A New Junction-Transistor High-Frequency Equivalent Circuit

A small-signal equivalent circuit for a junction transistor is presented which is applicable to alloy or grown types of p-n-p or n-p-n transistors, and which is valid from d-c up to twice the cutoff frequency. The equivalent circuit is in the form of four short-circuit admittances, each of which can be represented by a simple network of lumped elements constant with frequency. The derivation is based on physical principles and takes into account base widening and collector barrier capacitance. Equations for the equivalent circuit element values are given either in terms of physical parameters or in terms of six practical measurements. The four-admittance representation is given both for common-emitter and common-base connections, and a relation between the common- emitter and the common-base cutoff frequencies is derived and experimentally verified. Measurements of the real and imaginary parts of the four admittances as functions of frequency for several transistors show excellent agreement with the values predicted by the equivalent circuit