[Elecraft] Class - A operatiom (was K3 #441 received)

[N2EY at aol.com]

In a message dated 2/20/08 11:29:20 AM Eastern Standard Time, k0wa at swbell.net 
writes:


> isn't Class A not very efficient? 

That depends on how you define efficiency.

First off, an ideal Class A amplifier that is not overdriven and is dealing 
only with symmetrical waveforms draws the same DC power regardless of signal 
level. That means its efficiency goes up and down with signal level. Also means 
that the amp must be capable of dissipating the full DC input with no signal, 
or at low signal levels.

At full power, when amplifying sine waves, an ideal Class A amplifier can 
deliver 50% efficiency. IOW, on CW or FM/FSK, an ideal Class A amp can deliver 1 
watt of RF for every 2 watts of DC. So even at full power and ideal 
conditions, a Class A amplifier dealing with sine waves has to dissipate half the DC 
input.

Of course in the real world you don't get ideal efficiency. 35-40% overall is 
very good - I don't know if manufactured rigs are that good.

 You can get more power out of class AB1 or AB2 but you have to keep to the 
> linear portion of the curve to avoid distortion.  I guess what is happening 
> is that the rigs are not keeping to the linear portion of the curve because 
> of over drive by the operator or by the circuitry to compensate for something 
> or other.  A linear amp has to operated in the linear part of the curve. 
> Right?  So, what are these rigs doing?

The "linear part of the curve" is pretty limited for real-world devices. Yet 
to get max power, the whole curve is used.

It only takes a little bit of nonlinearity to create distortion products. For 
example, suppose an amplifier with 100W output has enough nonlinearity to 
produce a distortion product that is 1 watt. Such an amp's distortion is only 
20dB down. To reach 40 dB down, (.01%) the distortion product would have to be 
only 10 milliwatts. 

73 de Jim, N2EY




**************
Ideas to please picky eaters. Watch video on AOL Living.
      
(http://living.aol.com/video/how-to-please-your-picky-eater/rachel-campos-duffy/
2050827?NCID=aolcmp00300000002598)