[Amps] LDMOS availability

[Manfred Mornhinweg]

Bill,

>>> 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.
> 
> REPLY:
> 
> 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).

Manfred

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