Voltage-Controlled Electron Multipliers

The application of secondary-emission multiplication to conventional grid-controlled amplifier tubes is discussed from the viewpoints of practical voltage gain per stage of amplification, signal-to-noise ratio, and ultra-high-frequency applications. It is pointed out that the gain per stage is limited by the practical output current and the quotient of transconductance by current (N) and that electron multiplication increases the gain only as it permits the attaining of higher values of N. If the output current is assumed to be 20 milliamperes and N is taken as 1 milliampere per volt per milliampere, the output transconductance would be 20 milliamperes per volt-little, if any, better than could be achieved without multiplication. If N is assumed to be 11.6 (the theoretical maximum for conventional grid control with a cathode temperature of 1000 degrees Kelvin) the output transconductance could be greater than 200 milliamperes per volt per milliampere. Higher values of N might be attained by some other method of control. In this case, the ultimate limit of transconductance would be set by the difficulty in stabilizing the effective control-electrode bias voltage. The signal-to-noise ratio of the voltage-controlled multiplier is determined chiefly by the input system of the multiplier, the multiplier being a relatively noiseless amplifier following this input system. The noise level of the input system is determined by the input transconductance. If the use of a multiplier leads to reduced input transconductance, the noise level will be increased as compared with conventional tubes.