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This question is about theory facts of class A audio amplifier.

It is known that class A amplifier is usually biased so that an amplifier's quiescent point is in the middle of the amplifying device's load line. For class B amplifier is known that it is biased in such manner that quiescent point is sitting at the lowest point of load line (in cut-off region). In terms of practical terms, class A will be crossover distortion free, while there will be at least some crossover distortion seen in output signal of class B amplifier.

However, author G. Randy Slone had mentioned in a book named "High power audio amplifier construction manual" under section of distortion mechanisms that crossover distortion cannot be completely eliminated from class A amplifier.

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I wonder, whether his statement is really true, since it doesn't seems logical that crossover distortion could be recognized or measured in any way in class A amplifier. It seems reasonable for class A amplifier to completely eliminate crossover distortion since amplifying devices are fully biased and are conduction 360° of overall input signal. Does his statement holds true from any point of view? Because to me it seems pretty unrealistic for class A to generate even negligible amount of crossover distortion.

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  • \$\begingroup\$ It's hard to guess what the author meant without more context. Perhaps they were referring to crossover distortion introduced by a preamplifier? \$\endgroup\$
    – Elliot Alderson
    Jul 16, 2019 at 15:53
  • \$\begingroup\$ Regretfully voting to close as primarily opinions-based. Depending on how one chooses to define "crossover distortion" vs. plain old 3rd-order distortion, a true push-pull class A stage could be 100% one or 100% the other. \$\endgroup\$
    – TimWescott
    Jul 16, 2019 at 16:08
  • \$\begingroup\$ @ElliotAlderson No, he was referring to crossover distortion in output power stage. But does it make any difference, if we are discussing only crossover distortion? \$\endgroup\$
    – MucaGinger
    Jul 16, 2019 at 16:30
  • \$\begingroup\$ Probably refers to class A push-pull. \$\endgroup\$
    – Chu
    Jul 16, 2019 at 17:41
  • \$\begingroup\$ @Chu Even so, if bases (gates) are biased enough for both transistors to conduct full length of input sine wave (that is 360 degrees) then there should be absolutely no crossover distortion present in output sine signal, or should there be some? \$\endgroup\$
    – MucaGinger
    Jul 17, 2019 at 8:49

1 Answer 1

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In the complementary pair you've drawn, altering the bias point alters the amount of gain variation you get in the open loop amplifier.

In 'class A' operation, the bias current is large, both devices stay conducting, so the open loop gain variation is very small, but not zero. The VBE has to vary to vary the emitter current, and this is non-linear.

In 'class B' operation, the bias current is small, so the open loop gain variation is large. The output devices turn on and off, causing large changes in gain.

In each case, once feedback is closed round the loop, the closed loop gain variation is suppressed by the loop gain.

In each case, the closed loop gain variation, therefore the distortion, is not zero.

In the case of class B amplifiers, it tends to be audible to many people, hence the preference for AB biassing, higher current than a pure class B, but still much lower power than class A.

In the case of class A amplifiers, it's generally inaudible to everybody, even those with golden ears. With enough loop gain, it may even be tough to measure. However, you can see that it will never be zero.

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  • \$\begingroup\$ Why were you referring to open/closed loop gain? From the picture, it can be seen that output is taken from each device's emitter, therefore open loop gain equals approximately 1. \$\endgroup\$
    – MucaGinger
    Jul 16, 2019 at 16:27
  • \$\begingroup\$ Except if you were referring to gain in sense of how much input signal is reproduced at the output of both transistors (larger open loop gain, less crossover distortion?). \$\endgroup\$
    – MucaGinger
    Jul 16, 2019 at 16:29
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    \$\begingroup\$ Typical class A amplifiers do not use complementary devices in the output stage, so do not have crossover distortion. Class AB biases an amplifier to reduce the crossover. Typically here you might see 200W amp with 10-15W of it's operation as class A. This is helpful: electronics-tutorials.ws/amplifier/amplifier-classes.html though even here they don't explain properly how an AB is operating in Class A for low power outputs. \$\endgroup\$
    – Jack Creasey
    Jul 16, 2019 at 16:42
  • \$\begingroup\$ @Keno Yes, open loop gain is approximately 1, not exactly 1. It's this variation that causes the distortion. \$\endgroup\$
    – Neil_UK
    Jul 16, 2019 at 16:47
  • \$\begingroup\$ Variation of emitter follower's gain? But its gain is mostly constant and changes very little... And I still don't get it how this could cause crossover distortion, since it only occurs if base isn't biased enough for transistor to conduct for full length of sine wave. \$\endgroup\$
    – MucaGinger
    Jul 16, 2019 at 20:02

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