Colorsound Overdriver Input Stage, what topology is this?

[bushidov]

Just curious to know what is going on in this circuit:


Like how transistor Q1 feeds back into Q2 in the ways that it is doing?

It obviously isn't a straight forward non-inverting transistor amplifier.

[Rob Strand]

Without R6 there's no feedback so it's more or less a Fuzzface and DC feedback to stabilize the bias comes from R5.   The extra stuff on the emitter of Q1 would reduce the (open loop) gain compared to a Fuzzface.   Nonetheless the overall gain of the two stages is high.

The emitter of Q1 provides a place to provide feedback.  R6 provide the feedback and the closed loop gain is approximately 1 + R6/ (R1 in parallel with VR2) exactly like a non-inverting opamp.  The resistor R6 is AC coupled so it doesn't affect the biasing.

A similar circuit is this one,  (the second stage is replaces the NPN with a PNP)




However in this case the feedback is DC coupled so it stabilizes the bias and provides the feedback to set the gain.

Basically you start with a high gain amp and put feedback around it just like you do with opamps.  The feedback reduces or sets the gain.   The difference between opamps and transistor circuits is you have to be careful about biasing and you have many options to add the feedback because you have access to all the points in the circuit.

[idy]

Isn't R5 also feedback?
Typo and "Q2 feeds back to Q1?"

[Rob Strand]

Isn't R5 also feedback?

The "big" cap C3 shorts out the AC signal so there's no AC feedback but it does provide DC feedback.
If C3 is not present it's a whole different story.

(When there's no R6 the small feedback through R5 would have a small effect.)

[bushidov]

Thanks Rob! I was thinking it wasn't fuzz faced based until I read your explanation. I just needed that second set of eyes, I guess.