[Radio Wrangler]

You seem to have got the two circuits slightly crossed, Hugo.

In the normal cascode, the lower triode is a common-cathode amplifier, but its anode load is the input impedance of the common grid stage above it. This impedance is roughly the same as the lower triode's cathode impedance, so the gain from grid to anode for the lower triode is -1 roughly. The lower triode is giving no significant voltage gain. It still gives plenty of current gain and power gain. The upper valve of the cascode is a plain grounded grid triode. It's anode is provided with a load resistor and the modulated DC current in the totem pole (controlled by the lower valve) can develop an appreciable voltage swing. The top valve exhibits appreciable voltage gain, unity current gain and therefore appreciable power gain.

The benefit comes from the small swing of the lower anode. There is much less Miller effect than there would be with a single triode trying to do the job of the cascode.

In the original post circuit, the lower triode is a common cathode stage but its anode is loaded with a constant-current bias generator in the shape of the upper triode and its cathode bias resistor. This provides quiescent current for the lower stage, but it simulates a very high impedance source, and so the lower triode produces the absolute maximum voltage gain it can manage. Miller effect is rampant. As a neat trick the drive to the next stage comes off the top triode cathode and the top triode does double duty as both the constant current bias source and as a cathode follower buffering the output. In this circuit the bottom triode has large voltage gain the upper stage has unity voltage gain. Quite the reverse of the cascode. Far from minimising Miller, this circuit has it on steroids. Also, getting the DC bias running voltage of the lower triode anode is very tricky. Tricky enough to scare designers away.

David