Equivalent Passive 'T' Network

Author: Leonard Krugman

In the analysis of the transistor on a four-terminal basis, the entire device is treated as a sealed box with three external terminals e, b, c, designating the emitter, base, and collector, respectively. This basic four-terminal network is illustrated in Fig. 3-7 (A), in which the input signal is applied between emitter and base. The input signal E_g is taken from a signal generator that has an internal resistance R_g. The output circuit is between the collector and the common base and consists of a load resistance R_L. In the small-signal analysis which follows, it is assumed that the transistor is biased in the linear region of its operating characteristics. It is also assumed that the operating frequency is low enough so that the transistor parameters may be considered pure resistances, and the capacitive junction effects may be considered negligible.

Fig. 3-7. (A) The basic circuit for transistor four-terminal network analysis.
(B) Transistor equivalent "T" on a passive basis.
(C) Transistor equivalent "T" on an active basis.

The simplest method of approaching the analysis of the equivalent transistor circuit is by an equivalent "T" network with no internal generating sources (passive basis). This circuit is illustrated in Fig. 3-7 (B). Under these conditions, the transistor parameters can be completely specified by the following terminal measurements:

Notice that the forward transfer resistance is equal to the backward transfer resistance. This is typical of a four-terminal passive network. In the practical case, then, it is only necessary to measure r₁₂ or r₂₁.