> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Mon, 28 Jul 97 12:17:17 +0000
> >What is the difference between audio amplification and linear RF
> >amplification?
>
> Audio amplifiers use a 2-tube push-pull configuration with a transformer.
> Each tube supplies half of the waveform.
Not true, audio amplifiers can use any configuration. Most audio
amplifiers are single ended, unless high power is needed. In that
case they switch to AB class so the conduction angle can be shortened
and efficiency improved with minimal HARMONIC distortion.
>Tube-type RF linear amplifiers
> use a push-push configuration with a Pi-network/'flywheel' that fills in
> the other half of the waveform.
Also not true. RF amplifiers can be ANY configuration, they can
be push pull or push push. (Actually they pull-pull, and NEVER
push...but that is a different topic.)
>Audio amplifiers can amplify
> non-coherent waveforms and coherent waveforms. Push-push RF linear
> amplifiers only amplify coherent waveforms.
Not true. RF amplifiers can amplify waveforms that vary in
period or level randomly. If RF amplifiers could NOT amplify such
signals, our receivers would be VERY quiet. White noise other forms
of static are about as incoherent as it gets, and they zip right
through either an audio or RF amplifier.
> >Don't both require a linear amplifier, with the same
> >transfer characteristics? Aren't the mathematical models for analysis
> >the same?
>
> The models are different.
The models are EXACTLY the same, a Chaffe (fourier) analysis
works the same with any frequency. Only the goals are different.
In an audio PA, but intermodulation products AND harmonic distortion
are usually both important. That means we usually have to watch even
and odd order sinewave products.
In an RF amplifier, everything is generally less critical. We only
need worry about odd order intermodulation products. That's why a
push pull audio PA is often useful, but often makes no difference at
all in RF applications.
That is also why a properly sloped non-linear transfer characteristic
is often harmless, and actually can be desired, in many RF
applications.
73, Tom W8JI
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