Roger, and all,
I'm becoming attached to the idea that the word "linear" should be
banned from the advertising Ham Amps, unless they (the seller) can show
proof that the amp is truly linear, neglecting HOW Linear an amp must be
before we can call it linear. How often has an add for a 2-meter Amp
stated, "160 Watt 2-meter FM linear amp?
I agree. For a long time I have been trying to fight the wrong use of
"linear".
> A linear class C amp is quite a trick.
Yes, it's tricky, but it can be done. Variable bias controlled by an
error amplifying loop is one way to do it.
I would "guess" that instead of linear, those devices were run in deep
class C,
It turns out that many LDMOSFETs are not good for class C use, due to
restricted gate voltage range, and because the very peaky current
waveform results in less power output per device. That's why they are
almost invariably run in class AB.
80 to 90 percent efficient tells me they were either using one of the
more exotic forms of amplification, operating in deep class C, or a
combination. They were definitely not running class AB, or B in PP.
I have to correct you. I have built amplifiers myself that run in class
AB and are 80% efficient. Of course, these are not linear!!!
Many hams automatically associate "class AB" to "linear", and "class C"
to "nonlinear, FM only". But this is wrong!!! The ONLY thing expressed
by the class of operation is the conduction angle of the active devices.
NOTHING else. If you build an amplifier in which the active devices
conduct for 190 degrees of the RF cycle, and stay in complete saturation
for 150 degrees, you get a non-linear class AB amplifier having a high
efficiency in the range of 80 to 90%. And that's the mode of operation
used in many test circuits that appear in MOSFET data sheets.
MOSFETs have a soft, square-law onset of conduction. For that reason
they need to be biased into class AB just to exhibit a good gain! Even
when linearity is not required. If operated in class C, a much stronger
drive will be required, and the drain current will be far peakier, so
the power output and often even the efficiency will be lower than in
saturated class AB!
So, everybody, please forget anything you learned about class AB being
linear and class C being nonlinear. Even class A can be very nonlinear,
if a MOSFET is driven from nearly cut-off to saturation! But that's
still class A, because current remains flowing at all times...
Class A, AB, B and C only tell about the conduction angle of the
devices. Not about linearity. Sure, it's easier to achieve decent
linearity in class A and AB than in class C, but that's about all there
is to it!
Manfred
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