Probably a stupid question...
Dave
Hate to expose my ignorance, but... Is it the biasing? Or what? What I
have doesn't specify such on the packaging, and I am wondering.
TNX
Dave
Bob Myers
"Dave" <db5...@hotmail.com> wrote in message
news:13dgm6b...@corp.supernews.com...
> But what determines the gain, or amplification factor, of an N-channel
> JFET? Hate to expose my ignorance, but... Is it the biasing? Or what?
> What I have doesn't specify such on the packaging, and I am wondering.
Transistors, etc., themselves don't have a "gain" or
"amplification factor," unless you're talking about
some very specific parameters related to the inner
workings of the component (such as "beta" for a bipolar
transistor, which is sort of "gain" figure that relates the
collector current to the base current). But those
parameters are not directly related to the "gain" figure
that will be established for the complete circuit which
uses that device; THAT comes in from other factors,
such as the particular amplifier configuration involved,
the values of particular components used within that
circuit (which will, among other things, set the biasing,
so yes, that is involved) the impedance of the source
and load, etc., etc..
Bob M.
Dave
"Bob Myers" <nospam...@address.invalid> wrote in message
news:fb9p11$jd0$1...@usenet01.boi.hp.com...
Hmmm. Okay. Thank you very much for the reply and the information. It's
been so long since I dabbled in such I am beginning to wonder what I *do*
remember. I have a copy of Electronic Principles and am trying to work my
way through the transistors sections, but it is slow going. Thanks again,
and thanks for taking the question seriously. :)
Dave
Jim Thompson
My take... Bob M. = amateur BS artist ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Don Bowey
"Jim Thompson" <To-Email-Use-Th...@My-Web-Site.com> wrote:
Aren't you into the grape a bit early today. ;-)
Jim Thompson
wrote:
Nope. Tell me, Is there anything intelligible in that, "Transistors,
etc..." ??
Fred Bloggs
> But what determines the gain, or amplification factor, of an N-channel JFET?
> Hate to expose my ignorance, but... Is it the biasing? Or what? What I
> have doesn't specify such on the packaging, and I am wondering.
Your packaging should specify things like BVdss, gm, Idss, and Vp at a
minimum. The "gain" of the standalone device is understood to mean gm,
the transconductance, and this is the small signal ratio of
drain-to-source current per unit of gate-source voltage. The JFET is
ideally a voltage controlled current source: the current is
drain-source, the control voltage is gate-source, and gm is the control
factor. Very simple.
Dave
"Fred Bloggs" <nos...@nospam.com> wrote in message
news:46D88EEF...@nospam.com...
COOL! Thank you, very much. I was ready to have to analyze the entire
circuit of a project I built based on a commercial schematic that doesn't
seem to amplify small-signal RF very much. Your words are most encouraging.
Much appreciated.
Dave
Dave
Forgot to add "and I may still have to do that..."
Bottom line: I now have a starting point, from which to proceed forward.
Again, Thanks.
Joel Kolstad
> circuit of a project I built based on a commercial schematic that doesn't
> seem to amplify small-signal RF very much.
How much is "not very much?" And at what frequency?
Unlike say, the op-amps that Jim designs that give you something absurd like
90dB open-loop gain, many RF amplifiers only provide some 10-20dB (which is
~3x-10x voltage) gain.
Dave
"Joel Kolstad" <JKolstad7...@yahoo.com> wrote in message
news:13dh7fj...@corp.supernews.com...
"Not very much" means a barely perceptable difference in signal quality when
used with a standard portable shortwave radio, if any perceptablel
difference at all. At any freq in the shortwave spectrum, but mainly from
approximately 6 MHz to 12 MHz. The commercial product I was copying is
supposed to help pull in weak signals, but I have heard it doesn't do this
very impressively, so suspected the schematic I was working with was just
not designed very well. Makes use of two N-channel JFETs and a UHF high
speed switch, but left a lot to be desired on my part. I added a
Q-multiplier taken from Joe Carr's Practical Antenna Handbook, which
improved things, but I'm still not satisfied as I want it to work with the
on-board whip as well as it does with the 110' longwire antenna I currently
use it with. I am guessing I need another 20 or 30 db out of it, at least.
