An actively loaded bipolar differential pair (mirror load) has Early voltage and is…

PHYSICS · Subjective · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Semiconductor devices and analogue circuits

Inside the Instrument, Ch The differential pair and the current mirror

Problem

An actively loaded bipolar differential pair (mirror load) has Early voltage

V_A = 80\,\mathrm{V}

and is biased at tail current

I_{\rm tail} = 50\,\mu\mathrm{A}

. Someone argues: "The transconductance

g_m = I_C/V_T

is proportional to bias current, so raising the tail current fourfold, to

200\,\mu\mathrm{A}

, will raise the differential gain fourfold as well." Using

V_T = 25.9\,\mathrm{mV}

, compute the actual differential gain at both tail currents and find the error in this reasoning.

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