM...@electrooptical.net> wrote in message
> news:UNqdnf4z7q-lJofM...@supernews.com...
>>> The RF guys seem to think that an SWR of 2:1 is good enough.
>>>
>> RF amplifiers are always about 2:1 in my experience too. If you don't
>> like it, put in a pad. ;)
>
> A pad? As in, resistors?
>
> /Imagines Phil dragging his fingernails across a chalkboard ;-)

C'mon, I have nice fingernails.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203

Briarcliff Manor NY 10510 USA
+1 845 480 2058

[Phil Hobbs]

Not only is it possible in theory, it's commonly done in front ends.
Not Miller, which is capacitive, but a similar idea--you use a quiet
inverting amplifier to jiggle the opposite end of a resistor to make it
look smaller.

[Phil Allison]

"Jeroen Belleman"

** So the NF with a JFET might be 1 dB = no audible difference.

BTW:

The effective noise Z of a mag PU is about 4000 ohms, when RIAA
equalised. In real RIAA pre amps, JFETs have disadvantages
(ie non linearity, low gain & large basic parameter variations)
that outweigh any tiny noise advantage.




... Phil

[Robert Macy]

Linear Technology's LT1011 [I think it is] has around 1 nV/rtHz input
noise.

Again, from memory Supertex makes some FETs with less than 1nV/rtHz.

[Phil Hobbs]

The BF862 is around 0.7 to 0.8 nV, with the usual JFET noise corner at 1
kHz or so. The LT1028 and its ilk are around 0.9 nV, but the original
LT1028A has a nasty noise peak around 300 kHz that they don't tell you
about. The datasheet noise plot conveniently ends well below that, even
though it's a 100 MHz op amp. (Marketing again.)

The ADA4898 is a nice well-behaved part with around 0.9 nV noise.

[Michael A. Terrell]

Tim Williams wrote:
>
> "Phil Hobbs" ?pcdhSpamM...@electrooptical.net? wrote in message
> news:UNqdnf4z7q-lJofM...@supernews.com...
> ?? The RF guys seem to think that an SWR of 2:1 is good enough.
> ??
> ? RF amplifiers are always about 2:1 in my experience too. If you don't
> ? like it, put in a pad. ;)

>
> A pad? As in, resistors?

3 dB pads are common between RF circuits, to improve both matching &
isolation between stages. It was always a PITA to be slightly below the
gain spec, and not be able to remove a pad without causing other
headaches.

[Tim Wescott]

I said "power match" but should have said "impedance match". I believe
that Hayward stated that the reason is to make the various filters in the
system happy.

But your general tone of superiority makes it obvious that Wes Hayward,
and his work (neither books, nor, presumably, decades of Tektronix
spectrum analyzers) exist.

Silly me. And here I've been laboring under the delusion that I've used
those spectrum analyzers, and read some of the books, to boot.

[Mr Stonebeach]

On Feb 13, 2:23 am, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:> Not only is it

possible in theory, it's commonly done in front ends.
> Not Miller, which is capacitive, but a similar idea--you use a quiet
> inverting amplifier to jiggle the opposite end of a resistor to make it
> look smaller.

Exactly. The trick traces back to the vacuum tube era:
W.S.Percival, "An Electrically Cold Resistance",
the Wireless Engineer, May 1939, p. 237. It is necessary
as a standard practice in room-temperature front ends
of SQUID readouts, where the generator resistance indeed has
the 50-ohm Johnson noise *but* it is located in LHe.

Nowadays eg. the VCA2611 and AD8331 use the technique. I'm
in impression that most low-noise rf/microwave gain blocks
utilize that technique to move the noise match and power match
to roughly the same impedance.

The technique is effectively the same as the thought
experiment of damping an indicator needle, discussed in
many thermodynamics textbooks. I think I got first exposed
to the idea in the Kittel's book, without realizing how
widely it is applicable.

Regards,
Mikko

[Mr Stonebeach]

On Feb 12, 11:48 pm, John Larkin <jlar...@highlandtechnology.com>
wrote:

> If you connect a 50 ohm resistor, at room temp, to such an amplifier,
> it will cool the resistor. A little.

´
That's right, that was Nyquist's original thought experiment.
I have been wanting to demonstrate it, but it'd be tough to
get a measurable effect.

Actually, in Transiton Edge bolometers the Johnson noise
is modified because of the correlated temperature fluctuation
in the heat bath - because the electrical power present in
each upward (downward) voltage swing is drawn from (dumped
into) the thermal bath.

