Analysis and Design of Emitter Followers at High Frequencies

This paper deals with the performance of emitter followers at high frequencies. Under the assumption of a linear current gain of the form
\beta(s) = \beta_0/(1 + s\tau)
, equivalent circuits are derived for the resistive and capacitive loaded emitter follower, and the effect of the collector-to-base capacity is shown to be equivalent to a modified load. An explanation of feedback in the emitter follower is presented which is helpful in understanding the reason for instability in certain cases. The particular case of inductive drive and capacitive load is analyzed, and a simplified stability expression is derived which is useful at high frequencies. Compensation, in the form of a base RC network, is discussed, and design curves for this type of compensation are presented.