by Fred Nachbaur, Dogstar Music ©1998, 2001

So far we have a pre-amplifier with an open-loop passband gain of about 60 dB, with 3-dB corners at about 40 Hz and 2 kHz. (See

- a) Open loop gain, although quite high, cannot be considered infinite as in many solid state op-
amp devices.
- b) The circuit exhibits a large output-to-input voltage offset (on the order of 290 volts). Any AC
feedback elements between output and input therefore have to include DC blocking capacitors.
The alternative would be level-shifters, which would in my opinion be an additional complication
(and expense) without commensurate rewards in terms of performance.
- c) The DC voltage at the feedback input is non-zero (about 27 volts in practise), so again there is
a need for DC blocking. The point marked "ACG" (AC Ground) is provided for convenience, acting
as a virtual ground for AC, at the same DC voltage as the inverting input.

Keeping these restrictions in mind, we can use the formula for the classic non-inverting op-amp to approximate our gain with feedback. Note that the inverting input (-IN) has a 47K resistor (R113) to "AC Ground". This is our "default" value for input resistance to the feedback input. Let's call that resistance Ri, though it can be considerably higher, as needed. The bare-minimum feedback network would consist of just a single resistance (we'll call it Rf ) in series with a DC blocking capacitor between output and -IN. The theoretical gain with feedback would then be:

For instance, let's compute our gain if we connect a "bare bones" feedback network consisting of a 430K resistor in series with a DC blocking capacitor between "OUT" and "-IN". That is, Rf / Ri = 9.15, so our gain would be 10.15, or about 20 dB.

The feedback elements do not have to be pure resistances; the above formula could be generalized to include complex impedances.

The circuit's actual performance follows this predicted formula very closely, verifying that our gain-matching shortcut described earlier works just fine. See

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