RF Design magazine had an article on a class E amp for around 10 MHz,
single freq, years ago. See below. QST published one recently I believe.
Used to be you had to get a license from Nat Sokal and Design Automation to
do a class E amplifier (for a production transmitter) as he patented it.
Probably expired now. Basically using the MOSFET to switch, and doing some
waveform modification by including parasitic capacitances in the switch you
can improve the efficiency. By eliminating the losses of the shunt
capacitance on the output of the active device, you improve HF or VHF
efficiency. You build a class C amplifier, then add tuned circuits to
produce open or short circuits at the harmonic frequencies, flattening the
collector (or drain) waveform while current is flowing. With third harmonic
peaking, collector voltage approaches a squarewave. With second harmonic
peaking, the collector voltage becomes approx a half sinewave.
The efficiency can be very high, nearly 100% on a single freq optimized
amplifier. Meaning, most of the DC power is converted to RF. To be able to
tune it is tricky, as you can damage the devices if not careful. (at least
back in the 80's you could) My old boss told me he made a class E amplifier
deliver a few watts out of a plain old 2N2222 or 3904 or something common
like that. And you could hardly feel the heat.
One of the references below (Davis,Via) was supposidely about 500 Watt, 6
Mhz operation. I do not have that RF expo proceedings, have all the
others!#@$#.
Forget it for linear amplification.
--------------
Some of the foundation articles and references on class E that are in
English language are:
Sokal, Nathan and Alan, US Patent 3919656, issued 1975.
Sokal and Sokal, IEEE Jour of Solid State Circuits (JSSC) June 1975,
"Class E- a new class of high efficiency tuned single-ended switching power
amplifiers"
Sokal and Fredrick Raab, IEEE Jour of Solid State Ciruits (JSSC) Feb 1977,
"Harmonic Output of Class E Rf PA and load coupling network design"
Raab, (JSSC), April 1978, "Effects of circuit variations on the class E
tuned PA"
Sokal and Raab (JSSC), Dec 1978, "Transistor power losses in the class E
tuned PA"
Sokal and Sokal, RF Design Magazine, Summer 1980 edition,"Class E switching
mode RF power amplifiers"
(long article - good)
Solid State Radio Engineering, textbook by Herb Krauss (deceased), Fred
Raab and Charles Bostian. 1980, Pg 448-454. (Krauss and Bostian were
professors in my EE classes back then)
Kazimierczuk (JSSC) Aug, 1986 "Class E Tuned PA with Nonsinusoidal Output
Voltage"
Collins, Hinchliffe and Hobson, Electronics Letters (UK) (June 1987)
"Optimized Class E amplifier with Load variation"
C. Davis, W Via, RF Expo Proceedings (1990), "Designing a practical 500
watt class E PA."
D. Cripe, Broadcast Electronics, Inc., Technical paper presented at NAB a
few years ago,"Improving the Efficiency and Reliability of AM Broadcast
Transmitters through Class E Power"
QST (year or so ago?) Sorry cannot find it!
And then there's class EF, class DE, and class F, G, H and S (like D).
You'd do better making a class B push pull or class AB1 with transistors if
you must, unless you only want CW and wish to play with the tuning each
time from 7 - 21 MHz.
John
K5PRO
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