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 here, 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.
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:
- 160 Meters: 500pF per MOSFET
- 80 Meters: 250pF per MOSFET
- 40 Meters: 125pF per MOSFET
Example: on 80 meters, 4 MOSFETs in parallel would require 1000pF of shunt
capacitance.
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.
Don't be afraid to experiment a little with this value! MOSFETs
vary, and you may also find you have some reactance - inductive or
capacitive - reflected from your load, depending on tuning.
NOTE: It is possible to operate without this capacitor, if you have
PLENTY of voltage headroom. Without the external capacitor, you can
safely figure your peak voltage will be about 5 to 8 times the DC
applied voltage, depending on the internal capacitance of the MOSFETs,
and exactly how the RF amplifier is tuned. A 1000 volt MOSFET operating
at a *peak* applied voltage of 75 volts can be safely operated with no
shunt capacitor.
Make sure you choose a GOOD capacitor for the shunt
capacitor. The capacitor should have a low inductive component and
should be able to take plenty of RF current.
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.
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 2.5 to 3 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 you are using a step-up transformer with a 1:2 ratio,
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 2500V or better, a 3000V 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
application.
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.
