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If this seems too good to be true, that's because it is too good to be true. Yes, the gain is almost unity and the Zo is amazingly low, yet the circuit cannot deliver very much current into a low impedance load, such as a Grado headphone, 32 ohms. Imagine a car with 340 horsepower, yet which could only do 10 miles per hour. Surprisingly, if we try to output more than a few millivolts into the 32 ohm load, we will overdrive the circuit, as we will break out of Class A operation.
Here is what happens in detail. Any variation in the current flowing though the top triode will produce a variation in the voltage developed across the plate resistor. In turn, this voltage will be transmitted to the bottom triode's grid, which can only see a few positive volts before it is driven into positive grid voltage or, if the voltage swings negatively, it is completely turned off. The greater the value of the plate resistor, the easier it is to overdrive the bottom triode, as a smaller amount of current is needed to develop a large voltage change across the plate resistor.
On the other hand, if we make the plate resistor smaller in value, we gain dynamic headroom, but lose the stellar specifications. In fact, if we set the plate resistor to zero ohms, we end up with a classic Cathode Follower with an active load, i.e. the bottom triode. Of course, if the load we wish to drive is not a punishingly low 32 ohms, the headroom issue is much less of an issue. But if the load is a high impedance one, such as a 100k potentiometer, then we must ask: Why do we need to use a super low output impedance buffer?
Optimal White Cathode Follower
We found that too large a value plate resistor limits the potential output current from this buffer and that too low a value reduces the buffer's specifications. So the question is what would be the optimal value for a given load and a given desired out voltage swing?
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