Crossover distortion is a type of

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio s ...

which is caused by switching between devices driving a load. 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 loudspeaker crossover filter—a filtering circuit which divides an audio signal into frequency bands to drive separate drivers in multiway speakers.

Distortion mechanism

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

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s 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 V_BE 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 to 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.

Possible solutions

As with most kinds of distortion, another way in which crossover distortion can be reduced is through the use of

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

. By comparing the output to the desired output, and adjusting the input to correct for any error, we can significantly reduce distortion. This may be done with an

operational amplifier An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to ...

, as shown below, or with a discrete circuit. In the example shown, the operational amplifier is used to reduce the distortion of a push-pull pair. Operational amplifiers are differential voltage amplifiers with very high gain (sometimes modeled as infinite gain). In an ideal model, the output of the op amp is held such that both inputs of the op amp must be at exactly the same voltage. In this case, since the inverting input is directly connected to the output, the voltage at the non-inverting input is always equal to the voltage at the output and inverting input, hence eliminating distortion. With a more precise model of an operational amplifier (with non-infinite gain) distortion is reduced by a factor equal to the gain of the op amp. Most modern power amplifiers (including those used in hi-fi) employ both techniques, using both

class-AB In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...

in their output stages, and feedback, offering reasonable efficiency and good distortion figures.

References

{{DEFAULTSORT:Crossover Distortion Electronic amplifiers Audio amplifier specifications de:Verstärker (Elektrotechnik)#Verzerrungen

Distortion mechanism

Possible solutions

See also

References