[Image] Evacuated Envelopes                                         [Image]

The On-Line Valve Amp Magazine

DATE: AUGUST 1995

---------------------------------------------------------------------------

Class A, AB1, AB2, B...and A2 explained

Author: Scott Robinson

Date: 26 AUGUST 95

These classes of operation all have to do with how the devices (tube,
transistor, or FET) are biased. The number suffixes (-1 or -2) have only to
do with tubes. All descriptions assume push-pull except where otherwise
stated. Read on:

CLASS A-both devices are biased at half of the maximum current to be had on
a peak signal. Therefore, when one gets to twice the static current, the
other just cuts off. As a result, NO crossover distortion is possible,
since both devices conduct all the time. This is probably the best for
audio quality, but requires that the output stage dissipate heat equal to
or greater than the maximum available output power all the time. Your cat
will like a class A amp 'cause he/she can sleep on it and stay warm. I
beleive the Krell ss amps are operated this way. All single ended audio
amps are operated like this, because any other class of operation single
ended cips off some of one side of the waveform.

CLASS B--The devices are biased so that at idle, both are off. A positive
going signal turns one of them on; a negative signal, the other. The trick
is to get the splice between them at zero signal perfect. Any imperfection
makes crossover distortion, which is nasty stuff that generates listener
fatigue and gave early ss amps a bad name. Nearly all solid state amps are
class B, or nearly class B. They dissipate very little heat when no signal
is present, and less that class A even at about 70% signal, their hottest
condition. Read on to find out about the 'nearly class B' part.

CLASS AB--This class of operation is in between classes A and B. Both
devices are on near zero but one turns off somewhere between zero and
maximum signal. For transistor amplifiers, only a very small bias is needed
to get a good--but not perfect--splice. I called this 'nearly class B'
above. FET amplifiers like Haflers splice better with quite a bit of bias,
hence they run hotter with no signal. Efficiency less than B and better
than A and depends on how much bias is applied.

CLASS C--only used in transmitters and can be either single ended or
push-pull. The tube is only on for a small part of each sine wave cycle.
Only useful when driving a resonant circuit so that it works like pumping a
swing. Very efficient.

NUMBERS-1 and -2: These only apply to tubes. A -1 suffix means that the
grid is never driven positive so that grid curent will flow. A negative
driven grid will, for practical purposes, not conduct. A -2 suffix means
the gird is driven positive. A tube with its grid dirven positive can
conduct down to lower instantaneous plate voltage, giving substantially
higher output. The cost is again distortion, because there is a crossover
between grid current and no grid current, and unless the driver has a 0 ohm
output impedance, the signal will be affected to some degree. Most hi-fi
audio amplifers are actually class AB1, considered a reasonable compromise
between efficiency (hence cost) and sound quality. Many large public
address amps are AB2.

It is possible to construct an amplifer that is class A2..sort of, and some
old radios (1930's) do this. Neither tube cuts off but some distortion will
be introduced if the dirver circuit isn's a perfect voltage source, as I
said above.

And if this ain't a class act, I don't know what is!

---------------------------------------------------------------------------
tubes@hillier.demon.co.uk
© M.J.Hillier 1995