Double-feedback transistor amplifiers

[cctsim]

I was wondering if anyone has any reference text that explains how to deal with transistor circuits that have two types of feedback at the same type.

For example the Big Muff Pi stages or the Zvex SHO have both voltage-series and voltage-shunt feedback at the same time. I know how to work out
numerically the behaviour of such circuits using nodal ac analysis but that is not very insightful.

Thanks in advance.

[Rob Strand]

It depends what you want to achieve!  There's too many interactions to summarize.  You can focus on  AC aspects or the DC aspects (bias stability).

Often you just add a small emitter resistor to help stabilize biasing, set the feedback resistor (Rf) to set the bias then adjust the input resistor (Rin) to set the gain or input impedance.   If the Rc/Re ratio is usually much larger than the Rf/Rin and the gain will be close to Rf/Rin.  If your Rf/Rin ratio is high then the gain is more like Rc/Re.  You can add a third resistor from the base to ground which lets you adjust the bias.   When you add Rin it tends to be noisier.  The shunt feedback tends to produce an amplifier with a lower output impedance.

You can also tinker around with the design to try to reduce how much hum on the power rails get out on the out.

Often you want to pin down some things like gain, input and/or output impedance.  Then play around with the design to improve the the aspect you think are important by playing around with the other parts.   Try a few designs then pick the one you think is best.

You can go overboard analyzing this stuff and it doesn't achieve much - been there many times.    If a simple CE amp without Rf and Rin works why go further?

[cctsim]

Many thanks for your input.

The DC analysis is no problem. The AC aspects are where I have problems. 

The relationships that you mentioned between gain and ratios of Rc/Re and Rf/Rin seem to be useful design rules, thanks. You are probably right about over-analyzing things.