Re: [Amps] Re: Amps Digest, Vol 12, Issue 35

[Bill Fuqua <wlfuqu00@uky.edu>]

After looking at a web site http://www.cree.com/ftp/pub/appnote1.pdf
I realize that all these linearization techniques are used with linear 
amplifiers to make them more
linear except for one (Envelope Elimination and Restoration)
which is just like the DAF system using a class C amplifier and a modulator to
make the output waveform envelop just like the input.

These techniques are used to not only make a linear amplifier more linear 
but in some cases to
preserve phase relationship of signals that also have amplitude modulation. 
The problem with
solid state amplifiers is that the internal and effective internal 
capacitances change with
voltage causing phase amplitude dependant phase shift.  The polar loop 
linearization system
reduces this effect but does not improve efficiency of amplifiers. Linear 
amplifier is still needed.
It just does not have to be as linear as the ones we use in HF or VHF SSB 
work.

Class E amplifiers are so nonlinear that to transmit CW they have to 
modulate the power supply with
a soft turn on and off to prevent key clicks.

73
Bill wa4lav


At 11:34 PM 12/17/2003 +0000, you wrote:
> > **  I have yet to hear a Ham station on SSB who is getting 90% efficiency
>
> Neither have I. But that's not the point of the discussion I was trying to
> have. I was just trying to point-out that many hams are unaware of some of
> the developments in transmitter design which have taken place since the
> 1950s, and that there are techniques being commonly used elsewhere which we
> could use to our advantage.
>
> As it happens, switching mode amplifiers (class-E, F, and beyond) can work
> well to beyond the vhf region, but not as linear amplifiers. There's an
> active group using them for CW PAs on the low bands - usually using cheap
> general purpose power mosfets. Have a look at the Web.
>
> But, it is entirely practicable to make an ssb transmitter with a switching
> mode PA if you go about it the right way! There's a transmitter topology
> (patented by the remarkable, and sadly late, Vlad Petrovic)  - polar loop
> linearisation - which has been used in the past, and looks like it might be
> making a comeback for cellphone base transmitters. A 90% efficient ssb
> transmitter at HF could be a reality if someone had the time and backup to
> design it.
>
> Vy 73
>
> Chris
> GW4DGU
>
>
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