Chapter Summary

Amplifier configurations. A transistor amplifier can be built in common-emitter (CE), common-base (CB) or common-collector (CC) configuration. The CE amplifier gives high current, voltage and power gain with a 180° phase reversal; the CB amplifier has a current gain below 1 but a wide bandwidth and no phase shift; and the CC amplifier (emitter follower) has a voltage gain just below 1, a very high current gain and no phase shift, which makes it a buffer for matching a high-impedance source to a low-impedance load. A JFET common-source amplifier performs the same job with a field-effect device and also gives a 180° phase reversal.

DC and AC equivalent circuits (Section 3.4). Because DC bias and AC signal flow together, the amplifier is analysed in two parts. The DC equivalent (open all capacitors, remove AC sources) fixes the operating point through the divider voltage $V_2$, the emitter current $I_E$ and the collector–emitter KVL loop. The AC equivalent, or re' model (short the DC supply and all capacitors, then replace the transistor by an emitter AC resistance and a collector current source), yields the small-signal results: input impedance $R_1\|R_2\|\beta r_e'$, output impedance $R_C$ (or $R_C\|R_L$), voltage gain $-R_C/r_e'$, current gain $\beta$ and power gain $A_i \times A_v$, with $r_e' = V_T/I_E$ and $V_T = 26$ mV (Si) or 52 mV (Ge).

Power amplifiers (Section 3.5). A power amplifier delivers large power to a load, converting DC supply power into an AC output; its efficiency is $P_{out}/P_{in}$. By the position of the Q-point it is classed A (full cycle, 25–30% efficient), B (half cycle, 70–80%), AB (more than half a cycle) or C (less than half a cycle, above 80%). The Class B push–pull amplifier uses a phase-splitter input transformer and two transistors that each handle one half-cycle, but suffers cross-over distortion near the zero crossings; the Class C amplifier uses a collector-tuned LC load to select and amplify a chosen frequency. …