A series-pass regulator with an npn follower pass device supplies a photodiode…

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

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Problem

A series-pass regulator with an npn follower pass device supplies a photodiode amplifier at

I_L = 50\,\mathrm{mA}

300\,\mathrm{K}

. Its error amplifier is the loop's controlled source, with DC return ratio

T_0 = 1500

and a single dominant pole at

f_p = 50\,\mathrm{Hz}

, so

T(jf) = T_0/(1+jf/f_p)

. With the loop dead (

T=0

), the pass device presents its emitter resistance

r_e = V_T/I_L

to the output, and no other feedback path returns to this output node. ((a)) Using Blackman's formula, write the output impedance

Z_{\rm out}(jf)

in terms of

r_e

and

T(jf)

for this circuit, finding the short-circuit and open-circuit return ratios for this circuit. ((b)) Evaluate

Z_{\rm out}

at DC and at

f = 10\,\mathrm{kHz}

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