At A, an oscilloscope display of the signal output waveform from a properly operating class B power amplifier. At B, a display showing distortion in the waveform caused by overdrive.
Class A and AB1 power amplifiers do not, in theory, take any power from the signal source to produce significant output power. This is one of the advantages of these classes of operation. It is only necessary that a certain voltage be present at the control electrode (the base, gate, emitter, or source) for these circuits to produce useful output signal power. Class AB2 amplifiers need some driving power to produce ac power output. Class B amplifiers require more drive than class AB2, and class C amplifiers need still more drive.
Whatever kind of PA is used in a given situation, it is important that the driving signal not be too strong. If overdrive takes place, distortion occurs in the output signal. An oscilloscope can be used to determine whether or not an amplifier is being overdriven. The scope is connected to the amplifier output terminals, and the waveform of the output signal is examined. The output waveform for a particular class of amplifier always has a characteristic shape. Overdrive is indicated by a form of distortion known as flat topping.
In above figure A, the output signal waveshape for a properly operating class B amplifier is shown. In above figure B, the output of an overdriven class B amplifier is shown. Note that the peaks are blunted or truncated. The result of this can be distortion in the modulation on a radio signal, andalso an excessive amount of signal output at harmonic frequencies. The efficiency of the circuit can be degraded, as well. The flat tops of the distorted waves don’t contribute anything to the strength of the signal at the desired frequency, but they cause a higher-than-normal dc power input, which translates into a lower-than normal efficiency.