Question

VW 400 50 10 R2 VO CS 20pF vin The circuit above shows an opamp used in a non-inverting configuration. The plot above it shows the open-loop gain magnitude (solid line) and phase (dotted line) of the opamp. The opamp has 20pF of parasitic capacitance at its negative input due to cabling and bad PCB layout. a) Derive an expression for the frequency-dependent feedback parameter B(w) for this amplifier. b) What is the closed-loop gain of this amplifier at frequencies well below w = ? c) Estimate the frequency at which Aw)B(w) = 1. Explain your reasoning. d) At that frequency, what is the phase angle of the loop gain, 2A(w)(w)? e) Estimate the phase margin. Is the circuit stable, unstable or marginally stable? f) will increasing Rincrease or decrease the phase margin? Explain your answer. RC

Answer 1

RR F vo Ros a ahnlying virtual short circuit Voltage at terminal as in Vin applying Nodal at ViNXRod & Nin-vo a 2K Vinaros & Vin vo RK 2x Ros & Il Vin 109 QK, Voc B vir B = 3x (208 +1 109 2K

! closed looh gain vo 2x [ 2010 ] ogla vin = 1 x)o xiog 3 2x10x2 RSK grin decreases putting W = ask in gain er. (a) drastically as seen from the graph the magnitude hlot touches odd at w = soring C d at f- pornz Phase is approx 128° 08130
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