Drive and overdrive

Class-A power amplifiers do not, theoretically take any power from the signal source in order to produce a significant amount of output power. This is main advantages of class-A operation. The same is true for class-AB1  amplifiers. It is required that a certain voltage be present at the control electrode (the base, gate, emitter, or source).

Class-AB2 amplifiers need some driving power to produce alternating current power output. Class-B amplifiers require larger drive than class-AB2, and class-C amplifiers require more drive still.Whatever type of PA is used in the given situation, it is significant that the driving signal not be too strong. If overdrive takes place, there would be distortion in output signal.

An oscilloscope is used to determine that is the amplifier is being overdriven or not. The scope is connected to amplifier output terminals, and waveshape of the output signal can be examined. The output waveform for the particular class of amplifier has a characteristic shape. Overdrive can be indicated by a form of distortion called as flat topping.In the figure given to us the output signal waveshape for a properly operating class-B amplifier is shown. It looks like a output of a 1/2-wave rectifier, as the bipolar transistor or FET is drawing current for exactly half (180 °) of the cycle.

Figure-- At point A, waveshape at the output of a properly operating class-B amplifier. At point B, distortion in the output waveshape caused by overdrive.

In the given, output of an overdriven class-B amplifier is shown in figure. The wave is no longer a half sine wave, but instead, it depicts the evidence of flat topping. The peaks are truncated. The result of audio distortion in the modulation on the radio signal, and an excessive amount of energy at the harmonic frequencies.

The efficiency of a circuit can be degraded by the overdrive. The flat tops of distorted waves do not contribute anything to the strength of the signal at the desired frequency. But they cause a higher than the normal PC or PD value, which translates into the lower than normal efficiency Pout/PC or Pout/PD.A thorough discussion of overdrive and distortion in various amplifier classes and applications would require the whole book.