A bipolar transistor operating as a current source has small-signal parameters

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 Loop gain in real feedback circuits

Problem

g_m = 40\,\mathrm{mA/V}

r_\pi = 2.5\,\mathrm{k\Omega}

and

r_o = 50\,\mathrm{k\Omega}

. An emitter resistor

R_E = 500\,\Omega

is added between the emitter and ground, and the output resistance is measured looking into the collector. Someone argues: "Adding

R_E

closes a negative-feedback loop around the transistor. Negative feedback always reduces output impedance, so the collector's output resistance must fall below the bare value

r_o = 50\,\mathrm{k\Omega}

." ((a)) Find the error in this argument. ((b)) Calculate the actual output resistance seen at the collector.

Browse the problem library