I am still fairly new to RF however, so it goes slowly.
Thanks,
Dave
Jamie
Hows it going Jim? that's not like you :)
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Jamie
Well, I'll tried to shorten it and in simple terms..
JFETS have to be biased to shut them down, in other
words, they normally are in their state of flow/low-resistance with
the gate pulled to common for example.
With JFETS, you must lower the gate voltage below
the source voltage. This figure is in the specs of the
transistor data sheet of where the pinch off point is.
Since the FETS are voltage biasing devices and not
current biasing devices like bipolar, this is where
signal source types dictate's as to how the circuit
should be designed.
In the case of FETS, those with lower voltage specs
on the Gate-source (Vgs) that places it in the pinch
off state is generally those that will give you a higher
gain when designing around them.
So basically, a small (Vgs) for pinch off should give a
higher gain.
Now since fets have very high impedance on the gate, this
device is very good in cases where you need to reference
signals that generate a higher voltage but hardly not enough
current to drive a bipolar type input circuit with out effecting
the performance of the reference device.
There are other benefits to using FETS over bipolar like switches
and so on, but won't get into that.
P.S.
an Enhanced mode fets work the other way.
John Larkin
The datasheet should specify a typical transconductance at some drain
current, and probably has a graph of same. The common-source voltage
gain is nearly equal to the transconductance (gm, in Siemens)
multiplied by the load resistance (in ohms) in the drain circuit.
The operating current is a function of the gate-source voltage,
another spec or curve. Actually, gm is the derivative (slope) of the
Id versus Vg curve. In general, the higher the drain current, the
higher the transconductance.
A typical small-signal jfet might operate with the gate a half volt or
so negative relative to the source, and might have a gm of, say, 0.003
Siemens at 4 mA or so drain curent. If it dumps into a 2K drain
resistor, the voltage gain would only be about 6.
Jfets have low gains and, usually, terrible part-to-part
repeatability, with Idss sometimes specified over a 5:1 or even 10:1
range, which makes design tricky.
"Amplification factor" for a tube is transconductance times plate
resistance, which is the voltage gain you'd get with a
constant-current (very high impedance) plate load. The same idea
exists for a jfet, namely gm times the slope of the drain curve. I'm
guessing numbers like 25 or so for a typical jfet.
So, in general, jfets suck. They are handy in some niche applications,
like low-noise, high-impedance amplifiers.
John
Eeyore
Dave wrote:
You need to know the transconductance of the device. Also, the voltage gain will
vary according to the circuit in which it is operated.
A full treatment of the subject can be found in text books. There is no simple
one-line answer. You'll need to study to understand.
Graham
Eeyore
Jamie wrote:
> Dave wrote:
>
> > But what determines the gain, or amplification factor, of an N-channel JFET?
> > Hate to expose my ignorance, but... Is it the biasing? Or what? What I
> > have doesn't specify such on the packaging, and I am wondering.
>
> Well, I'll tried to shorten it and in simple terms..
Bwahahahaha ! Your 'answer' was anything but short and didn't even answer his
question.
> JFETS have to be biased to shut them down,
He didn't ask about 'shutting them down' did he ? It's called 'pinch off' by
electronics engineers btw.
> in other
> words, they normally are in their state of flow/low-resistance with
> the gate pulled to common for example.
> With JFETS, you must lower the gate voltage below
> the source voltage. This figure is in the specs of the
> transistor data sheet of where the pinch off point is.
> Since the FETS are voltage biasing devices and not
> current biasing devices like bipolar, this is where
> signal source types dictate's as to how the circuit
> should be designed.
