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But doesn't this current feedback circuit look like some preexisting tube circuits? Indeed it does, as a single triode makes a very poor Op-Amp. The key difference is that this circuit was designed with current feedback operation in mind, which lent a focus and direction that is usually missing in tube designs.
The circuit below illustrates the transistor equivalent topology. It too is single-ended throughout. (The push-pull version was actually given in a previous schematic.)
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But adding both the capacitor and the necessary biasing resistors would tend obscure the operating principles in an already complicated circuit. (In fact, the MOSFETs could be replaced by many EL509s, if sufficient drive voltage is available and the input tube's cathode is raised by 50 volts.)
Tony, I hope this proves useful. For the rest of our readers, please send in your views on including pure solid-state circuitry.
//JRB
More Information
To learn more about current-feedback amplifiers, search out issues 30-3 and 30-4 of Analog Dialogue, wherein writer Erik Barnes does a beautiful job of explicating current feedback. (Tony, this means you.)
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Hybrid Power Buffer
For those few tube fans who haven't in disgust stop reading after the first mention of transistors, here is your reward: a tube front-end hybrid power buffer that can be run in Class-AB or Class-A. As shown at the right, the triode's own cathode bias voltage sets the bias for the top MOSFET. The potentiometer sets the DC offset. Only N-channel MOSFET are needed, which is a blessing in that are no true matched sets of P-channel and N-channel MOSFETs available, whereas matching two N-channel devices is easy. Ultimately, the DC coupling of the cathode to gate is probably not a good idea, as a coupling capacitor would provide a safer operation.
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