lowering Q1 gain

mordechai · March 10, 2011, 10:07:42 AM

I have several PNP silicon transistors with an average gain of 150-180. I'd like to use them in the Q1 position...what sort of resistor to ground should I connect to the emitter of such a transistor to bring the gain down closer to 80-100?

Johan · March 10, 2011, 01:45:05 PM

Q1 of what?...
..for a basic answer, not knowing anything else...devide the collector R with emitter R to estimate amplification

mordechai · March 10, 2011, 03:38:21 PM

Oh, sorry! Q1 of a Fuzz Face circuit! I've read a few posts here that running a resistor from the emitter to ground can cut down on gain, but don't know the right value to try...

jasperoosthoek · March 10, 2011, 04:04:57 PM

The emitter of Q1 is directly connected to ground

. Don't you mean the base of Q1?

Adding resistors here and there won't 'really' change the gain. It won't be the same. It might give pleasing results, nothing wrong with trying.

I know it's common practice to place a resistor/pot at the input. I think it is called a smooth control.

clsnacky · March 10, 2011, 05:31:53 PM

Do a search using the term "piggybacking". It has worked for me.

mordechai · March 10, 2011, 05:52:24 PM

Quote from: jasperoosthoek on March 10, 2011, 04:04:57 PM
The emitter of Q1 is directly connected to ground . Don't you mean the base of Q1?

Adding resistors here and there won't 'really' change the gain. It won't be the same. It might give pleasing results, nothing wrong with trying.

I know it's common practice to place a resistor/pot at the input. I think it is called a smooth control.

Jasper, I know that the emitter usually goes to ground, but I've seen some schematics where an additional resistor is added between the emitter and the ground to reduce the gain -- of course, the gain of the transistor itself doesn't change, but the effect of the gain is apparently reduced. On the YAFF circuit, this is done with both Q1 (with a 100 ohm resistor) and Q2 (with a 220 ohm resistor before the fuzz pot). I don't want to change the gain on my Q2 but I do think that lowering the gain effect of Q1 would be helpful. However, I don't know if a 100 ohm resistor would be sufficient to bring the Q1 from its current gain rating down to something more akin to 80-110 or so, and I wanted to see if anyone had an opinion on possible choices that could accomplish this.

Gus · March 10, 2011, 05:53:39 PM

jasperoosthoek · March 10, 2011, 06:52:18 PM

Doh! I know what you mean now.

A simple way would be to put it in a DMM and test it with a (500 ohms if you can find it) trimpot. Just dial in the correct value.

I did this with a piggyback transistor and a 10k trimpot. I posted it here last year. But I came to realize that it didn't work for me. I'll stick with germanium.

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lowering Q1 gain