[Amps] Low Input SWR Amps Networks

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Wed, 03 Nov 2010 13:30:51 -0500, donroden at hiwaay.net wrote:

>Please explain to the choir why the two aren't necessarly the same.

REPLY:

I don't completely understand your question but maybe I can answer it
anyway. On an amplifier where the input RF goes directly to the cathode
with no network of any kind, there is no impedance matching, and in fact
the impedance may vary greatly over the 360 degrees of the RF cycle.

If you place a parallel resonant circuit from that line to ground, there
is still no impedance matching, but now you have added some Q which
helps provide the "flywheel" effect to smooth out the variations in
impedance over the RF cycle. There is still no matching of impedance, so
whatever the native impedance of the tube is, that's what you get.

From that point, you can tap down on the coil, either the input or the
cathode, to do some impedance matching. If the tube's native impedance
is greater than 50 ohms, you would tap the input down. If lower than 50
ohms, tap the cathode down. Basic transformer action, although with a
resonant circuit instead of a simple inductor.

Not to complicate the issue, but you can also "tap" a capacitor just
like you can "tap" an inductor if the circuit Q is high enough. If you
replace the single capacitor with two in series, move the ground point
to the junction of the two caps, move the input to one end of the coil
and the output to the other end of the coil, you have just created a
pi-network input matching circuit. Confuset? Draw it out on paper and
see. 

Does that answer the question?

73, Bill W6WRT