Vinante et. al. have used a SQUID and feedback to cool
a metal bar weighting a ton into microkelvin range of
temperatures (although the above wording cheats a bit).

Regards,
Mikko

[Mr Stonebeach]

On Feb 13, 3:48 am, Phil Hobbs

<pcdhSpamMeSensel...@electrooptical.net> wrote:
> LT1028A has a nasty noise peak around 300 kHz that they don't tell you
> about.  The datasheet noise plot conveniently ends well below that, even
> though it's a 100 MHz op amp.  (Marketing again.)

Indeed. That undocumented peak spoiled our noise-cancelling
readout back in 1994. There's another peak at 2.5 MHz. We had
to move to the AD797.

Regards,
Mikko

[Jeroen Belleman]

On 2013-02-12 23:52, radam...@gmail.com wrote:
> In theory it's possible to synthesize a 50 ohm resistor using the
> Miller effect, and end up with a resistor that has less than 4KTR
> noise. Assuming the amplifier you use for the Miller effect is
> ultra-low-noise. This could buy you 3db in the limit, assuming you
> used this resistor as a termination. Don't know how practical this
> really is.

It's quite practical. I've built amplifiers with 50 Ohm matched
inputs and 300pV/rtHz input-referred noise. I use them in the
beam trajectory measurement system of a particle accelerator.

The signal source isn't resistive. It's a capacitive position
pick-up. I'd have preferred to use Hi-Z amplifiers directly on
the pick-ups, but the radiation would kill them.

>
> Back to the OP's topic, generally real products use JFETS. You can
> get them with less than 2 nv/root-hz, I think. Some of those parts
> have probably gone obsolete.

A BF862 does 0.8nV/rtHz. There are lower noise JFETS, but none
with Yfs/Cg as good as this one. Well, perhaps some RF devices
can beat that, but those lose out on 1/f noise.

Jeroen Belleman

[Mr Stonebeach]

Hi Tim,

I did not sense that Phil objected Haywards conclusion
about moving the noise match and impedance/power match
to the same Z. I think that's completely feasible and
done on a regular basis.

I think Phil's point was that the noise match is more
important than the power match. If you cannot squeeze
as much power gain out of your stage as you would with
the optimal power match, you can always add stages. Gain
is cheap. But if you spoil your SNR with non-optimal
noise match, there is no way you could get rid of the
added noise in the subsequent stages. It'll sit in
your signal forever.

The above does not contradict with this fact: if
the earlier stages want to see a 50-ohm amplifier input
(e.g. for proper freq response of filters, termination
of directional couplers etc.), you better arrange such
an input or risk screwing the whole circuit. In this
case (as you indicated) the 50-ohm input is not needed
for the maximum power transfer, but for proper damping
of the previous stages.

Regards,
Mikko

[Phil Hobbs]

I didn't see your followup post that Mikko is replying to, but he's
right, I didn't intend to contradict you. I'm not a big 50-ohm RF guy
myself--I generally buy stuff for that, and certainly until I do the
math myself, I'll take the word of somebody who has.

When it comes to front ends, my shtick is salvaging good performance out
of inconvenient corners of the design space, and impedance matching is
usually a very peripheral issue.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203

Briarcliff Manor NY 10510 USA
+1 845 480 2058

[Phil Hobbs]

The familiar TIA is sort of the logical conclusion of that process, i.e.
cranking the gain up as high as you can to make the input resistance
close to zero, but that isn't always what you want.

Front ends are fun.

[John Larkin]

On Tue, 12 Feb 2013 23:43:34 -0800 (PST), Mr Stonebeach <r...@wmail.fi> wrote:

>On Feb 12, 11:48�pm, John Larkin <jlar...@highlandtechnology.com>
>wrote:
>> If you connect a 50 ohm resistor, at room temp, to such an amplifier,
>> it will cool the resistor. A little.
>�
> That's right, that was Nyquist's original thought experiment.
>I have been wanting to demonstrate it, but it'd be tough to
>get a measurable effect.

Two electrically-connected resistors have an equivalent thermal conductivity,
through their Johnson noise, but it's many orders below the thermal conductivity
of any electrical conductor. The effect is probably unmeasurable.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

[Robert Macy]

On Feb 12, 6:48 pm, Phil Hobbs

THANKS!!!
first 1028, not 1011 Brain dead here.
second, I got bit by that spike! Not having any instrumentation of
value had to 'noodle' out what was wrong and did not understand until
now.

