over 5 dB. Nonetheless, the efficiency is excellent, and class-C operation is common inCW, RTTY, or FM radio transmitters.PA efficiencySaving energy is a noble thing. But in electronic power amplifiers, as with many otherkinds of hardware, energy conservation also translates into lower cost, smaller size andweight, and longer equipment life.Power inputSuppose you connect an ammeter or milliammeter in series with the collector or drainof an amplifier and the power supply, as shown in Fig. 24-7. While the amplifier is in op-eration, this meter will have a certain reading. The reading might appear constant, or itmight fluctuate with changes in the input signal level.PA efficiency44324-7Connection of a current meter for dc power input measurement.The dc collector power input to a bipolar-transistor amplifier circuit is the prod-uct of the collector current in amperes, and the collector voltage in volts. Similarly, foran FET, the dc drain power input is the product of the drain current and the drainvoltage. These power figures can be further categorized as average or peak values.(This discussion involves only average power.)The dc collector or drain power input can be high even when there is no signal ap-plied to an amplifier. A class-A circuit operates this way. In fact, when a signal is appliedto a class-A amplifier, the meter reading, and therefore the dc collector or drain powerinput, will not change compared to the value under no-signal conditions!