6.1 Introduction

In the prior chapter we discussed the general operational characteristics of amplifiers including voltage gain, input and output impedance, compliance, distortion and so forth. In this chapter we shall focus on the analysis of small signal amplifiers, specifically, their voltage gain and input/output impedances. As we will be performing a small signal analysis, we will not be concerned with compliance, maximum load power, device dissipation or the like. There is no specific definition of small signal versus large signal but for our purposes we shall define small signal as output signals that are well below the clipping limit and with power dissipation of no more than a few hundred milliwatts for either the load or transistor.

There are two popular techniques used to analyze BJT amplifier circuits. One is through the use of hybrid parameters. There are four different hybrid parameters. We have already seen one of them, the forward current gain, h_fe. We simply call it β. The other three are h_ie , the input impedance; h_oe, the output admittance; and h_re , the reverse voltage gain. The second letter of the subscript (the “e” in h_fe) indicates it is for the common emitter configuration (that is, input applied to the base, output taken at the collector and the emitter at the common ground).

The second approach uses r’ parameters (pronounced “r prime”). The r′ approach is sufficient for all of our analyses and, given an understanding of Ohm’s law, KVL and KCL, produces straightforward equations for circuit gain, input impedance and the like. As a consequence, we shall focus on the r′ system.