Fet source capacitor, why?

[Eirik]

I've noticed that in fet gainstages there is a capacitor across the source resistor.

Why?

Will the size of the cap influence the sound in any way?

Do the emitter cap in a fuzz face play the same role?


Eirik

[aron]

This is from another forum, but it's from R.G. so I'm borrowing this post:

http://www.harpamps.com/micKcircuits/FET-source-follower.html

[davebungo]

Simply put, it increases the gain of the stage for AC signals which would otherwise be subject to a reduction in gain due to negative feedback.

[R.G.]

I've noticed that in fet gainstages there is a capacitor across the source resistor. Why?

In a FET gain stage, the available gain at the drain approached Rd/Rs where Rd is the drain resistor and Rs is the source resistor. The capacitor lets the DC biasing conditions remain the same on the source, but effectively short circuits the source resistor for higher freuquencies, so it makes the signal gain higher. The frequency where the gain is full is the rolloff frequency of that source resistor and the cap: F = 1/(2*pi*Rs *C)

Will the size of the cap influence the sound in any way?

Yes. Frequencies below F = 1/(2*pi*R*C) will have lower gain, down to the gain of Rd/Rs; so it can cut bass frequencies if you make the cap small enough.

Do the emitter cap in a fuzz face play the same role?

Yes.

[WGTP]

And to state the obvious, this is has maximum tweakablilty potential for everything from FF to Obsidian to BSIAB to ROG amp sim's to etc.   

[Eirik]

Thanks guys

[Connoisseur of Distortion]

bootstrap!