Defining gain in JFET stages.

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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briggs
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Post by briggs »

As I work this out now I use the formula:
Taken from runoffgroove.com, Fetzer Valve wrote:Av = 0.54 * (Vcc/|Vp| - 2), where Vcc is supply voltage Vp is the pinch off voltage of the FET and Av is the overall voltage gain..
Takes a lot of measurements!

Are there any other simpler methods that are as accurate?
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Post by lolbou »

Av= -gm x (Ru//Rd//rds)

with:

rds=1/gds

Rd: drain resistor (between +V and drain)
Rs: source resistor (between drain and ground)
Ru: output resistor (from drain coupling capacitor to ground)
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Post by briggs »

cheers, what do the other terms represent? gm, rds, gds....
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Post by lolbou »

gm is the jfet transconductance (also called gfs)

gds is the output conductance, which is the invert of rds (drain-source on resistance), also called gos.

both are given in mS, sometimes in uS...
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Post by briggs »

many thanks lou 8)
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Post by lolbou »

sometimes you can't read these data directly on datasheets, but you might get them from the graphics (slope of a straight line)...
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Post by briggs »

I download datasheets which have it all on there. Just playing with a few 2N5457 jfets and a compressor circuit 8)
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Post by lolbou »

Few explanations to anybody... It might make the formula much clearer:

Ru is the output load.

The value of Ru//Rd//rds is R = (Rd x Ru x rds) / (rds.Ru + Rd.Ru + Rd.rds)

The gain is negative (see that minus?) because the output is 180° out of phase with the input.

In an upper post, I spoke about Rs. You see here that its value doesn't change the voltage gain since it doesn't appear in the formula...
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Post by RnFR »

i think it's about time i learned this stuff! thanks for the explanation.
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Post by briggs »

I'm going to start writing up various little easy tutorials on things like Jfets, mosfets, bjt, opamps and rc filters. I've learn't this stuff very very slowly, with a lot of help from various guys around here. It would be nice to give summit back 8)
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Post by RnFR »

i think we would have to immortalize you into stompbox sainthood if you did all that!

i've seen what looks like some really good stuff along those lines on some japanese sites. too bad all i can understand are the diagrams! i found them by going to fffworks and following some links around.

like this-

http://homepage3.nifty.com/ichian/analo ... alent1.htm

maybe it could give you some ideas? some simple explanations putting these things to practical use always helps as well.
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Post by briggs »

I would do it to be the basic of basic, like learning guitar to write songs - you don't need scales, just the chords. Give people the tools they need to design and make their own stuff. It would be awesome if we could get some of that japanese site translated, it looks like a great resource!

The fetzer valve article on runoffgroove is one of my favourite reads - I'm forever back and forth to that for ideas. I'll get some stuff worked out and see if I can squeeze some quick and easy guides out. First up - basic RC filters and the equation to work out corner frequencies!
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