From: tomb@hplsla.hp.com (Tom Bruhns)
Newsgroups: rec.audio
Subject: Re: Class A/B/whatever... What does it mean?
Date: 21 Jul 92 22:56:19 GMT
Organization: HP Lake Stevens, WA

jmd@bear.com (Josh Diamond) writes:

>So, let the technical explanations (and debate) continue.  I would
>personally like to know how class A vs. B vs. AB fits together with
>Tube vs. Transistor.  Grant states that the comment in the original
>response about "complementary tubes" is babble.  Why is this not
>valid?

Tubes are available in only one "polarity"; transistors in two
(NPN and PNP, or N-channel and P-channel).  Tubes would more
commonly be connected push-pull through a transformer, though a
stacked arrangement as is used commonly with transistors is
possible.  Some transistor amps have transformer-coupled outputs,
with the devices driving opposite sides of a center-tapped winding,
as with tubes, but that's uncommon, and almost unheard of in high
fidelity amps.  With transistors, though the devices that conduct
the bulk of the output current may all be one polarity, typically
NPN, one side is usually arranged with other driver transistors so
the net "looks" like a PNP, while the other side with its
corresponding drivers "looks" like an NPN.

The definitions of A, AB, B, C, and D classes is pretty general,
and doesn't depend on the stage being single-ended or push-pull
nor does it depend on the active devices being tubes or transistors.
A:  all active devices in nominally linear region for all of the
    input waveform.  Relatively low efficiency and easier to
    achieve low distortion.
AB: each active device in nominally linear region for more than
    half but less than all of a full-output-power sinewave.
    Remains class A for powers up to some percentage of full
    power.  Better efficiency than A, especially for low output 
    power, at the expense of increased distortion problems at power
    levels where active devices aren't on for the full cycle.
B:  each active device in nominally linear region for just half
    of (symmetrical) signal waveform.  (generally turned "off"
    for the balance.)  Efficiency is high, but distortion
    products from devices turning off come into play even at
    low power levels.
C:  each active device in nominally linear region for (well)
    under half of (symmetrical) signal waveform.  This class is
    useful only where amplitude information need not be preserved,
    as in RF power amplifiers.  Very efficient for this
    application.
D:  Amplification takes place by switching the output rapidly
    between two rails (plus and minus power supplies, generally),
    and modulating the percentage time spent at each rail to
    provide an average output proportional to the input; that
    switched output is then filtered with passive components.
    Obviously, the switching frequency must be considerably
    higher than the highest frequency to be amplified.  The
    active devices are either full "on" (yielding low voltage
    drop, and therefore low dissipated power in the device) or
    full "off" (yielding low current, and low power); efficiency
    is very good, and linearity can be quite good but requires
    a lot of care:  the devices behave nominally the same over
    the entire cycle, but because RF switching frequencies are
    involved (for an audio power amp), little gremlins get in
    pretty easily.

Any of these can be "push pull" or "single-ended".  However, for a
class C amp to be useful, it normally would be dealing with a
narrow-band signal and using a tuned circuit to carry the output
through its full cycle, independent of push-pull of single-ended.
Class B or AB must be push-pull so at least one side is in the
active region at any instant, or else it must be operated in a
narrow-band mode like the class C if it's single-ended, so a
tuned circuit completes the cycle when the device is "off".  In
the latter case, the B or AB amp is suitable for applications
requiring varying amplitude output, unlike the class C amp.  Class
A is suitable for linear amplification either single-ended or
push-pull.  Broadband power amplifiers (particularly audio) are
generally run push-pull.  Class D amps can be single-ended or
push-pull; you can consider a switching power supply to be a
class D amp.  I believe the commercial ones done for consumer
audio power applications have all be push-pull.

Finally, I believe you can apply the amplifier classes to tubes,
bipolar transistors, FET's, and even things like fluidic amps.
Newsgroups: rec.audio