A two-pole amplifier has and a first (dominant) pole at

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Feedback, operational amplifiers and analogue signal processing

Inside the Instrument, Ch Multistage amplifiers, the Miller effect and output stages

Problem

A two-pole amplifier has

A_0 = 6\times10^5

and a first (dominant) pole at

f_{p1} = 530\,\mathrm{Hz}

, set by the capacitor bridging the gain stage. Suppose a redesign of the input stage's mirror load moves the second pole from an original

320\,\mathrm{kHz}

down to

f_{p2} = 90\,\mathrm{kHz}

, with

A_0

and

f_{p1}

unchanged. Use

f_u \approx \sqrt{A_0f_{p1}f_{p2}}

for the new unity-gain frequency and

\arctan(f_{p2}/f_u)

for the phase remaining before

-180^\circ

there. ((a)) Find the new

f_u

. ((b)) Find the phase remaining before

-180^\circ

f_u

. ((c)) State whether pulling the second pole down to

90\,\mathrm{kHz}

has improved or worsened this margin compared with an original value of

1.8^\circ

, and say why in one sentence.

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