Really sad, because Linear parts are usually 'normal'

[John Larkin]

On Tue, 12 Feb 2013 20:48:45 -0500, Phil Hobbs
<pcdhSpamM...@electrooptical.net> wrote:

>On 2/12/2013 8:34 PM, Robert Macy wrote:
>> On Feb 12, 3:52 pm, radams2...@gmail.com wrote:
>>> In theory it's possible to synthesize a 50 ohm resistor using the Miller effect, and end up with a resistor that has less than 4KTR noise. Assuming the amplifier you use for the Miller effect is ultra-low-noise. This could buy you 3db in the limit, assuming you used this resistor as a termination. Don't know how practical this really is.
>>>
>>> Back to the OP's topic, generally real products use JFETS. You can get them with less than 2 nv/root-hz, I think. Some of those parts have probably gone obsolete.
>>>
>>> Bob
>>
>> Linear Technology's LT1011 [I think it is] has around 1 nV/rtHz input
>> noise.
>>
>> Again, from memory Supertex makes some FETs with less than 1nV/rtHz.
>>
>
>The BF862 is around 0.7 to 0.8 nV, with the usual JFET noise corner at 1
>kHz or so. The LT1028 and its ilk are around 0.9 nV, but the original
>LT1028A has a nasty noise peak around 300 kHz that they don't tell you
>about. The datasheet noise plot conveniently ends well below that, even
>though it's a 100 MHz op amp. (Marketing again.)
>
>The ADA4898 is a nice well-behaved part with around 0.9 nV noise.
>
>Cheers
>
>Phil Hobbs

The price you pay in bipolars like the LT1028 is current noise. In the
non-inverting config, you wind up with a 20 ohm feedback divider. I
sometimes have the 1028 drive a follower opamp which in turn drives
the feedback divider, so the feedback current doesn't make thermal
tails in the LT1028.

--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com

http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators

Custom laser drivers and controllers

Photonics and fiberoptic TTL data links

[Phil Hobbs]

On 2/13/2013 11:51 AM, John Larkin wrote:
> On Tue, 12 Feb 2013 20:48:45 -0500, Phil Hobbs
> <pcdhSpamM...@electrooptical.net> wrote:
>
>> On 2/12/2013 8:34 PM, Robert Macy wrote:
>>> On Feb 12, 3:52 pm, radams2...@gmail.com wrote:
>>>> In theory it's possible to synthesize a 50 ohm resistor using the Miller effect, and end up with a resistor that has less than 4KTR noise. Assuming the amplifier you use for the Miller effect is ultra-low-noise. This could buy you 3db in the limit, assuming you used this resistor as a termination. Don't know how practical this really is.
>>>>
>>>> Back to the OP's topic, generally real products use JFETS. You can get them with less than 2 nv/root-hz, I think. Some of those parts have probably gone obsolete.
>>>>
>>>> Bob
>>>
>>> Linear Technology's LT1011 [I think it is] has around 1 nV/rtHz input
>>> noise.
>>>
>>> Again, from memory Supertex makes some FETs with less than 1nV/rtHz.
>>>
>>
>> The BF862 is around 0.7 to 0.8 nV, with the usual JFET noise corner at 1
>> kHz or so. The LT1028 and its ilk are around 0.9 nV, but the original
>> LT1028A has a nasty noise peak around 300 kHz that they don't tell you
>> about. The datasheet noise plot conveniently ends well below that, even
>> though it's a 100 MHz op amp. (Marketing again.)
>>
>> The ADA4898 is a nice well-behaved part with around 0.9 nV noise.
>>
>> Cheers
>>
>> Phil Hobbs
>
> The price you pay in bipolars like the LT1028 is current noise. In the
> non-inverting config, you wind up with a 20 ohm feedback divider. I
> sometimes have the 1028 drive a follower opamp which in turn drives
> the feedback divider, so the feedback current doesn't make thermal
> tails in the LT1028.
>
>

Good idea. My fave at the moment is that PNP-wraparound-BF862 follower
running into the - input of an ADA4898 or 4899 (a 500 MHz, +-5V part).
But that has about sqrt(2) times more voltage noise and doesn't have the
good drift of a LT1028. That's often a good trade for 1 pA input bias
current.

[Mr Stonebeach]

On Feb 13, 5:28 pm, John Larkin

<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> Two electrically-connected resistors have an equivalent thermal conductivity,
> through their Johnson noise, but it's many orders below the thermal conductivity
> of any electrical conductor.

The Wiedemann-Franz law, yes. But it doesn't apply if you have
superconductors
available. They conduct electricity but not heat. Or more accurately:
the
electron-mediated heat conductivity freezes out, but phonon-mediated
remains.

