The RF amplifier: circuit values, MOSFET ratings and operational conditions
This section describes the various components used in class E RF amplifiers,
and how the values of these components affect amplifier operation.
Note: Complete class E RF amplifiers are presented on this site, with all circuit
and component values specified. These are tested, proven circuits that
have been reproduced many times.
If you are building a class E amplifier with a different configuration
(numbers of MOSFETs, frequency, etc.), it is possible to figure your RF
values by an educated guess and check method, using the component values presented
here as a rough guide, or via complex calculations.
There is some very good design software available that will calculate the
component values for common class E amplifier layouts. Check the
Design Tools Section for more information.
The Shunt Capacitor
The function of the shunt capacitance is to reduce the peak voltage
appearing across the MOSFET when the device is in the off state, and to spread the
width of the "off" pulse. The shunt capacitor is also part of the output
matching network. This capacitor must be a high quality, high current component.
Typically, multi-layer ceramic capacitors work best, however silver-mica
capacitors can also be used in this function.
The actual value of the shunt capacitor is important for a couple of
reasons. First, if the capacitance is too small, you will see a
very high RF peak voltage across your MOSFETs. If the value is too large,
the efficiency can suffer. This is not an extremely critical value, and your transmitter
will work over a wide range of values.
A good rule of thumb for FQA11N90 MOSFETs operating at 45 volts, approximately
1 to 1.2 Amperes for MOSFETs in parallel is as follows:
Example: on 80 meters, 4 MOSFETs in parallel would require 1000pF of shunt
- 160 Meters: 500pF per MOSFET
- 80 Meters: 250pF per MOSFET
- 40 Meters: 125pF per MOSFET
The shunt capacitor value is correct if the peak RF voltage across the MOSFETs
during the "off" cycle is around 3.5 times the DC voltage applied to
the stage. If the voltage is higher than this value, you may need to increase
your shunt capacitor.
The best capacitors as of this writing for shunt capacitor use are
Multi Layer Ceramic capacitors made by American Technical Ceramics. The
100C series of capacitors from ATC is particularly good. AVX also makes
good, high current multilayer ceramic capacitors.
Tuning Capacitor Voltage Rating
The series tuning capacitor is subject to very high RF voltage, and
several thousand volts is not uncommon. Using a lower inductance and
higher capacitance in the resonant circuit will reduce the voltage
across the tuning capacitor somewhat. A useful rule of thumb for
figuring the tuning capacitor voltage rating is 3 to 5 times the peak
to peak RF voltage fed to the resonant network plus a safety factor. As
an example, you normally expect to see 500 volts peak across your class
E MOSFETs, and if you have 2 modules in series in your RF amplifier,
you will see 1000 volts peak to peak across the secondary of the
transformer, which is the input voltage to the resonant network. The
tuning capacitor should have a 3000V or better, a 4000V or 5000V rating.
MOSFET Voltage Rating
The peak voltage across the MOSFETs is going to be a little less
than 4 x the DC applied voltage for a proper class E transmitter. This
will vary somewhat with tuning and your exact circuit. If you have very
low, or NO shunt capacitor, the ratio of peak RF voltage to applied DC
can be 6x or 8x the DC or MORE. For class e transmitters with proper shunt
capacitors, figure 4x the DC, plus a safety factor. If you're
running 40 @ 5 amps of carrier, and expect to modulate 150% positive,
your power supply voltage is going to be 100 volts. So, your MOSFETs
are going to see 400V at a minimum. Use AT LEAST a 600 volt
MOSFET, and I would personally use an 800V or 1000V MOSFET in this
MOSFET Current Rating
The current rating of the MOSFET should be at least three times the Maximum
DC current expected. Again, if you're running 40 @ 5 amps of carrier,
and expect to modulate 150% positive, your power supply voltage is
going to be 100 volts. Base your calculations based on 100V. So, your
DC current at 150% positive modulation will be 12.5 amps. Figure your
total MOSFET current rating (all MOSFETs in parallel) should be at
least 36 amps.
Your efficiency will be better if you run less current, or use
MOSFETs with a higher current rating (or MOSFETs in parallel). The R
D-S on (Resistance Drain-Source when the MOSFET is on) becomes a big
factor, particularly as currents increase.