The Cathode Follower

Author: N.H. Crowhurst

The basic common-plate (cathode follower) circuit

The third way to connect a tube makes the plate common. We do this by connecting the 20000-ohm resistor in the cathode circuit. Now the change of -5 volts to -10 volts between grid and cathode results, as before, in a change in current through the tube between plate and cathode, from 10 milliamperes to 5 milliamperes. Thus the cathode will fluctuate between 100 volts and 50 volts, respectively, due to the different currents flowing in the 20000 resistor.

Adding these voltages together, we can find what input voltages we have to use to get these output voltages. With -5 volts between grid and cathode, the voltage from cathode to ground is 100 volts, so the voltage from grid to ground must be (100-5) or 95 volts. With -10 volts from grid to cathode, the cathode to ground voltage is 50 volts, hence the grid to ground voltage must be (50-10) or 40 volts. Thus, this circuit requires an input fluctuation between 95 and 40 volts (55 volts) to get an output fluctuation of only 50 volts, between 50 and 100. How can this be amplification?

The answer is that the cathode follower does not give voltage amplification. Because the grid input does not require any current to cause the output circuit to give a current fluctuation of 5 milliamperes, we can see that this arrangement can be regarded as a current amplifier. Because the output voltage at the cathode is almost the same as the input voltage at the grid (although more current fluctuation is available at this voltage), this circuit is called a cathode-follower, the idea being that the cathode voltage follows the grid voltage.