Vacuum tubes, on the other hand, only exist as positive types, i.e. they only increase conduction with a positive increase of grid voltage. Having the option of polarity is the single greatest advantage to solid-state devices, as it allows for symmetrical circuit design and for topologies simply not realizable with vacuum tubes.
  Mixing the two technologies helps level the playing field. The following circuit is an inverted cascode. The MOSFET's source must be at some positive voltage relative to its gate in order for it to conduct any current. This voltage varies from one MOSFET to another, but it is usually between +2 to +5 volts. By carefully choosing the right device we can greatly simply the biasing of the triode by using this voltage to set the cathode bias of the tube.

Inverted Cascode first stage

  Let's resume the thread about how a very poor PSRR figure can be an asset in the right circuit. In the above circuit we see the second triode configured in a Grounded Cathode amplifier. The PSRR figure for this amplifier is determined by the voltage divider defined by the value of the plate resistor and the rp of triode, which the unbypassed cathode resistor increases.

pg. 5

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