A two-stage amplifier driving an ultrasound transducer has ,

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

Inside the Instrument, Ch Stability and compensation

Problem

A two-stage amplifier driving an ultrasound transducer has

g_{m1} = 4.0\,\mathrm{mS}

g_{m2} = 5.0\,\mathrm{mS}

, and a tail current

I_{\rm tail} = 200\,\mu\mathrm{A}

. A pole at a node the compensation capacitor

C_c

cannot move sits at

f_x = 600\,\mathrm{kHz}

. The amplifier is used as a unity-gain follower. Following the chapter's design rule,

C_c

is chosen so that this pole costs

45^\circ

of phase at

f_u = g_{m1}/2\pi C_c

, taken as the loop's crossover, and a series nulling resistor

R_z

removes the right-half-plane zero. Take the dominant pole's contribution at crossover as

90^\circ

and neglect any further output pole's contribution there. ((a)) Find

f_u

and

C_c

. ((b)) Find

R_z

. ((c)) State the phase margin this design aims for, and the overshoot it implies. ((d)) For a

12\,\mathrm{V}

peak output, find the slew rate and full-power bandwidth.

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