Technical Information


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Output Circuit Values & MOSFET ratings

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Construction Overview

Simple 400 Watt
RF Amp for
80 meters

VFO for 160 & 80 meters

Pulse Width Modulator and power supply

24 MOSFET RF Amplifier - Step by Step

Analog Modulator (Class H) and power supply

Class E Kits
and Parts

Radio Engineering Associates

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Parts for Class E Transmitters

I have MOSFETS, parts, boards, kits, etc. for class E transmitters. Check (click here for) The Parts Page at the Class E Web Site for more information.


I have used various MOSFETs in the output stages of the transmitters described in this document. I have used MOSFETs made by IXYS, Motorola, Fairchild and Mitsubishi. The IXYS MOSFET is a very good MOSFET, however small quantities, the IXYS IXFH12N100 MOSFETs can cost up to $25 each. This is fairly expensive, as MOSFETs go, and less expensive alternatives are available. The Fairchild FQA11N90 (900V., 11A.) seems to be the best price/performance part available coming in at around $2.80 each, and I am using this part in all new designs for 40, 80 and 160 meters. Another good alternative MOSFET for 160 and 80 meters is the FS14SM-18A (900V., 14 A.) made by Mitsubishi, if the Fairchild MOSFET is temporarily unavailable.

As of this writing, the Fairchild MOSFETs used in both the RF amplifiers are available at Newark Electronics and directly from Steve, WA1QIX.

You do not have to use the same MOSFETs which I used! Other MOSFETs can be used. Use the rules for figuring the peak voltage across, and current rating of the MOSFET (see above) when determining if a particular device will work. Make sure you leave an adequate safety factor in the event of amplifier mistuning, antenna system failure or other anomalies. The 1000V MOSFETs I used in my transmitter provide more than a 100% safety factor, and I have never had a component failure. 600V MOSFETs will probably work, however the safety factor is considerably reduced. It would be advisable, if using 600V MOSFETs to drop the voltage from 40V carrier to 30 or 35V, with a corresponding reduction in the total power supply voltage from 100V to 75 or 80V.

Other Parts

Ferrite Toroid Cores: Available from CWS/Bytemark FB-43-1020, $2.00 or less each in small quantities.

Tuning Capacitor: Use a good high voltage capacitor. These are available from various, new suppliers. I will add the names and web sites as they become available. Use at least a 3500 volt, and preferably a 6000 volt capacitor. Use vacuum fixed capacitors across smaller value variable capacitors if more capacitance is needed.

Loading Capacitor: A 3 or 4 gang "broadcast" variable will work fine - 360pF per section, all sections in parallel. RF Parts is one source, however these are generally readily available.

The Shunt Capacitors:Better than doorknobsare multilayer ceramic capacitors of the high current variety. ATC (American Technical Ceramics) makes some VERY good capacitors. I have used the ATC 100C series of capacitors with great success. Costs vary, however a single ATC100C series capacitor will handle 12 amperes of RF current, so only one capacitor is required per class E module. Costs vary from between $8.00 and $11.00 in small quantities for 1000pF units. Contact American Technical Ceramics directly.

The RF bypass capacitor - should be a good, mica or other low-loss capacitor. RF Parts or surplus. This capacitor handles a high amount of RF current, and will get very hot if it is not a good capacitor.

The Source Resistors (class H modulators): Check Mouser, Newark Electronics and Digikey. If you get the ceramic square units, they are quite inexpensive - less than $1.00 each.

The other parts (ICs, resistors, small power supply components, etc.) are readily available from suppliers, including Radio Shack.

It is no problem whatsoever to build the modulator without an etched board. I have built several using "dead bug" construction. Bob, Blaine and other folks have built the modulator using perf-board construction. Don't let this hold you up!

Power Supply Components

The Power Transformer for the class H modulator should be 96VCT @ 8A, available from:
Torroid Corporation of Maryland, part # 782.482
120/240V primary, dual 48V secondaries (put these in series), weight 13 lbs
cost: around $110.

You can also use an isolation transformer (with 2 secondaries, if a center tap is needed, such as with the class H modulator) with either a Variac to lower the primvary voltage, or you can put a low voltage transformer secondary in series with the primary, to buck the line voltage.

Rectifier: 35A minimum @ 200V. available from:
Newark Electronics, part # MCCM5006 50A units,
cost: around $3.00

Filter Capacitors: For capacitors connected in series, should be 60,000uFd minimum for each bank @ 70VDC.
Check DEBCO electronics, and other sources. Fair Radio also has large capacitors. Put multiple capacitors in parallel to get the desired capacitance.
Cost varies from $1.00 for a 15000uF 63V unit (which I am using quite successfully - 5 in parallel for each bank, total of 10) to $5.00 each. for larger units.

Relays: Easy to get large units from places like Hosfelt Electronics, CPI surplus and even Radio Shack. Cost varies, but I have never paid more than $8.00 for a 25A DPST relay.

Be SURE to build the overload circuitry which is documented in the schematic for the modulator/power supply.

Other Parts: Heat Sinks - you can really use anything which is the correct size. RF Parts for new units, however there are several other surplus outlets which have heat sinks. For a 6 FET transmitter (350 watts), your heat sink should be around 96 square inches. Put a little fan on it to keep things cool. A larger sink is no problem, and a smaller one will also work. As long as the FETs don't get too hot, you're fine.

MOSFET insulators: I've been using Sil-Pads, which work very well and do not require any silicone grease. Get at Digikey, part # BER178-ND

The modulator heat sink needs to be a serious heat sink - 96 square inches minimum, with a fan. Larger is better (isn't that the nature of most things :-)

Unetched PC board - available from many surplus sources, and Radio Shack.

Copper flashing for making bus bars, etc. Home Depot.

Anyone who has other sources of parts, please contact me.

Comments? Contact me at:
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