On Apr 2, 9:20 pm, Owen Duffy wrote:
"K7ITM" wrote in news:1175574896.245112.244360
@y80g2000hsf.googlegroups.com:



On Apr 2, 4:52 pm, Cecil Moore wrote:
K7ITM wrote:
...
seen a great many examples of it. I gave a few of them earlier. I've
also worked on the design of broadband RF amplifiers which are
designed specifically to be 50 ohm resistive sources, through the use
of feedback to set that impedance. You don't need brute-force

Tom, I suggest that amplifiers with a specific equivalent source
impedance to low tolerance don't happen by accident or by magic of a PI
coupler, they require design measures that are not usually applied to
amplifiers for SSB telephony.

Absolutely. The amplifier designs I referred to took some work to get
"right."

The assertions here that "conjugate matching" occurs naturally as a by-
product of peaking the PA is not consistent with the realities of design
of amplifiers with specific equivalent source impedance.

If I understand you correctly, I again fully agree. The design goals
of ham, and for that matter most commercial, RF power amplifiers put
things like decent efficiency, low distortion and stable operation
well ahead of any consideration to design to a particular output port
source impedance. Rarely is that ever even considered, since it
doesn't matter.

The amplifier configuration I used as an example was a single ended class
B valve RF linear with PI coupler for SSB telephony for comparison with
Walt's test. A push pull class B bipolar transistor amplifier with
broadband transformer coupling and high pass filter will behave
differently, and some of those designs include negative feedback which
reduces the equivalent source impedance. (I am not talking ALC here, ALC
is in the form of dynamic power control rather than reducing the
equivalent source impedance, though you could be fooled by some steady
state tests into thinking it has reduced the equivalent source
impedance.)

Owen

I would point out that negative feedback does not necessarily reduce
the output impedance. Voltage-derived negative feedback does, but
current-derived negative feedback increases output impedance. If you
analyze the difference between a grounded-cathode and a grounded-grid
amplifier from the point of view of negative feedback, you will see
that the grounded grid amplifier has higher source impedance, viewed
at the plates. The driver source impedance in the cathode circuit
effectively monitors the cathode current (which is very nearly equal
to the plate current), and generates negative feedback to the grid-
cathode voltage as a result. Adding a small cathode resistance to a
grounded-cathode amplifier will have a similar effect; the effect on
the impedance seen at the plates is much greater than the resistance
placed in the cathode circuit. What you see through a pi or other
coupling network depends on that network, as shown in the examples I
posted earlier today, and generally will be much different than what
happens at the plates.

Actually, the comparison of a grounded-cathode and a grounded-grid
amplifier is a good illustration of how the plate source impedance and
the optimal load impedance are unrelated. The change between those
two types, for a given tube and given plate voltage, has a minor
effect on optimal load impedance, and a major effect on plate source
impedance.

Cheers,
Tom