Solid-state
Global Feedback Buffers
     The MOSFET is certainly easier to drive than the transistor and potentially it offers more rugged service, with negative temperature idle current changes. Still, transistors have their advantages: they are generally much cheaper and they offer lower distortion and output impedances. If transistor are chosen, then a third stage will probably be necessary to meet the required current gain, as each transistor has a finite amount of current gain, referred to as its h_FE (beta). For example, if an output transistor has a h_FE of 100 and this device draws 3 amps of current, then its base will draw 30 mA of current. Then this 30 mA might be handled by a driver transistor also with a h_FE 100, which would need to draw .3 mA at its base. In turn, this .3 mA could be handled by an input transistor with the same h_FE of 100, which would reduce the needed input current to a mere .003 mA, a total DC current gain of 1,000,000.
      (For those that use the tube exclusively, the wide tolerance variation revealed in solid-state spec sheets will prove disconcerting. I have received emails from worried readers who are tormented by a tube being 5% off the published curves. Transistors usually have +/- 300% range of actual h_FE.)

     Still, this high power buffer deserves to be built and tested. Additionally, creating the high-voltage bipolar power supply is not that difficult as it can be derived from the existing low-voltage power supply. Adding four extra power supply capacitors and two more diode bridges allows the creation of new rail voltages equal to roughly three times that of the low-voltage rails. In this example, the +/-30 volts rails can also generate +/- 90-volt rails.

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