QRP Labs QCX-mini, Assembly Instructions Manual

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4.10 Class-E Power Amplifier
A Class-E power amplifier is
awonderful thing. It has a very
high efficiency, sometimes over
90%. This has several important
benefits:
a) Since not much power is
dissipated, we can use
smaller (and cheaper)
transistors
b) So little power is wasted as
heat that the requirement
for a heatsink is reduced or
eliminated
c) During transmit the radio
requires less current, so
the drain on a battery is
less – important for people
who want to operate portable.
A Class-E Power Amplifier contains a resonant circuit at the frequency of operation, so it is
only suitable for single-band use. A lot has been written about Class-E, much of it is very
technical and mathematical.
Some excellent background reading are two papers by Paul Harden NA5N:
http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF1.pdf and
http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF2.pdf
Paul NA5N describes two defining features of Class-E:
1) Use of a square-wave drive to reduce switching losses: the transistors are either on,
or off… no lossy region in between
2) Reducing the effects of the transistor capacitances. Class-E has a resonant tuned
circuit. The capacitance of the transistors, normally an unpleasant lossy aspect, is
now a part of the tuned circuit.
Class-E also has a reputation for being difficult to achieve. All those intense mathematics
Google will help you find, don’t help. In reality, once you realize the secret – it is not so
difficult. Calculation of the impedance of a resonant circuit is simple, and there are many
online calculators which will do the job for you. For example,
http://toroids.info/T50-2.php which allows you to type in the operating frequency, and the
desired resonant circuit impedance. Then the calculator computes the required inductance,
capacitance, and the number of spires required for a certain toroid (in our case we use a
T37-2).
The Class-E design process is simple. Choose the output impedance. We choose 50-ohms,
because this is the input impedance of the Low Pass Filter we will use. The online
calculator will tell you what inductance is needed, and how many spires to wind on the
toroid. The online calculator also tells you the required capacitance to bring it to resonance
at the operating frequency. Here we resort to experiment, because it is a little difficult to
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