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Re: [Amps] Old Moto RF Decks

Subject: Re: [Amps] Old Moto RF Decks
From: Manfred Mornhinweg <>
Date: Tue, 02 Dec 2014 20:06:14 +0000
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Now when is Freescale going to offer a 2-30 or 2-50 mHz device that has high efficiency and very low IMD at the same time and be targeted at the linear SSB amp market?

Freescale provides you the best FETs they can. It's up to _you_ to come up with an amplifier circuit that delivers that high efficiency and linearity!

Expecting to take a device and connect just the drive signal, power supply, output line, and maybe a bias source, to get a clean output at high efficiency, unfortunately exceeds the possibilities of _any_ active device, be it solid state or empty state!

A tube can be linearized to some extent by adding additional grids, and cleverly shaping the electrodes. But it still needs lots of idle current to provide low IMD, and thus will have poor efficiency. One could do something similar with FETs, by adding a lot of source ballast resistance. That costs in efficiency and in gain, but does improve linearity a whole lot. Instead by using a plain device and building a clever circuit around it, you can get _both_ high efficiency and good linearity. At least in principle. It can get complicated, though.

If you want to keep things simple, the easiest way to reduce IMD in solid state amps is to give them lots of negative feedback. To do that, you have to start with excess gain! 30 years ago that was somewhat hard to do. But nowadays there are FETs that have huge power gain in the HF range. Given that the typical ham needs an amp having just 12dB gain, you can burn up the FET's excess gain via negative feedback, getting improved linearity in a class AB amplifier.

But the efficiency will still be modest. If you want 80-95% efficiency along with good linearity, it's time to look into all the complexity of class E amplifiers with modulated switching power supplies, and a DSP box wrapped around. And at that point you don't really need a FET with any particularly good "natural" IMD spec! You just need an RF-capable switch.

So, your teasing Freescale to give you a super-linear, efficient FET with two coax connectors and a power plug on it, will hardly lead to any success! It lies in the nature of the thing... ;-)

I can't deny, though, that I would like such a special FET myself.


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