TRANSMITTERS
Battery-powered transmitters need to be carefully designed
to minimize current draw. This absolutely kills any idea of
using a linear amplifier -- unless you are a magician at
power control. So all of the transmitters I've ever used
have been switching PA's. The best thing about these is
that when you aren't transmitting, the current drain can be
just a few milliamps. And, being digital, you can just shut
power to the whole circuit if you want and it draws NO
current.
My favorite is the design used in TAPS. It uses V-FET power
transistors to drive a push-pull output transformer and
simple digital logic to drive the V-FETs. It is pretty easy
to design an output transformer to produce 100-150 watts
from a 12-15V power supply. Frank Evans, then at Oregon
State University, showed me how to build these power amps
back in the 1970's, when power MOSFET's were pretty new.
Since then I've learned a bit about designing transformers
and come up with a 'standard' design that I use over and
over.
The difficult part of a switching amplifier is that the
output is a square wave. Some substantial portion of the
output power is contained in the odd harmonics of the
primary frequency. Thus it is mandatory that a filter be
deployed between the output windings and the load. On the
other hand, it is also going to be necessary to somehow
match the output impedance of the transformer (I use 50
ohms, usually) to that of the load. The simplest way to
match impedances is with an L-network. Properly selected,
the matching network can also be a low-pass filter. Two for
the price of one!
I plan to pull the design data out from the TAPS manuals,
pull together what I've learned about transformers (you'd
be surprised how few folks know how to design one), and a
bit about L-networks and put it all here. But I haven't
done it all yet, obviously!