But it still is tough...

Regards,
Mikko

[Phil Hobbs]

But you'd still be transferring k(T_hot-T_cold) per hertz just via
I**2*R, even with superconducting wires, right?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

[Mr Stonebeach]

On Feb 13, 11:25 pm, Phil Hobbs

<pcdhSpamMeSensel...@electrooptical.net> wrote:
> On 02/13/2013 02:34 PM, Mr Stonebeach wrote:
> > On Feb 13, 5:28 pm, John Larkin
> > <jjlar...@highNOTlandTHIStechnologyPART.com>  wrote:
> >> Two electrically-connected resistors have an equivalent thermal conductivity,
> >> through their Johnson noise, but it's many orders below the thermal conductivity
> >> of any electrical conductor.
>
> >    The Wiedemann-Franz law, yes. But it doesn't apply if you have
> > superconductors
> > available. They conduct electricity but not heat. Or more accurately:
> > the
> > electron-mediated heat conductivity freezes out, but phonon-mediated
> > remains.
>
> >    But it still is tough...
>
> >    Regards,
> >             Mikko
>
> But you'd still be transferring k(T_hot-T_cold) per hertz just via
> I**2*R, even with superconducting wires, right?

Yes, exactly, you would. The cooling effect via the Nyquist
mechanism would be there, and the effect would not get counteracted
by the heat backconduction. As John argued, with non-superconductors
any temperature difference (due to the Nyquist mechanism) would tend
to equilibrate via ordinary heat conduction.

My colleaques one floor up have actually been experimenting with
related effects.

Regards,
Mikko

[Spehro Pefhany]

On Wed, 13 Feb 2013 11:34:02 -0800 (PST), Mr Stonebeach <r...@wmail.fi>
wrote:

Thermal conductivity of superconducting Nb is not so bad. About 0.25*
RRR at 4.2K. Of course you can make the "wires" extremely thin since
the currents are very low.

[josephkk]

On Tue, 12 Feb 2013 11:04:19 +0200, upsid...@downunder.com wrote:

>On Mon, 11 Feb 2013 23:40:21 -0800 (PST), RealInfo
><therig...@gmail.com> wrote:
>
>>Hi all

>>
>>In an article about low noise PU preamp for magnetic stylus

>> it was written that the bipolar transistors were chosen for their low noise figure .My question is how exactly a noise figure of a single bip[olar transistor is defined measured .
>
>I assume by the context that by PU you are actually referring to some
>audio pick-up preamplifier.
>
>If your intension is to achieve best power match (source impedance =
>load impedance) then some grounded base amplifier is the best choice
>(a big grounded base 2N3055 or a half doxen grounded base transistors
>in parallel).
>
>However, typically magnetic pick-ups are designed for much greater
>load impedance to give a flat (after RIAA correction) frequency
>response.
>
>So what do you exactlly want to do ?

And you are responding to this scammer WHY?

?-(

[Mr Stonebeach]

On Feb 14, 12:35 am, Spehro Pefhany

<speffS...@interlogDOTyou.knowwhat> wrote:
> Thermal conductivity of superconducting Nb is not so bad. About 0.25*
> RRR at 4.2K. Of course you can make the "wires" extremely thin since
> the currents are very low.

At 4.2K Nb is still quite close to its transition temperature.
Normal
electrons (which do carry heat, as per two-fluid model) freeze away
exponentially
when the temperature is lowered below the transition. At half-Tc the
normal electron fraction still contributes significantly on the heat
conductivity.

Regards,
Mikko

[Glenn]

On 12/02/13 21.09, Tim Wescott wrote:
...

> I've seen suggestions (in, for example, "Radio Frequency Design" by
> Hayward) that you can get simultaneous power match and noise match in an
> amplifier using transformer feedback (he called it "advanced feedback
> methods"). I'm not sure if this gets you below that magic T/2 noise
> temperature, though -- I haven't even done the math on the things, much
> less built them and tried them out.
>

Hi Tim

Do you mean the Norton amplifier?:


Lossless Feedback Amplifiers: Theory and Advanced Techniques:
http://www.ko4bb.com/Manuals/08)_Misc_Ham_Equipment/Trask_-_Lossless_Feedback_Amplifiers.pdf
"...
Perhaps the single most significant development in high dynamic range
amplifiers has been that of the lossless feedback amplifier. Conceived
and patented by David Norton and Allen Podell of Adams-Russell (2), this
to- pology is often referred to as a Norton amplifier, and sometimes as
noiseless feedback.
..."

/Glenn