Gain of common-cathode triode stage with FB

Thomeeque · Post by **Thomeeque** » 04 Jan 2008, 11:26

Hi!

Can somebody tell me approximate value of voltage gain of circuit above ( Ideally with some calculation-steps break-down

). All caps are 47nF, Vcc around 300V.

Is it -5 (just wild guess

)?

Thanks!

AL · Post by AL » 04 Jan 2008, 15:46

Try this:

http://www.aikenamps.com/CommonCathode.htm

AL

Thomeeque · Post by **Thomeeque** » 04 Jan 2008, 16:01

Thanks, but before posting my question I have gone *exactly* through this page briefly and have not found answer to my question

As you can see, on my picture is additional negative feedback branch, which is not described there (or is it?). Maybe it's obvious or something but unfortunately not to me (I'd say it's approximately -R_FB/R1 as with OpAmp, but I can be easily wrong, I'm very weak in amp theory).

KippeKiller · Post by **KippeKiller** » 04 Jan 2008, 22:23

Hi,

maybe this will help you, must be somewhere between aiken and there.

http://www.freewebs.com/valvewizard/localfeedback.html

regards

Mathias

Thomeeque · Post by **Thomeeque** » 05 Jan 2008, 01:06

Yeah, this one is perfect, thanks!! So it really can be roughly approximated as inverting op-amp stage with negative feedback, even from input impedance point of view (Av = -R_FB/R1, Zin = R1), gain is slighlty less, input impedance slightly bigger - approximated values would be -5 and 100kOhm, precise would be probably around -4 and 120kOhm (just estimating, I'm not going to count it now, I don't need precise values). Nice! I'm smarter again