Probably a stupid question...

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

Reply to
Dave
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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.

Reply to
Bob Myers

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

Reply to
Dave

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

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

Aren't you into the grape a bit early today. ;-)

Reply to
Don Bowey

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

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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.

Reply to
Fred Bloggs

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

Reply to
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.

Reply to
Dave

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.
Reply to
Joel Kolstad

"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

Reply to
Dave

Hows it going Jim? that's not like you :)

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Reply to
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.

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Reply to
Jamie

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

Reply to
John Larkin

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

D from BC

Reply to
D from BC

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

Reply to
Dave

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

No, Jim, actually I'm a pro at it....;-)

Bob M.

Reply to
Bob Myers

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

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.

Reply to
Bob Myers

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

Reply to
Bob

Well, I love true full people :)

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Reply to
Jamie

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