A two-stage amplifier has open-loop poles at and and open-loop DC gain

PHYSICS · Subjective · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Feedback, operational amplifiers and analogue signal processing

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Problem

A two-stage amplifier has open-loop poles at f1=200 Hzf_1 = 200\,\mathrm{Hz} and f2=1 MHzf_2 = 1\,\mathrm{MHz} and open-loop DC gain A0=8×104A_0 = 8\times10^4. It is used as a non-inverting amplifier at two closed-loop gains, first 1010 and then 4040, each set by an accurate resistor divider so that the loop gain at DC is T0=A0/(closed-loop gain)T_0 = A_0/(\text{closed-loop gain}) in each case. (a) Before calculating, predict which of the two closed-loop gains will show the larger step-response overshoot, and which will be the more accurate against a drift in A0A_0. Give the reasoning in one sentence each. (b) For each gain setting, find T0T_0, the closed-loop natural frequency fnf_n, the damping ratio ζ\zeta, and the fractional gain sensitivity 1/(1+T0)1/(1+T_0) to a change in A0A_0. (c) State whether the calculation confirms the prediction, and summarise the trade-off in one sentence.

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