21.1: Theory Overview

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An ideal common emitter amplifier simply multiples the input function by a constant value while also inverting the signal. The voltage amplification factor, \(A_v\), is largely a function of the AC load resistance at the collector and the internal emitter resistance, \(r’_e\). This internal resistance is, in turn, inversely proportional to the DC emitter current. Therefore, if the underlying bias is stable with changes in beta, the voltage gain will also be stable. The circuit will appear as an impedance to the signal source, \(Z_{in}\). This impedance is approximately equal to the base biasing resistor(s) in parallel with the impedance seen looking into the base \((Z_{in(base)})\) which is approximately equal to \(\beta\) \(r’_e\). Consequently, the amplifier’s input impedance may experience some variation with beta. In contrast, the circuit’s output impedance as seen by the load is approximately equal to the DC collector biasing resistor.

From a practical standpoint, input and output impedance cannot be measured directly with an ohmmeter. This is because ohmmeters measure resistance by sending out a small “sensing” current. The DC bias and AC signal currents will interact with this current and produce an unreliable result. Instead, impedances can be measured indirectly through a voltage divider effect. That is, if the voltages of both legs of a voltage divider can be measured and the resistance of one of the legs is known, the remaining resistance may be determined using Ohm’s law or the voltage divider rule.