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Re: [Amps] LDMOS availability

Subject: Re: [Amps] LDMOS availability
From: Manfred Mornhinweg <>
Date: Thu, 08 Jun 2017 02:34:36 +0000
List-post: <>

A linear class C amp is quite a trick.
Yes, it's tricky, but it can be done. Variable bias controlled by an error amplifying loop is one way to do it.


If you're varying the bias to make it linear, it's no longer Class C.

I beg to differ. I don't see why varying (modulating) the bias turns it into something else than class C. I maintain that as long as the bias modulation I do is such that the active device conducts over less than 180 degrees of the RF cycle, it's still class C.

Imagine a very simplistic theoretical case: You have a transistor or tube that has a straight transfer curve, and you modulate the bias proportionally with the drive signal amplitude, starting from cutoff bias, so that at any signal amplitude the device conducts for the exact same 120 degrees. And you keep the drive level below the saturation point. This amplifier would be linear, it would be class C, it would have varying bias, and it would be somewhat more efficient than a class AB amp.

Of course practical transistors and tubes don't have that straight transfer curve, and also you want to drive the amp well into saturation to further increase efficiency. This is where the bias modulation must become a little more complex, for example by deriving the bias from an error amplifier comparing input to output amplitude. The actual bias voltage will then vary in a complex way with amplitude. But the amplifier can still operate fully in class C.

The amplitude linearity of such a linearized class C amplifier can be far better than that of a conventional class AB amp. Phase distortion is another matter, though. Such amplifiers are best implemented with devices whose capacitances don't change much (tubes), or devices that have capacitances low enough that the phase modulation resulting from capacitance fluctuations isn't too bad (UHF-capable LDMOSFETs at HF).


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