> I always thought that class C meant using pulses to excite a tank
> circuit to resonance, and class D is some form of pulse modulation
> with the output signal derived from the pulses average.  Class D
> usually implies filtering to get rid of the high frequency pulses, but
> is intended to produce some arbitrary waveform, not to resonate a tank
> circuit.

In my experience class C amps do drive the output transistor as hard as
possible for less than 180 degrees of the input signal, but they have a
couple of obvious items that make them special, at least at a second look:

First, all class C amps have a loaded Q of at least 2 and often 10. This
means that the collector voltage becomes negative vs. the emitter (and
base) half of the time. Then, the current phase vs. voltage phase in a
loaded RLC resonant tank circuit does not allow usual Ton Toff etc
calculations. Only impedance and S parameter etc calculations make sense.
And last, the C21 and Miller capacitances of the transistor play a
significant role in the operation of the circuit, requiring
'neutrodynation' (ac positive feedback) in some applications.

The bottom line is, that getting some unknown transistors to work in class
C requires an inordinate amount of lab work with a spectrum analyzer,
especially to remove any 'birdies' and parametric oscillations that may
appear and to get harmonics under control. This has been my experience so
far. Also unspecified transistors will often try hard to oscillate by
themselves in this role (and self destruct if allowed to). All modern
designs I've seen move away from class C to class AB (for low harmonics)
or D (switching) using push-pull outputs and untuned balun transformers.
The only people who still use class C are probably amateurs and fixed
frequency stations (like remote control Txes and radio stations). The
reward for all the effort is low cost, simplicity, and good efficiency.

Peter

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