[Radio Wrangler]

So imagine you're the bit of circuitry looking into the grid of a triode. You have some value of source resistance, being the preceding stage.

You see the grid-cathode capacitance as a simple capacitor to ground (assuming the cathode resistor is fully decoupled) Easy.

You see the grid-anode capacitance, but this is not so easy. The anode isn't decoupled to ground. Worse still, the anode voltage shoots off in the opposite direction to the way you are driving the grid. Even worse, it shoots off gain times as far as you drove the grid.

Say a valve is set up to give a gain of 10 just as a convenient number.

You drive the grid 1v more positive with your signal. The anode voltage falls 10v (gain of ten inverting, remember) so the coltage on the grid-anode capacitance changes by 11 volts So the current and charge it took to get there is 11 times as much as if the same capacitance had gone to ground.

This is the current that a capacitor to ground would have taken if it had been eleven times the value of Cag. So the anode-grid capacitor is looking to you eleven times bigger than the valve data sheet quotes. This is one of the consequences of the Miller Effect.

So if I look at the total capacitance the valve presents at the grid, a large part of it is gain dependent. If I move the grid bias voltage around, I vary Gm and so I vary the gain.... and the gain varies the total capacitance seen at the grid

So I can use it as a voltage controlled capacitor... useful for applying AFC to an oscillator.

The magnified capacitance can be seen to a much smaller effect at the anode, but tuning this way would be much weaker. OK if that's what you want.

By applying capacitors and resistors to amplifiers you can simulate inductors. Even wilder, you can simulate NEGATIVE resistors. THis latter is one of the routes into understanding the modes some parasitic oscillations run in. But these areas do need maths to simplify them.

And a cascode amplifier works because the anode of the bottom valve is almost an RF ground, with very little voltage gain. Miller doesn't happen. And the top valve has its grid at RF ground, so no full Miller here.

David