[Amps] Efficient amplifier design

[Manfred Mornhinweg]

Cathy,

> How much easier does the design become if
> we select an upper frequency bound of 15 MHz instead of 30 MHz?

To give a simple-to-grasp figure, I would say it becomes roughly 3 times 
easier. That's because a lower frequency limit opens the doors to use 
many more devices, and also because all problems related to stray 
inductance, device capacitance, etc, get smaller. In addition the 
bandwidth will become smaller, given that we are still starting at 1.8MHz.

Just one example: When I played with cheap switching MOSFETs in TO-220 
cases for linear HF amp use, I ended up with pretty few really usable 
devices that would cover up to 30MHz. There were a lot of cheaper and 
more powerful devices, which I discarded simply because their practical 
frequency limit was too low. With a 15MHz limit, many of them could have 
been used.

In fact my own use for an amp would be mostly confined to 40 and 80 
meters. But then it's something like a given mindset, that an HF amp 
should cover ALL of the HF spectrum. 160m, while outside HF, is easy to 
add, so why not include it. But I leave 6m out. An amplifier covering 
160-6m has twice the bandwith of a 160-10m one, which significantly adds 
to the complications, and 6m is VHF... so let's leave it out.

Of course there are many hams who will want 6m too, but many others 
don't need 6m coverage. Or like you, might be happy with 160-20m coverage.

With homebrew amps, a practical thing to do is to base the upper 
frequency limit on a "best effort" basis: Make sure it works on the 
bands you want most, and try to extend coverage to the higher bands as 
much as is easy enough to do, but don't despair if some of the highest 
bands are left out!

Hams have been doing that from the beginnings. I remember having seen 
circuits using the 833 tube that were usable only on the lower HF bands.

Manfred



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