Crossover distortion
Crossover distortion is a type of distortion which is caused by switching between devices driving a load, most often when the devices (such as a transistor) are matched. It is most commonly seen in complementary, or "push-pull", Class-B amplifier stages, although it is occasionally seen in other types of circuits as well.
The term crossover signifies the "crossing over" of the signal between devices, in this case, from the upper transistor to the lower and vice versa. The term is not related to the audio crossover—a filtering circuit which divides an audio signal into frequency bands.
The image shows a typical class-B emitter-follower complementary output stage. Under no signal conditions, the output is exactly midway between the supplies (i.e., at 0 V). When this is the case, the base-emitter bias of both the transistors is zero, so they are in the cut-off region where the transistors are not conducting.
Consider a positive-going swing: As long as the input is less than the required forward VBE drop (≈ 0.65 V) of the upper NPN transistor, it will remain off or conduct very little - this is the same as a diode operation as far as the base circuit is concerned, and the output voltage does not follow the input (the lower PNP transistor is still off because its base-emitter diode is being reverse biased by the positive-going input). The same applies for the lower transistor but for a negative-going input. Thus, between about ±0.65 V of input, the output voltage is not a true replica or amplified version of the input, and we can see that as a "kink" in the output waveform near 0 V (or where one transistor stops conducting and the other starts). This kink is the most pronounced form of crossover distortion, and it becomes more evident and intrusive when the output voltage swing is reduced.
Less pronounced forms of distortion may be observed in this circuit as well. An emitter-follower will have a voltage gain of just under 1. In the circuit shown, the NPN emitter-follower and the PNP emitter-follower will generally have very slightly different voltage gains, leading to slightly different gains above and below ground. Other more subtle forms of crossover distortion, stemming from slight differences between the PNP and NPN devices, exist as well.
In the case of a class B/AB amplifier, crossover distortion can be reduced by using a slight forward bias in the base circuit such that the transistors are idling at a small output current. The forward bias causes the circuit to operate in class-AB mode, so both transistors are slightly on during crossover. This can reduce or eliminate the characteristic kink of crossover distortion, although other types of crossover distortion will remain.
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