[Amps] HV MOSFETs for RF

[Manfred Mornhinweg]

Cathy,

To your list of other services needing linear amps you can add analog TV 
transmitters (where still used), digital TV transmitters, certain 
medical equipment, marine and aeronautical HF communications, military, 
various scientific uses, satellite transponders, and on and on...

> What components do they typically use for their final output,
> high-power devices?  

Whatever they can find that does the job! Tubes in olden times or at 
extremely high power, bipolar transistors after that, then it was 
VDMOSFETs and planar DMOSFETs, then LDMOSFETs, and now MOSFETs made from 
composite semiconductors are being used.

 > Could hams use the same approach?

Of course. And they have been doing it for ages.

> How do they
> get around the efficiency and cooling problems we've been discussing?

There are two approaches: Either live with those problems, or use some 
high efficiency scheme to avoid them.

The "live with them" approach typically means using devices with high 
power dissipation rating at quite low power output. For example, check 
the datasheet for the BLF188XR LDMOSFET. It starts giving a table of 
typical HF and VHF applications, most of which are non-linear, highly 
efficient ones with power outputs of 1200 to 1400W along with 
efficiencies up to 85%. And then there is a single, lone linear 
application mentioned: A digital TV transmitter on VHF, delivering 225W 
output power at 29% efficiency! That's how they do it.

And the other group uses high efficiency techniques, like the Doherty 
method, Kahn's EER, or direct baseband PWM.

In this context it's interesting to remember a snippet of ham history: 
In 1974 the amateur satellite AMSAT Oscar 7 was launched. This satellite 
carries a mode B transponder built by Karl Meinzer (DJ4ZC) and Werner 
Haas (DJ5KQ). This transponder uses Kahn's EER, called HELAPS (High 
Efficiency Linear Amplification by Parametric Synthesis) by the 
builders. Karl made this project part of his Ph.D, in 1973. The specs 
are pretty impressing: Operating in the 2m band, with 50 kHz bandwidth, 
many simultaneous signals going through a single transmitter, -40dB IMD, 
and 10W PEP output at no more than 5W average power consumption for the 
whole transponder! Talk about efficiency! Of course this is possible 
only because of a low average-to-peak ratio, but still it's pretty 
impressive.

The document with schematics is available, and is essential reading on 
this matter, along with Kahn's publications. And AO-7 is still working 
whenever the sun angle permits, 43 years after being launched! Which 
proves that this technology can be quite reliable...

So, don't believe that EER on the ham bands is something newfangled and 
outrageously modern, that needs to be developed from scratch. It has 
been with us for many decades, and the only thing we need to do is come 
up with a practical, inexpensive, good, hopefully reproducible design, 
that puts out legal limit power, using some of the recent high power 
devices!

Manfred

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