6.7 Summary

A simplified AC model of the bipolar junction transistor consists of a controlled current source in the collector and a dynamic resistance in the emitter called r’_e . This resistance is a function of the DC bias current, I_C; the higher the biasing current, the lower the resistance. Fluctuations in this resistance can lead to waveform distortion. Swamping, also known as emitter degeneration, is a technique used to reduce distortion and stabilize gain. The basic idea is to add a fixed resistor in series with the emitter so as to buffer or “swamp out” the changes of r’_e .

There are three basic AC amplifier configurations: common emitter, common collector and common base. The common emitter configuration produces a voltage amplifier with high gain and intermediate input impedance. It also inverts the signal. Because it exhibits both voltage gain and current gain, it has a potential for high power gain. The common collector configuration is known as a follower because its output follows the input. It produces a non-inverting voltage gain of one and exhibits high input impedance and low output impedance. Therefore, it is useful as either an input buffer or as a final drive stage to a low impedance load. The common base configuration exhibits high non-inverting voltage gain. It has a low input impedance and a high output impedance.

The Darlington pair is a two-transistor configuration that may be treated as a single device. As such, it exhibits a doubling of both V_BE and r’_e , and a very large ß.

In order to achieve higher gains, multiple stages may be cascaded. Their gains multiply together to produce the combined system gain. The stages may be coupled through capacitors or via a capacitor-less direct coupling technique that can improve performance while reducing component count.