[Amps] LDMOS availability

[Manfred Mornhinweg]

Roger, and all,

> I'm becoming attached to the idea that the word "linear" should be 
> banned from the advertising Ham Amps, unless they (the seller) can show 
> proof that the amp is truly linear, neglecting HOW Linear an amp must be 
> before we can call it linear.  How often has an add for a 2-meter Amp 
> stated, "160 Watt 2-meter FM linear amp?

I agree. For a long time I have been trying to fight the wrong use of 
"linear".

 > 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.

> I would "guess" that instead of linear, those devices were run in deep 
> class C,

It turns out that many LDMOSFETs are not good for class C use, due to 
restricted gate voltage range, and because the very peaky current 
waveform results in less power output per device. That's why they are 
almost invariably run in class AB.

> 80 to 90 percent efficient tells me they were either using one of the 
> more exotic forms of amplification, operating in deep class C, or a 
> combination.  They were definitely not running class AB, or B in PP.

I have to correct you. I have built amplifiers myself that run in class 
AB and are 80% efficient. Of course, these are not linear!!!

Many hams automatically associate "class AB" to "linear", and "class C" 
to "nonlinear, FM only". But this is wrong!!! The ONLY thing expressed 
by the class of operation is the conduction angle of the active devices. 
NOTHING else. If you build an amplifier in which the active devices 
conduct for 190 degrees of the RF cycle, and stay in complete saturation 
for 150 degrees, you get a non-linear class AB amplifier having a high 
efficiency  in the range of 80 to 90%. And that's the mode of operation 
used in many test circuits that appear in MOSFET data sheets.

MOSFETs have a soft, square-law onset of conduction. For that reason 
they need to be biased into class AB just to exhibit a good gain! Even 
when linearity is not required. If operated in class C, a much stronger 
drive will be required, and the drain current will be far peakier, so 
the power output and often even the efficiency will be lower than in 
saturated class AB!

So, everybody, please forget anything you learned about class AB being 
linear and class C being nonlinear. Even class A can be very nonlinear, 
if a MOSFET is driven from nearly cut-off to saturation! But that's 
still class A, because current remains flowing at all times...

Class A, AB, B and C only tell about the conduction angle of the 
devices. Not about linearity. Sure, it's easier to achieve decent 
linearity in class A and AB than in class C, but that's about all there 
is to it!

Manfred


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