stipulated that Gm´ = Gm as a condition of satisfaction in the quest for the optimally valued Ra, and we found that Gm´ = (mu + 1)/(Ra + rp). In effect, what we have actually done by specifying the correct value for Ra is to balance the push-pull aspect of the circuit, which includes each triode offering the same output impedance to the load. Consequently,
    Zo = 1 / 2Gm,
or
    Zo = rp / 2mu.

Conclusion
   We find once again that we cannot get something for nothing: spectacularly low output impedance came at the price of a disappointingly low input overload voltage and a miniscule output current ability. But what  we did get, when we gave the White Cathode Follower the optimal plate resistor value to work with, was a buffer circuit twice as good as a textbook Cathode Follower: half the output impedance and a symmetrical output current swing with twice the output current swing than a single triode Cathode Follower.

      Also known as the Anode Follower, the Plate Follower is in many ways the inverse of the Cathode Follower. The Cathode Follower preserves the phase of the input signal; the Plate Follower, inverts. The Cathode Follower's output is taken at the cathode; the Plate Follower's output, at the plate. The Cathode Follower's input impedance is extremely high;  the Plate Follower's input impedance, relatively  low, as it is equal the value of resistor R1. And finally, where the Cathode Follower can only aggressively pull the output more positively; the Plate Follower, can only aggressively pull the output more negatively.
     Still, the Plate Follower makes a fine buffer and boasts some very desirable features: a ground potential input, adjustable gain, and low heater-to-cathode differentials. The need for a coupling capacitor or a connection to a high voltage input is eliminated in the Plate Follower, as the grid is, in DC terms, at the ground voltage. Unlike the Cathode Follower, whose gain always falls short of unity, the Plate Follower can achieve unity gain output, or if

pg. 8

<PREVIOUS

NEXT >