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Since we looked into an SRPP MOSFET amplifier, we should pay the same favor to the White cathode follower. The TCJ optimal version of the White cathode follower has covered many time in these pages, so I won’t bore you with too many details. In a nutshell, we want the circuit to yield the biggest, most symmetrical current swings, so we use an optimal chosen sense resistor to drive the bottom tube. In this case, the tubes have been replaced by MOSFETs. The 0.65-ohm resistor is equal to the inverse of the MOSFET’s transconductance, so as the current flow this resistor varies, the bottom MOSFET receives the appropriate drive voltage. And as we have set this resistor to equal the inverse of the transconductance, it is effectively loaded by an equal impedance, once the feedback loop is attached. This means that the amount of power supply noise at the top MOSFET’s drain will equal half of the power supply’s noise.

Now, the two triodes in series also define a 50% voltage divider, so at their midpoint, the power supply’s noise will be halved as well. Cascading the input circuit into the output stage results in the top and bottom MOSFET seeing the same amount of power supply noise, which means that the power supply noise cancels at the output — another nice trick. The 6GM8 is used to good advantage here and the two 1M resistors are there to protect the output stage when the tube is warming up or absent.

The next amplifier displays the same fundamental topology, but instead uses a high-voltage power supply, coupling capacitor and 6DJ8s. Note the 100µF capacitor that connects the top triode’s plate to the 40-volt power supply. If this capacitor went to ground, the PSRR would be much worse. Think Aikido, not Zen. Note, both circuits require that the output stage be run in strict class-A operation, as the White cathode follower output stage can only work if the top MOSFET is always conducting; in this case, about 1A at idle.

The next circuit is a buffer/amplifier that provides no voltage gain. The tube’s job is to provide current to drive the MOSFET’s heavy input capacitance and to provide a high input impedance. The output stage can be run in a lean class-AB or a rich class-A. A DC servo loop would be a good addition and the ±90 power supply could be parasitically derived from the ±30 power supply.

What would happen if the output were shorted to ground? The tube would still follow the input signal and the output stage would undergo huge current swings. If the power supply were left floating, the swings would drive the power supply’s center tap up and down in anti-phase to the input signal. The amplifier shown below makes this topology clear. The buffer has become an amplifier with much gain and a high output impedance, making it a perfect candidate for feedback. But where do we apply the feedback loop?