[Lowfer] Class C v Class E

[Johan H. Bodin]

Alan Melia wrote:
> The minus point
> is that Class E is a "single frequency" or at least narrow band mode at LF
> and also it is "single power". It is possible to operate a design at lower
> supply voltages but the efficiency degrades and of course the output
> impedance hence the matching to any filter changes.


Hi Alan,

the impedance does not change much with supply voltage. Of course the 
transistor's Cout varies somewhat with supply voltage but at LF, where the shunt 
C1 is usually much larger than the variation of transistor Cout, the tuning 
variation is very small. As long as the transistor acts a voltage independent 
"hard switch", the RF output voltage is proportional to DC input voltage and the 
efficiency is independent of supply voltage. This linear relationship makes 
class E amplifiers very good for high level AM modulation. The biggest problem 
with AM on low RF frequencies is the AF lowpass filter formed by the DC feeding 
choke and the amplifiers equivalent DC load resistance.

73
Johan SM6LKM