It has to do with the plate load impedance that the tube sees. In order to
handle the modulation peaks the load on the tube must show the proper
impedance to allow the proper plate voltage swing for that power.
When the load is tuned for the peak power, as it must be to avoid flat
topping while allowing the full peaks to develope, then the load is not
optimum for the lower power of the carrier so efficiency suffers at carrier
power.
If the amplifier is tuned for greater efficiency at carrier power then the
tube will run out of plate voltage swing range when it tries to reach
necessary peak power that the audio peaks demand. Flat topping will result
which produces distortion and splatter.
The 50% efficiency change in a class B amplifier happens to work out to 50%
because of the parameters that the tube must operate at in class B.
In the class A tube operation the efficiency change is greater but again is
necessary to allow proper development of peak envelope power.
Because of the 100% conduction angle of plate current in class A, input
power remains constant regardless of signal input or output so the
efficiency change range will be greater than with class B.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of KA5MIR
> Sent: Tuesday, November 14, 2006 10:34 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Class A for AM
>
> Hello Gary,
> Isn't that 50% difference only true because we cause it to be tuned that
> way? We could tune it to be more or less different between carrier and
> pep
> but we cause it to be 50% for the sake of AM linearity.
>
> If we didn't cause the 50% difference in class A or any case, wouldn't
> that
> mean the AM signal was not amplified linearly? Not that it would
> necessarily be a bad signal. In fact, we can affect modulation percentage
> through the amplifier if other limitations are observed.
>
> This class A RF thinking is different.
>
> Jeff/KA5MIR
>
> > Using a class A amp as an AM linear I don't think the 50% efficiency
> > change rule applies as it does with a class B linear amplifier. With a
> > class B amplifier the efficiency difference between maximum peak
> envelope
> > power output and carrier output is right at 50% difference.
> > In other words if the efficiency of the class B amp is 70% at maximum
> PEP
> > then it will be 35% efficient at carrier power level of the AM signal.
> > (carrier at 1/4 power of PEP)
> >
> > With class A amplifiers I think it is stated that the efficiency must
> not
> > exceed 50% of full output efficiency but it can be less, and by looking
> at
> > the fact that plate input power does not change at all from zero power
> out
> > to maximum power out on a class A amp the efficiency would not be a 50%
> > change but something greater.
> >
> > In the original example of 25% efficiency at full output (maximum PEP)
> > then carrier output efficiency would be 6.25% with the class A
> amplifier.
> > It would also seem that that % would change with a class A2 amplifier
> > slightly because grid current would only flow on modulation peaks.
> >
> > 73
> > Gary K4FMX
> >
> >
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