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What makes the triode a good choice for a single device amplifier is that it will simultaneously exhibit a consistent gain, a fairly low output impedance, and a fairly high PSRR. This is so because, unlike the MOSFET, transistor, pentode, FET, the triode benefits from having a built-in feedback mechanism in the form of plate resistance: a change in plate voltage will result in a change in current drawn through the triode. Whereas, the alternative devices might realize vastly greater gain, the precise amount of gain would vary from sample to sample to a much greater degree than would the triode. Furthermore, the output impedance for these devices is usually so high that it is in effect equal to he impedance of the load resistor as they are close to being constant current devices, which results in them having an extremely high output impedance. In addition, any other device can only yield a much poorer PSRR than the Grounded Cathode amplifier, once again because of the plate resistance, which will serve to define a bottom resistor of a voltage divider whose top resistor is the plate resistor.
Cascode Amplifier
The Cascode amplifier works by realizing all of the bottom triode's transconductance by isolating it from the plate resistor and load resistance. This allows the grid to fully control the current through the triode and thus define a gain across the plate resistor. This increased gain, which can easily exceed the amplification factor of the triode, comes at a price; increased distortion and virtually no power supply rejection (PSRR). This circuit enjoyed great popularity during the 1980's. It gave the tube audio designer lots of gain and something new for the marketing people to distort in an ad. Today, the circuit is not as popular, as it often makes for a more transistor-like sound. Still, it remains a good choice when low input capacitance or high gain is required.
Cathode Follower
If the Grounded Cathode amplifier is the quarter horse of tube circuits, then the Cathode Follower is the work horse of tube circuits. It is used to match a signal from a high impedance source to a low impedance load. The gain is always less than unity and the output impedance is roughly the reciprocal of the transconductance and the PSRR is roughly equal to the inverse of the mu. Here the triode does not enjoy any marked advantage over the MOSFET, pentode, or FET, as each of these alternative devices usually have a much greater transconductance. The transistor, on the other hand, suffers from the disadvantage that it is a current driven device; whereas the triode is voltage driven, which results in a high input impedance.
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