7.7 Summary

Class A operation is defined as having collector current flow for 360° of the cycle. This means that a single output device can be used to amplify the entire input waveform. To determine the maximum signal swing, or compliance, an AC load line is used. This is similar to a DC load line and plots all possible transistor voltage and current coordinate pairs. The efficiency of the class A amplifier tends to be low. The maximum theoretical efficiency is only 25%. Further, the amplifier draws full current from the power supply regardless of whether or not a signal is present. As a consequence, the transistor runs hottest when there is no signal. With an applied signal, some of the power formerly dissipated within the transistor is shifted to the load.

Loudspeakers offer a complex impedance as a load. As such, they are more challenging than simple resistive loads and may require the output transistor to be rated for a higher-than-normal power dissipation.

Heat sinks are used to efficiently move heat from the transistor’s internal structure to the surrounding air. The thermal effectiveness of a heat sink is measured by thermal resistance, θ. The lower the value of θ, the more effective the heat sink is at transferring heat from the transistor to the surrounding air. For high power applications where a good deal of heat is generated, heat sinks are augmented with forced air cooling.