[Amps] Old Moto RF Decks

[Carl]

Manfred, I didnt become proficient in hollow state amplifier design by not 
extensively studying, and experimenting with linear circuits that were as 
strange in the 50-60's as SS is to many now.

OTOH there were enough old design tubes available to place a somewhat clean 
SSB signal on the air until tube manufacturers got up to speed and equipment 
manufacturers started using them and/or developed FB circuits for the 6146.

SS QRO development has been slow and commercial user requirements have 
driven it, HF is a small market these days and the money is at VHF and up.

Ive no problem with 12-15dB gain when 100-200W of drive is available, just 
think of it as GG SS (-;

My own R&D experience has all been at 10 gHz and much higher, mostly 
commercial and military but not at QRO levels!

Your special FET might not be that far off the way developments are going, a 
new startup may already be working on it.

Carl
KM1H


----- Original Message ----- 
From: "Manfred Mornhinweg" <manfred at ludens.cl>
To: <amps at contesting.com>
Sent: Tuesday, December 02, 2014 3:06 PM
Subject: Re: [Amps] Old Moto RF Decks


> Carl,
>
>> 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.
>
> Manfred
>
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