Re: Class F amplifiers


From: Dana H. Myers (myers@bigboy.West.Sun.COM)
Date: Wed Jun 19 1996 - 13:06:22 EDT

Laura Halliday wrote:

> On return from my recent European trip I found the May issue of RF
> Design waiting for me, including an article on Class F amplifiers.
> These are a enhancement of Class D amplifiers, which are extremely
> efficient - as long as the transistors can switch fast enough. The
> necessary switching speeds are difficult to achieve at higher
> frequencies, so a Class F amplifier adds tuned circuits to the output
> to emphasise harmonics, resulting in a better approximation to a
> square wave in the output circuit and improved efficiency. I wonder if
> this might be just the ticket for cheap, highly efficient radios for
> the higher HF bands once we start getting sunspots again...

This is done using a quarter-wave line in conjunction with a
resonant output circuit, right? The output matching section is
connected to the power FET drain using a quarter-wave line. The
output circuit presents a low impedance at harmonics of the
operating frequency, and the quarter-wave line inverts this
impedance for the odd harmonics.

I've wanted to try this at 6m or 2m for sometime; at lower frequencies,
the quarter-wave line is way too long to be practical.

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