You make a lot of sense here. You're right now that I think about it.
The whole conjugate match idea is based on linear amplifier designs. And
for a class AB or B amp, you do not have a linear situation.
I have to admit I am beginning to be a bit embaressed in all this! In my
college days, the onlt type of amplifiers that were ever covered in
detail were class A (and I specialized in RF at U of I Urbana!). And in
my design engineer days for Motorola, all I designed were Class A
amplifiers. So my paradigm is Class A and that is all I think about.
Now I have to sit and rethink things a bit. I am beginning to see why
the input impedance of the tube does vary with drive.
To quote Pooh Bear, "Think, think, think, think......." :-)
>[Richard W. Ehrhorn]
>Oh, boy! I've been biting my lip and staying out of the discussion of Zin
>variation versus drive power - and I agree with George.
>BUT I can't resist commenting on the "conjugate match" issue, even though I
>do know better.
>Unless the basic electrical engineering principles that I was taught in
>EE101 (probably out of Terman or Ryder) have been repealed, Thevinen and
>similar equivalent circuit principles on which the "conjugate matching"
>idea is based are specifically defined as applying only to LINEAR devices
>In class AB, B, and C power amps such as discussed here THE TUBES (or
>transistors) ARE NOT LINEAR DEVICES. Plate current flows over much less
>than 360 degrees of the rf cycle and is OFF the rest. The tube is as much a
>switch as a linear device.
>In fact, since the source R and load R are equal in a conjugate match, half
>the total available power is dissipated in Rs and only half can be
>delivered to RL: maximum theoretical efficiency = 50%. A tube running class
>A draws relatively uniform Ip over the entire rf cycle; it IS a linear
>device. If I remember right, maximum efficiency typically realized in a
>class A amp is around 30%.
>IMHO the most fundamental way to define the function of the input matching
>network of a class AB2 or B amp is that it (plus all the circuitry
>intervening between it and the plate/collector/drain of the driver devices)
>must transform the input impedance of the power tube(s) into whatever load
>impedance those driver devices must see to deliver their desired output.
>Alternately, since nearly all transceivers are designed to work best into a
>nominal 50 ohm resistive load, the power amplifier input network should
>present a 50 ohm resistive input.
>I know there are many fans of the conjugate matching idea out there, but no
>one has explained credibly to me how a 50 ohm SOURCE resistance would allow
>more than 50% of the total drive power to be delivered to the load. The
>remarkable concept of a NON-DISSIPATIVE Rs has been offered, but it's sort
>of beyond my comprehension.
>During preparation of the amps chapter of the 1995 ARRL Handbook I included
>this argument, but the issue apparently had been so controversial that
>League diplomats thought it best to forgo that discussion. Probably I'll do
>the same hereafter!
>73 to all! Dick W0ID (ex-W4ETO)
"A life lived in fear is a life half lived."
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