Crossover distortion

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Crossover distortion is a type of distortion which occurs in complementary Class-B amplifier stages.

[edit] Explanation

Input-Output characteristic of a Class-B complementary emitter follower stage.

The term crossover signifies the "crossing over" of the signal from the upper transistor to the lower and vice-versa. Crossover distortion can be suppressed by using a slight forward bias in the base circuit such that the transistors are idling at a small output current and also by the use of negative feedback. The forward bias causes the circuit to operate in class-AB mode.

Most modern power amplifiers (including those used in hi-fi) employ class-AB in their output stages as it offers good to very good efficiency and distortion figures.

[edit] Distortion mechanism

Crossover distortion.

The image shows a typical class-B emitter-follower complementary output stage. Most of the accompanying circuitry has been omitted for clarity. Under no signal conditions, the output is exactly mid-way between the supplies (i.e., at 0V). When this is the case, the base-emitter bias (voltage) available both transistors is zero and so they are in the cut-off region, i.e., the transistors are not conducting.

Consider a positive going swing: As long as the input is less than the required forward V_BE drop (≈ 0.65V) 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 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 ±0.65V 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 0V (or where one transistor stops conducting and the other starts). This kink is known as crossover distortion and it becomes more evident and intrusive when the output voltage swing is reduced.

See also: Electronic amplifier