# [Amps] Class A for AM

Gary Schafer garyschafer at comcast.net
Wed Nov 15 11:17:29 EST 2006

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> -----Original Message-----
> From: Tom W8JI [mailto:w8ji at w8ji.com]
> Sent: Wednesday, November 15, 2006 4:42 AM
> To: Gary Schafer; amps at contesting.com
> Subject: Re: [Amps] Class A for AM
>
> > I was just reading some info on class A amplifiers.
> > It seems that a typical triode class A1 amplifier (no grid
> > current) the
> > efficiency is in the range of 20% to 35%.
>
> I was using the theoretical maximum. The practical maxmium
> could be any value less than that (and proably will be).
>
> > So I guess it depends on how the class A amp is configured
> > as to the maximum
> > efficiency it would produce. I have always used the figure
> > in the area of
> > 33% as a ball park value but obviously that can be
> > different.
>
> I haven't designed that many class A amplifiers where
> efficiency was an issue, so I can't really contribute any
> *typical* measurements like I can with class AB. Efficiency
> would have to be some value less than 50% on modulation
> peaks.
>
> > 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.
>
> Let's think about that. Input power is constant. Efficiency
> would be the standing dissipation minus the average RF
> output power.
>
> With 500 watts carrier power in the sidebands at 100%
> modulation would be 250 watts. As I recall the formula is
> Paudio= m^2*Pcar/2
>
> so with 100% modulation and 500w carrier:    1^2 * 500/2
> =250 watts in the sidebands. That's 750 watts when 100%
> modulated, and 500 watts with just carrier. So efficiency
> change would be a factor of  1.5 when going from carrier to
> 100% sinewave modulation. So my 50% to 25% assumption was
> wrong, as is the "doesn't change" assumption. In one example
> with 25% eff on carrier, the actual eff value would be 25%
> carrier to 37.5% with 100% sinewave modulation.
>
> I think this makes more sense.
>
> We have to consider the average or effective power of the
> signal, not the peak power, when considering heating.  A
> 100% sinewave modulated carrier requires an additional  50%
> of the carrier power to have 100% modulation. So the class A
> amplifier runs cooler with modulation because input power is
> constant and output power increases.
>
> 73 Tom
>

Yes tom, as far as heat dissipation goes I would think that would be in the
ball park. But actual efficiencies that the amplifier needs to operate at
would be figured at the peak envelope power level verses the carrier power
level for proper operating parameters.

73
Gary  K4FMX

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