[Radio Wrangler]

Say your triode doesn't make enough miller capacitance for your application.

Easy!

You can just add an off the shelf capacitor component from grid to anode, and the valve will multiply its capacitance by its gain. So you can make quite large variable capacitances this way.

Now a pentode has multiple grids between the control grid and the anode. These shield the grid from capacitance coupling from the anode. So they start with very low Cga, but what there is, gets multiplied by the gain as the Miller effect. As above, this can be enhanced by fitting a capacitor from g1 to anode, and the maguic of multiplication happens again. You don't need to use a triode, and you get the advantage (if you want it) of the pentode's higher gain.

Anyway, once you see the business of the capacitor not being to ground, but to an inverted and gained-up version of the signal, you're home and dry and you can figure out all the other variants that crop up.

On the principle of know thine enemy, this also tools you up on the methods of dodging the Miller effect to get better gain/bandwidth.

The miller effect goes on in transistors as well. Pretty much the same mechanism, but with a very nasty twist

The collector/base capacitance is a reverse biased diode junction, and as the voltage across it varies, so does the width of the depletion region, whch acts as the dielectric of the capacitance.... so Ccb is non-linear. Yikes! AND it gets multiplied up by the Miller effect. So non-linear Ccb gives a non linear effect to the Miller effect. Intermodulation distortion, here we come!

If doing very low distortion design with semiconductors, you really need to know the countermeasures for this. There are some and they work well. Good transistor designs can be done, but there are traps for the unwary.

David

David