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24 MOSFET RF Amplifier - Step by Step

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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.

Diagram showing two class E topologies

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.