Basic Frequency Control Circuit
In the following schematic, why can't R4 be taken away and the feedback network pass output feedback through C2 and R2?
In a perfect world with ideal op-amps, this appears to work fine since with the pot at mid-position, the gain is -1 at all frequencies* (after all, C1 blocks any dc from reaching the op-amp, so the infinite gain after point 'a' has nothing to amplify). The problem is that op-amps have input offset voltage and input bias current.
Offset voltage is a small voltage that appears to exist between the + and - inputs. Bias current is a small current which flows into the + and - inputs. The offset voltage will cause the output to change but the capacitor C2 blocks this change from feeding-back to the input and cancelling the effect. Similarly the bias current causes C2 to charge-up, causing the output voltage to increase.
Eventually, the output voltage reaches the supply rail and the op-amp saturates.