Quote:
Originally Posted by G8HQP Dave
Quote:
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Originally Posted by Argus25
However, there is no voltage gain from the upper valve, as its cathode and grid are at fixed potentials.
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In a cascode almost all the voltage gain comes from the upper valve. It cathode is not fixed at all, as it is connected to the lower anode. |
If one regards the upper valve as a voltage follower, its cathode attempting to follow the grid voltage and the grid is at signal ground, if it were perfect in that application the plate voltage of the lower valve would not move at all. But since it has a source resistance, looking into the cathode of the upper device (valve), of around a few hundred Ohms typical, I agree it will move. In transistor cascode though, the collector voltage of the lower device is pretty well rock solid voltage wise, so I may have been thinking more of that than the valve case.
Also, because the two devices are in series across the power supply, any dynamic changes in the plate or cathode current, in either device should be the same and solely due to the gm of the lower valve or device. I cannot see how the upper device, valve or transistor, is able to increase the dynamic changes of the lower device's plate (or collector current) beyond that provided by the lower device's gm. Therefore, in terms of contributing to the circuit's overall gain, the upper device can only contribute unity voltage gain in my view. I guess to sort that out, the gm of the upper device (or say the transistors hfe) could be doubled in a test circuit to see if the overall voltage gain changed. Have you tried that in a circuit simulator ?