[net.audio] Amplifier classes A and AB

labelle@hplabsc.UUCP (WB6YZZ) (04/04/84)

   The QUESTION. Can anybody hear "class ab distortion" on a properly
  designed amplifier.

   The answer-  NO!

sjc@mordor.UUCP (04/05/84)

In answer to the query about amplifier classes A and AB:

Bipolar transistors are most nearly linear in the middle of the "active
region", and much less linear near the "saturation" (that is,
completely turned on) or "cutoff" (completely turned off) regions.

Amplifier output stages usually feature two transistors, one supplying
current to the speaker during positive portions of the waveform and
another "sinking" (receiving) current from the speaker during negative
portions.

Ideally, only one of these two need be on at any instant. Letting the
supply-transistor turn off completely while the sink-transistor is
working, and vice versa, saves energy. Class B amplifiers do exactly
this.

Because of the nonlinearly near cutoff, however, this produces a
discontinuity whenever the signal crosses zero, because the second
transistor doesn't turn on quickly enough after the first turns off.

Class AB involves biasing the transistors so that neither one cuts off
completely. During the negative portion of the waveform, for example,
the supply-transistor continues to pass a small amount of "idle"
current.  This makes it run just slightly warmer than it would in class
B operation. Class A involves biasing the transistors so that neither
one ever comes close to turning off. This makes them run considerably
warmer than they would in either class B or class AB.

Advocates of class AB operation claim they can use negative feedback to
reduce any remaining discontinuity to insignificance; advocates of class A
operation disagree.--Steve

djl@fisher.UUCP (Dan Levin N6BZA ) (04/07/84)

If you are really interested, check out the _Amateur_Radio_Handbook_
from the library.  There is a very good section on amplifier design.
Published annually by the ARRL (American Radio Relay League).

-- 
			***dan

{ihnp4 | decvax | ucbvax}!allegra!fisher!djl
The misplaced (you call *that* a mountain ?!?!?) Californian