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Probably a stupid question...

D

Dave

Jan 1, 1970
0
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.

TNX

Dave
 
B

Bob Myers

Jan 1, 1970
0
Dave said:
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.
 
D

Dave

Jan 1, 1970
0
Bob Myers said:
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.

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
 
J

Jim Thompson

Jan 1, 1970
0
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

My take... Bob M. = amateur BS artist ;-)

...Jim Thompson
 
J

Jim Thompson

Jan 1, 1970
0
Aren't you into the grape a bit early today. ;-)

Nope. Tell me, Is there anything intelligible in that, "Transistors,
etc..." ??

...Jim Thompson
 
F

Fred Bloggs

Jan 1, 1970
0
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.
 
D

Dave

Jan 1, 1970
0
Fred Bloggs said:
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.

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
 
D

Dave

Jan 1, 1970
0
Dave said:
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

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.
 
J

Joel Kolstad

Jan 1, 1970
0
Dave said:
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.

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.
 
D

Dave

Jan 1, 1970
0
Joel Kolstad said:
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.

"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
 
J

Jamie

Jan 1, 1970
0
Dave said:
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.

TNX

Dave
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.
 
J

John Larkin

Jan 1, 1970
0
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.

TNX

Dave

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
 
E

Eeyore

Jan 1, 1970
0
Dave said:
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.

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
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
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

D from BC

Jan 1, 1970
0
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.

TNX

Dave

The gain depends on which spice model you are using. :)


D from BC
 
D

Dave

Jan 1, 1970
0
Dave said:
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.

TNX

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
 
E

Eeyore

Jan 1, 1970
0
Dave said:
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
 
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