Op Amp ckts with Capacitor Feedback Network

124

01:40:54

Close the SPST switch. 
It looks like capacitors can be used for feedback in op amp circuits. With a capacitor feedback network, the formula for gain for the non-inverting configuration seems to be
Vo = Vi(1+C1/C2).
With a resistor feedback network, the formula is
Vo = Vi(1+R2/R1).
I don't know if this is used anywhere or if it works IRL (I haven't tried it on the breadboard yet, but why shouldn't it work?), but I thought I should share it since I've never seen anything like this before.

published 6 years ago

hurz

6 years ago

DC undefined nodes are a real problem. OpAmps do need across capacitors extra resistors to not start or drift into undefined DC voltages.

westelaudio

6 years ago

Might work for AC but for DC I highly doubt it. EC models op amps with infinite input impedance and caps without DC leakage (and both would drain the charged caps IRL).

hurz

6 years ago

if we implement integration with opamps we do need a resistor across the capacitor to set the start conditions (from math point of view C=0) otherwise we might get with parasitics and nonlinearities demodulating effects which will cause DC drift. Caps are not linear and might cause very ugly integration behaviour. So means charge in one direction quicker then in other way around. Minimal, but an integrator does not forget and quickly sums up little errors to big ones.

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