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Re: [Amps] MOSFET amp for 80 m

Subject: Re: [Amps] MOSFET amp for 80 m
From: Angel Vilaseca <>
Date: Sat, 21 Jul 2018 12:29:56 +0200
List-post: <>
Hi, Manfred.
Thanks for your answer. By the way, thanks also for the many projects on your website. I built your ESR meter last year, and right away, it allowed me to repair several SMPS I had collected over the years!

About the MOSFET 80-meter amp: Sorry, the schematic cannot be displayed on this list, but you can see it here:
OK, since it is a push-pull, the calculation must be done with twice the supply voltage. I did not tnink of that.
But could it also be said, as you wrote on this page of your website:

"When Q1 conducts at the peak of its semicycle, it will have 1.3V on its drain, making 12.5V between Vcc and its drain. That places 12.5V on one half of T2. Due to the tight coupling, that forces 25V to appear across the entire T2, making Q2's drain go up to 26.3V. The primaries of T1 will see a total of 25V."

Or are these two ways to express in fact the same phenomenon?

Also, the other thing I could not understand was the high output impedance that the author of the project claims: 42 ohms. As Bill Turner writes in another post: "The output impedance of the amp should be very small, close to zero and the load impedance around 50 ohms. "
That is exactly what I thought too!
Instead, this amp uses no output transformer, only a 1:1 balun. How can this possibly work?

Vy 73

Angel HB9SLV

Le 18.07.2018 à 21:49, Manfred Mornhinweg a écrit :

you are forgetting that this is a push-pull amplifier.

If the MOSFETs were perfect (zero saturation voltage), then the available voltage swing would be ±13.8V for each, which equals 9.76V RMS. Between the two drains that's twice as much, so 19.5V RMS. This voltage applied to 50 ohm gives 7.6 watts.

Real MOSFETs do not swing fully to ground, but an IRF510 on 80 meters, at such low power and current, gets pretty close. So the maximum unsaturated output might be 7W. The author's spec of 5-6W seems conservative compared to that.

In saturation that amplifier should produce significantly more power, but of course it's no longer linear when driven that hard.


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