schamberlain@wateng.UUCP (Ray Chau) (04/27/84)
I haven't been on the net for a while, but in response to the
the enquiry on the difference between the two amplifier classes mentioned
above; read on.
Basically, class A amps are traditionally associated with low distortion
and voltage gain stages. This is because the particular device is
always operating in the "active mode". (ie. base-emitter is forward biased
and collector-base is reversed bias). Typically in low power stages (preamps,
pre-preamps,etc.) the use of common emitter, common base is popular.
The classification of this as class refers to the finite presence of
current in the collector (due to base-emitter always being turned 'on').
The above situation is termed active mode.
However, for current gain (power amplifiers) the above situation is normally
further fed into a push-pull arrangement. This is where most of the distinction
for the particular classes arises. Note: other classes exist.
For 90% of todays' amps (transformerless and complimentary supply rails),
the so called standard is class AB.
Before tackling the above 'label', I'll digress a bit more to class B.
Class B does in fact exist today in some "toy phonographs"(ie. the type
in a carrying case. I know, cause my little sister now has a the most
robust---5 A-----cleanest sounding beast alive due to a class AB upgrade).
By this I mean that at least one of the transistors cuts off = non active =
non linear. Usually both take devices (one pulls and one pushes) take turns
driving current. All is fine, as long as they do not do so simultaneously.
The problem with class B is that at '0' output they have a gray area
(1.4 volt range) where things are cut off or as the saying goes;non active.
In class AB this is circumvented by having the devices both on in this gray
area. In fact if you have them on enough (ie in terms of current) you
might qualify for some persons arbitrary definition of Class A.
In short;
Class B : no signal--- no current in output current in devices
Class AB : " --- finite cureent in.....
Class A : " ---- more current, but more heat
Note that switching distortion is all the rage as tiny amounts
are more easily percieved by our hearing.
Secondly, in feedback systems, this gray area has low loop gain ie low corrective abilities with the attendant demand of feedback correction
in 0 time (ie look at the waveforms).
Lastly, record clamps and attendant weight === pressure increase
have been shown to be beyond the Rockwell hardness limits within
resonalble bearing life. This means wear, wear, wear .........................................................................................................................Like the man said 'its been too long, but not nearly
long enough'
Ray