An AC Cathode-Follower Circuit of Very High Input Impedance

A cathode-follower circuit of very high-input impedance using ordinary tubes is described. At the coaxial cable input, the capacitance to ground is less than 0.3μμf from below 100 to above 5×104 cps. The magnitude of the input resistance is more than 4×109 ohms from below 100 to 4700 cps; above 3200 cps, its sign is negative and it decreases with frequency. A comparison of theoretical and experimental results is given and a stable modification of the circuit discussed with which it is possible to obtain zero or negative input capacitance.