# More op-amp circuits

Posted on Feb 4, 2014

The circuits looked at so far depended for their functioning on linear feedback. The magnitude of the signal returned to the negative input was always strictly proportional to that of the output voltage. The result was that within the limits set by the op-amp, the magnitude of the output voltage is proportional to that of the input signal. Often w

e want to create a more complicated response. For example, During this lab you will examine two circuits, one in which non-linear feedback is used to achieve a particular response or transfer function, the other uses non-linear feedback to stabilize the amplitude of an oscillator. It is most convenient to analyze its function by considering separately what happens for Vin>0 and Vin < 0. Figure 2a shows the equivalent circuit for Vin > 0. In this case the diode D1 is reversed biased and effectively an open circuit, D2 is conducting. The feedback loop is effectively a single resistor, from the output of OA2 to the negative input of OA1. OA1 and OA2 act together as a single op-amp and the complete circuit acts like a simple voltage follower, Vout = Vin. There is a complication with this circuit. At sufficiently high frequency, both OA1 and OA2 introduce 90 phase shifts, which together with the 180 shift achieved by feeding the signal back to the input of OA1, would make the circuit unstable. A small capacitor, C, is used to counteract this. Assemble the circuit, initially with C = 100 pF, and check that it works as advertised for various wave forms (f = 1 kHz). Increase the frequency until you can clearly observe the problems that arise when the circuit switches over from Vin < 0 to Vin > 0 mode. What is the typical switching time Can you improve this by changing C Observe that without C the circuit becomes...

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