Question 6A374

Grid-bias modulation (a form of amplitude modulation) typically employs Class B or Class C RF power amplifiers. These amplifier classes are chosen for their high efficiency, but their efficiency is not constant; it generally increases with the instantaneous output power. * **At zero modulation:** Only the carrier signal is present, meaning the amplifier is operating at a lower average power output. Since efficiency improves with higher output power in Class B/C amplifiers, the efficiency will be at its lowest point (minimum) when only the carrier is transmitted. * **At complete modulation:** During the peaks of the modulation envelope, the amplifier is called upon to deliver its maximum instantaneous power output (up to four times the carrier power at 100% modulation). At these peaks, the amplifier operates at its highest efficiency. Therefore, the efficiency of a grid-bias modulated stage will be minimum at zero modulation (carrier only) and rise to a maximum at complete modulation (modulation peaks). Options A and B describe two sides of the same characteristic behavior for such an amplifier.