Cathode followers and feedback amplifiers with high-capacitance loads

Abstract

Investigation, both theoretically and experimentally, was made into the mode of operation of cathode followers and feedback amplifiers with high-capacitance loads. The capacitance loaded cathode follower compares unfavorably with the resistance loaded cathode follower, particularly regarding operation with fluctuating input signals. Input overdrive of a capacitance loaded cathode follower causes production of transient voltages in the output circuit. Threshold of input overdrive is considerably lower than in resistance loaded cathode followers, especially at high audio frequencies. Also, harmonic distortion is increased at high frequencies and high-frequency response drops off. Experimental analysis of the voltage relations between the input and output circuits was made, and guides for design were developed. It was found that a symmetrical EB-ZP curve is the most important factor in development of a cathode follower for driving high-capacitance loads where transient distortion is to be avoided.HE cathode follower is a useful device for coupling a high impedance source to a lower impedance load. Its properties are due to feedback, which operates in a near-ideal manner if the load is a pure resistance with a relatively shunt reactance. However, where the load includes a relatively low shunt reactance, the feedback conditions are modified and the device no longer behaves as de~ired.l-~ This discussion is concerned with the problem of maintaining high cable driving capability of cathode followers, with low distortion of fluctuating input signals, over the audio frequency range.The limit of input signal voltage for low distortion under steady-state conditions is governed by plate-current cutoff and grid-current threshold points. However, sudden changes of the input signal amplitude often causes a low-frequency transient signal in the output circuit, the duration of which is determined by the RC time constant of the output circuit. This effect usually occurs at a much lower input voltage than is necessary to reach plate current cutoff or grid current threshold under steady-state conditions. Capacitance in the load magnifies these effects. Fig. 1 shows the circuit of a triode vacuum tube operated as a grounded plate amplifier, or cathode follower, with its load impedance in the cathode side of the circuit. Circuit parameters are suitable for operation in the audio frequency range. Eo is taken from the cathode through a coupling capacitor if i t is desired to block the dc from the load. The grid is biased a few volts negative relative to the cathode, and X, is usually in the order of megohms depending upon the input impedance desired.When a sinusoidal input signal is applied, negative feedback occurs in the grid-cathode circuit, so t h a t Eo is slightly lower peak-to-peak amplitude than Ei,. The difference in the vector quantities Ei, and Eo is applied between grid and cathode. With a resistance load, there is no phase shift between Ei,, and Eo, therefore E,k is the EP Fig. 1-Cathode follower schematic circuit. algebrai...