Roy Lewallen wrote:
K7ITM wrote:
. . .
In Class C, the active device is hard-on for a relatively small
fraction of the cycle. It always drives the output to about two times
the supply voltage, peak to peak, if it's properly designed for low
dissipation in the active device. . .

I've designed saturating RF amplifiers of a few watts output in which
the device is on for well over half the cycle, and which have an
efficiency of around 90%. The peak-peak collector voltage is nearly 40
volts when run from a 12 volt supply. The high efficiency implies
relatively low dissipation in the active device. But perhaps this falls
outside some definitions of class C.

Roy Lewallen, W7EL

A flyback switching RF amplifier, Roy? ;-) One can also use active
devices that pull hard to ground and to a rail, delivering a square
wave output at high efficiency. That square wave can then be filtered
with a network that presents a high impedance for harmonics and passes
the fundamental, and achieve very high efficiency. A square wave is
nice because the even harmonics are theoretically zero, and in practice
can be much lower than the odd, so the filter doesn't have to have
super-steep cutoff. I guess that would be some variation on class D.
It's actually the way the HP8640B generates its signal: divide-by-2
stages from the master oscillator, followed by filters. I suppose it's
really best to describe the operation of any amplifier in some detail,
and not just rely on "Class A" or the like.

Cheers,
Tom


Cheers,
Tom