QRP Labs QCX-mini, Assembly Instructions Manual

Page: 106 / 133

know what the output capacitance of the transistor is. The device capacitance varies
depending on supply voltage and whether it is on or off. A simple experiment is required,
adding different small capacitances to the circuit, and measuring the efficiency (measure
supply voltage and supply current to calculate power input; then measure RF power output.
Divide one by the other to get the efficiency). It is easy to find what additional capacitance is
required to peak the efficiency. The resonance is quite broad and non-critical.
In this implementation, three BS170 transistors are used in parallel. The BS170 is
inexpensive and small, but is rated for 500mA drain current and up to 830mW of
dissipation. Per device. Three in parallel provides plenty of capability to achieve a 5W
output on a single band.
There are always minor variations between device characteristics from one transistor to the
next. If these were bipolar NPN transistors, we would not be able to parallel them in this
way. If one transistor takes more of the load and starts to heat up, its resistance further
decreases and this causes it to get even hotter. This process is known as “thermal
runaway” and results (quickly) in destruction of the transistor. Emitter resistors are used to
help balance the load. But
with MOSFETs, their
resistance INCREASES as
the temperature goes up – so
there is an inherent self-
balancing when multiple
devices are used in parallel,
without any need for
additional balancing resistors
which would increase
component count and waste
some power.
This oscilloscope screenshot
shows the classic Class-E
waveform. Please ignore the
ringing due to poor set-up of
the ‘scope probes etc. The
lower (blue) trace is the 5V squarewave at the gate of the BS170 transistors. The upper
(red) trace is the voltage at the BS170 drain. It peaks at approximately 40V in this example.
This measurement was done with 12V supply and on 40m (7MHz).
The important point to note is that when the BS170 are switched ON (the gate voltage is
5V), the drain voltage is zero. When the BS170 is OFF the drain voltage pulses nicely to a
large amplitude. Class-E!
The summary: Class-E is actually quite easy to achieve in practice! Perhaps all the
complicated mathematics might help to squeeze out another % or two of efficiency. But for
practical purposes, it’s a wonderful building block to use in a single-band CW transceiver.
QCX-mini assembly Rev 1.05_Fr
106