Sorry, looked up the wrong tube in the wee hours on the morning.
But, that was not my real point.
In an amplifier with a plate tank circuit with Q>10 the Miller
capacitance is not
going to change much. Miller Capacitance is where we have the effect of a
much larger
capacitance on the input of an amplifier than the real input capacitance,
This is because of the feedback capacitance and the voltage across it is
much greater than
just the input voltage. The voltage across the feedback capacitance will be
the output voltage plus
the input voltage ( in the case of a inverting amplifier). This magnifies
the effective capacitance by
the amplifier's gain plus 1.
If the output voltage continues to oscillate, even if the amplifier is
not functioning at that moment,
this AC voltage is still across the feedback capacitance and the current
continues to flow. The
class room analysis of Miller Capacitance is not only a class A amplifier
but one with a
simple resistor as load, not a tuned circuit (Flywheel).
Look at it this way, Take two synchronized signal generators. One
driving one end of the
capacitor and one driving other. If you have one adjusted to one volt and
the other zero and you had a capacitive
reactance of 1 Ohm, you would have 1 Amp of current. Now adjust the other
to 1 volt and have its phase set 180 degrees
from the other. Then across the capacitor you would have 2 volts. Now even
though signal generator #1
is 1 Volt RMS the current thru the capacitor is 2 Amps. Set #2 to 100
volts, then the current is 101 Amps.
The capacitive reactance as signal generator #1 sees it is 1/101 of the
actual capacitance.
Or the capacitance that signal generator #1 sees it is 101 times the actual
capacitance.
In an amplifier with resonant circuits on the output the sinewave
continues to even when the active device is
inactive for any portion of the cycle.
It is interesting that even though reactance seen by the "tube" is
resistive the reactive components play a big
part in these circuits. There is nothing magic about amplification that
magnifies the feedback capacitance, it is just
the mean by which another voltage is applied to the "output" side of the
capacitor and the reactive parts fill in
the gaps of the sine wave that would otherwise exist produce harmonics.
Miller effect applies not only to just capacitance but to any
feedback impedance and to inverting and
non-inverting amplifiers ( just have to change sign). The case of a
non-inverting amplifier is even more interesting.
This just occurred to me when I looked at just the final couple of
comments on the topic. I have been too
busy dealing with other stuff and getting things ready our Club's Hamfest
that I have not been following the thread.
73
Bill wa4lav
At 10:10 AM 8/12/2012 -0700, you wrote:
>ORIGINAL MESSAGE:
>On Sun, 12 Aug 2012 01:51:55 -0400, WA4LAV wrote:
>
> >. That is shunted by the
> >tube's input capacitance, 15.7pF.
>
>REPLY:
>Where did that figure come from?
>
>According to the Eimac 4CX1500B data sheet, the input capacitance in grounded
>cathode mode is between 75 and 88 pF.
>
>73, Bill W6WRT
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