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06-10-2021, 06:54 PM
Hi Murray
Welcome to the forum
Yes, cascodes really need a buffer on their output to yield their advantages.
When it comes to mixing semiconductors with tubes, there is a lot of variation and in most cases - especially where the tube count is low - it is almost always for advertising.New York Audio Lab had a little hifi preamp that had a jfet input cascoded with a triode driving a mosfet buffer. A dual-triode made stereo but the unit was advertised as a "tube preamp". In this example, the jfet had lower-noise than the tube and the mosfet had a much lower output impedance than a cathode-follower would have. The whole thing was powered from a wall-wart.
A cascode is specifically where two active gain elements are series-connected via their primary current paths; the lower element accepts input signal on its control pin; the upper element has its control pin tied to a fixed voltage; the output is taken from the upper element's open primary current path. This is a general description. The term "cascode" originates in tube days where the plate of the first tube drives the cathode of the second tube, where the second tube's grid is tied to a voltage reference. As soon as you superimpose a signal on the second element's control pin, the circuit is no longer a cascode; rather, it becomes a push-pull output stage, not in the sense of being able to drive a speaker but certainly with enhanced drive compared to a plate output.
The voltage gain of a tube is the same using the grid as input or the cathode as input. Nominally the voltage gains "multiply" according to old texts but real world shows more of an addition or a "multiplication with significant error" - hehe. Tubes never do things as we expect, which is why we like them.
Welcome to the forum
Yes, cascodes really need a buffer on their output to yield their advantages.
When it comes to mixing semiconductors with tubes, there is a lot of variation and in most cases - especially where the tube count is low - it is almost always for advertising.New York Audio Lab had a little hifi preamp that had a jfet input cascoded with a triode driving a mosfet buffer. A dual-triode made stereo but the unit was advertised as a "tube preamp". In this example, the jfet had lower-noise than the tube and the mosfet had a much lower output impedance than a cathode-follower would have. The whole thing was powered from a wall-wart.
A cascode is specifically where two active gain elements are series-connected via their primary current paths; the lower element accepts input signal on its control pin; the upper element has its control pin tied to a fixed voltage; the output is taken from the upper element's open primary current path. This is a general description. The term "cascode" originates in tube days where the plate of the first tube drives the cathode of the second tube, where the second tube's grid is tied to a voltage reference. As soon as you superimpose a signal on the second element's control pin, the circuit is no longer a cascode; rather, it becomes a push-pull output stage, not in the sense of being able to drive a speaker but certainly with enhanced drive compared to a plate output.
The voltage gain of a tube is the same using the grid as input or the cathode as input. Nominally the voltage gains "multiply" according to old texts but real world shows more of an addition or a "multiplication with significant error" - hehe. Tubes never do things as we expect, which is why we like them.
