Analog and mixed signal design:Input impedance of a common emitter bipolar differential amplifier with emitter degeneration

Use of the emitter coupled bipolar differential amplifier is prolific. In addition a good way to stablize gain and bias stability is the use of a emitter degeneration resistor. This post simply presents, without proof, what happens to the input impedance of the differential device when degeneration is used. First one has to know the rpi of the bipolar small signal model. This is calculated as: Beta0/gm. Where Beta0 is the dc gain of the bipolar. If no degeneration is used, this is the input impedance of the transistor. When a degeneration resistor is used then the impedance rises significantly. The rise in input impedance is: (Beta + 1)*Re. Here Beta is the current gain at the particular bias point and frequency and Re is the degeneration resistor. Therefore the total input impedance rises to rp1+(Beta+1)*Re. For other items of interest please visit our website at http://www.signalpro.biz.