> In the case of FETS, those with lower voltage specs
> on the Gate-source (Vgs) that places it in the pinch
> off state is generally those that will give you a higher
> gain when designing around them.
> So basically, a small (Vgs) for pinch off should give a
> higher gain.
> Now since fets have very high impedance on the gate, this
> device is very good in cases where you need to reference
> signals that generate a higher voltage but hardly not enough
> current to drive a bipolar type input circuit with out effecting
> the performance of the reference device.
> There are other benefits to using FETS over bipolar like switches
> and so on, but won't get into that.
More stupid waffle from Jamie the clown as ever.
I notice you couldn't even provide a simple answer like gm.Rl
Graham
D from BC
The gain depends on which spice model you are using. :)
D from BC
Dave
Thank you, all who provided information on the necessary details of what I
was ultimately asking about. More complex than I realized, but now I have
some idea as to what I am asking. And thank you specifically, Jamie, for
getting to the heart of the matter in a cogent and intelligable form.
I have some studying to do, but I at least know better what I am looking
for...
Again, many thanks.
Dave
Eeyore
Dave wrote:
> "Dave" <db5...@hotmail.com> wrote
>
> > But what determines the gain, or amplification factor, of an N-channel
> > JFET? Hate to expose my ignorance, but... Is it the biasing? Or what?
> > What I have doesn't specify such on the packaging, and I am wondering.
>
> Thank you, all who provided information on the necessary details of what I
> was ultimately asking about. More complex than I realized, but now I have
> some idea as to what I am asking. And thank you specifically, Jamie, for
> getting to the heart of the matter in a cogent and intelligable form.
>
> I have some studying to do, but I at least know better what I am looking
> for...
It's also a lot more complex at Radio Frequencies. Device capacitances come
heavily into play.
Graham
Bob Myers
"Jim Thompson" <To-Email-Use-Th...@My-Web-Site.com> wrote in
message news:4ktgd31rcf9qrrurq...@4ax.com...
>
> My take... Bob M. = amateur BS artist ;-)
>
No, Jim, actually I'm a pro at it....;-)
Bob M.
Bob Myers
"Jim Thompson" <To-Email-Use-Th...@My-Web-Site.com> wrote in
>
> Nope. Tell me, Is there anything intelligible in that, "Transistors,
> etc..." ??
>
Well, that would depend on just what you think was being
asked in the original post, doesn't it? I made the assumption,
based on what was being asked and how, that the OP was
under the impression that the way you made an amplifier was
to pick up a transistor that was labelled "Gain of 10" or some
such, and that's all there was to it.
If that assumption was incorrect, my apologies. But what did
YOU think was being asked?
Bob M.
Bob
> >> I was ready to have to analyze the entire
> >> circuit of a project I built based on a commercial schematic that doesn't
> >> seem to amplify small-signal RF very much.
>
> > How much is "not very much?" And at what frequency?
> "Not very much" means a barely perceptable difference in signal quality when
> used with a standard portable shortwave radio, if any perceptablel
> difference at all. At any freq in the shortwave spectrum, but mainly from
> approximately 6 MHz to 12 MHz. The commercial product I was copying is
> supposed to help pull in weak signals, but I have heard it doesn't do this
> very impressively, so suspected the schematic I was working with was just
> not designed very well. Makes use of two N-channel JFETs and a UHF high
> speed switch, but left a lot to be desired on my part. I added a
> Q-multiplier taken from Joe Carr's Practical Antenna Handbook, which
> improved things, but I'm still not satisfied as I want it to work with the
> on-board whip as well as it does with the 110' longwire antenna I currently
> use it with. I am guessing I need another 20 or 30 db out of it, at least.
> I am still fairly new to RF however, so it goes slowly.
Connecting a preamp directly to a receiver will not have a usefull
effect at all unless the preamp has a lower noise figure than the
front end
of the receiver. The noise figure, not the gain, is usually the
limiting factor.
