High Frequency Equivalent Circuit

Author: Leonard Krugman

Because of these factors, representation of the transistor high-frequency operation by any linear four-terminal equivalent network is at best a rough approximation over any substantial frequency range.

This is especially true if the circuit is to be reasonably representative of the physics of the transistor, and if the number of circuit parameters are to be kept within reasonable limits. One form of equivalent circuit, suggested by Dr. W. F. Chow of the General Electric Company, has worked out well. This involves the insertion of a low pass R-C filter network in the low frequency circuit, derived for an equivalent current generator in the collector arm (αi_e). The modified equivalent circuit illustrated in Fig. 7-3 takes into account the variations of r_c and C_c with frequency. This circuit provides a fair representation of transistor performance through the range below the a cut-off frequency. If the operating frequency is greater than f_c, the low pass filter must be replaced with an R-C transmission line.

Fig. 7-3. Transistor high-frequency equivalent circuit.