A class AB push-pull stage runs from rails of into a load

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Semiconductor devices and analogue circuits

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

Problem

A class AB push-pull stage runs from rails of

\pm20\,\mathrm{V}

into a

4\,\Omega

load, with an idle current

I_Q = 100\,\mathrm{mA}

. Each output transistor has an emitter resistor

R_E = 0.22\,\Omega

, and each side has its own sense resistor and current-limit transistor. Take every base-emitter voltage, including the

V_{BE}

multiplier's and the limit transistors' thresholds, as

0.60\,\mathrm{V}

, and ignore the resistors' drops except in parts (b) and (d). ((a)) Choose

R_{\rm sense}

so that the current limit sits

20\%

above the peak current of a full-swing sine into the load. ((b)) Find the bias voltage the

V_{BE}

multiplier must hold at idle, and the ratio

R_1/R_2

that gives it. ((c)) Find the upper bound on the pair's dissipation that the heatsink must be sized for, and compare it with the pair's dissipation at full output. ((d)) At the full-swing peak current, how much of the output swing is lost across one side's

R_E

and

R_{\rm sense}

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