When the preamp does have a usfull effect you will only notice
an increase in signal quality when the received signal would be
near the noise floor of the receiver without the preamp.
Shortwave is noisy, when listening to a signal that stronger than
about
10dB more than the sensitity figure of your receiver you will not
get any improvement from a preamp.
If connecting a preamp directly to a shortwave receiver has a
significant
effect it generally means that your receiver is deaf.
Preamps are usefull where there is significant loss in the cable
between the antenna and the receiver but only if the preamp is
at the antenna, not at the receiver.
Adding 30dB of wideband gain is unlikely to have a usefull effect. It
is likely to overload your receiver and produce lots
of noise due to intermodulation and harmonics when you
drive the receiver into nonlinearity.
Bob
JimW52
>
>> JFETS have to be biased to shut them down,
>
> He didn't ask about 'shutting them down' did he ? It's called 'pinch off'
> by electronics engineers btw.
>
What biasing is required to shut you down? Just curious :-)
Jim
John Larkin
Ignore him. He has joined AlwaysWrong and Phyllis in the
all-insults-no-content mode. He's a usenet pain slut, determined to be
despised by everyone.
John
JimW52
Ha - I should have realised it - a touch of "English" should do it!
Jim
Michael A. Terrell
Anything from .22 to .357 should work.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
Jamie
There is no shut down for him, He has avalanched.
Jamie
Well, I love true full people :)
qrk
You might have a situation where you have lots of external
interference like power line arcing noise. A preamp won't help that
situation either.
---
Mark
LVMarc
> But what determines the gain, or amplification factor, of an N-channel JFET?
> Hate to expose my ignorance, but... Is it the biasing? Or what? What I
> have doesn't specify such on the packaging, and I am wondering.
>
>
> Dave
>
>
The simplest description of how these things amplify goes like this:
inside the package they have black magic
when you wiggle one of the lead , another one wiggles with more vigor.
biasing, without proper biasing and voltage and current limits, things
get hot and the magic black smoke escapes.
after the smoke is released, the part never amplifies again!
I have seen this myself with my own eyes!
The more scientific descriptions, use transconductance, gm, to describe
the Io/Vin relation ship.... a lot of background on operational
bisiaing is required for any operation at all.
Marc
Dave
Replies interspersed...
"Bob" <bo...@mailinator.com> wrote in message
news:1188649675.0...@50g2000hsm.googlegroups.com...
The noise figure of the active antenna seems to be extremely low. The
components have a noise figure of something like 2 db each, and (since I
added the q-multiplier) it does pull in signals that are otherwise
unavailable to the receiver alone (even with the 110' longwire attached.)
>
> When the preamp does have a usfull effect you will only notice
> an increase in signal quality when the received signal would be
> near the noise floor of the receiver without the preamp.
> Shortwave is noisy, when listening to a signal that stronger than
> about
> 10dB more than the sensitity figure of your receiver you will not
> get any improvement from a preamp.
Hmm. Okay. Might explain why it doesn't work any better than it does.
>
> If connecting a preamp directly to a shortwave receiver has a
> significant
> effect it generally means that your receiver is deaf.
>
> Preamps are usefull where there is significant loss in the cable
> between the antenna and the receiver but only if the preamp is
> at the antenna, not at the receiver.
>
Again, might explain...
> Adding 30dB of wideband gain is unlikely to have a usefull effect. It
> is likely to overload your receiver and produce lots
> of noise due to intermodulation and harmonics when you
> drive the receiver into nonlinearity.
>
> Bob
>
Hmmm. Well, thanks for the input. I may have been on a wild goose chase.
What would you recommend for increasing the sensitivity of my receiver? I
am planning on a directional antenna, to help eliminate extraneous noise
etc. What about that?
Many thanks,
Dave
Richard Henry
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
> Jim Thompson wrote:
> > On Fri, 31 Aug 2007 14:19:03 -0500, "Dave" <db5...@hotmail.com> wrote:
>
Have you changed your medication regimen lately?