Class-AB2 and class-B amplifiers take some power from the input signal source. En-gineers say that such amplifiers require a certain amount of drive or driving power tofunction. Class-A and class-AB1 amplifiers theoretically need no driving power, al-though there must be an input voltage.The class-C amplifierA bipolar transistor or FET can be biased past cutoff or pinchoff, and it will still work asa power amplifier (PA), provided that the drive is sufficient to overcome the bias dur-ing part of the cycle. You might think, at first, that this bias scheme couldn’t possibly re-sult in amplification. Intuitively, it seems as if this could produce a marginal signal loss,at best. But in fact, if there is significant driving power, class-C operation can work verywell. And, it is more efficient than any of the aforementioned methods. The operatingpoints for class C are labeled in Figs. 24-4 and 24-5.Class-C PAs are never linear, even for amplitude modulation on a signal. Because ofthis, a class-C circuit is useful only for signals that are either full-on or full-off. Contin-uous-wave (CW), also known as Morse code, and radioteletype (RYTY) are examplesof such signals. Class-C PAs also work well with frequency modulation (FM) becausethe amplitude never changes.A class-C PA needs a lot of drive. The gain is fairly low. It might take 300 W of ra-dio-frequency (RF) drive to get 1 kW of RF power output. That’s a gain of only a little442 Amplifiers24-6A class-B push-pull amplifier.