> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> To: "'amps'" <amps@contesting.com>
To: <amps@contesting.com>
> Date: Thu, 4 Sep 97 08:03:19 +0000
> >I'm not sure what you mean by the "output impedance of the
> >transistor". How was that parameter determined?
>
> In the same way as ra is for a pentode - from delta Vc over delta Ic.
I thought so.
That parameter is NOT what we measure looking back into
the PA. It would be like matching the resistance change of a tube or
voltage in a power source by holding the grid voltages steady for an
instant of time and perturbing the anode voltage. In a tube it is a
measure of the stiffness of the anode to cathode resistance change
for FIXED grid bias voltages as the anode voltage is moved.
Bruene "assumed" the plate resistance of the output device "looked
like" the source impedance, but if you look at his graphs in QST
you'll see the ETO PA crossed ZERO reverse generator SWR at about
1100 watts. That would be the point of conjugate match, and if you
measured anode efficiency at that point it would be highest, and
larger than 50%.
By the way triodes, tetrode and pentodes...and even my solid state
amateur rigs, have a higher source Z than the conjugate of the load Z
when drive is reduced. Measuring efficiency reveals efficiency peaks
at one level of drive ( in solid state fixed tuned rigs it is
sometimes beyond the rating of the devices, but an external tuner
will bring it back in a safe area) and decreases either side of that
value. Source impedance always increases (in non-class A PA's) as
drive is reduced, but if a reverse power generator is applied and
the tank adjusted for minimum reflected power while the PA
is driven normally with a fixed level of drive, maximum efficiency
will be obtained.
If drive is reduced until the PA device moves into class A, the
source impedance simply stops changing. At least it did this in
about 15 PA's I tested, and a few PA's Maxwell tested.
> I gather that's the way Spice does it as well.
No, SPICE works perfectly if you do a correct model. There is at
least one paper published on that subject.
73, Tom W8JI
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