GB673258A - Improvements in signal amplitude control systems employing semi-conductor devices - Google Patents

Abstract

673,258. Semi-conductor amplifiers. RADIO CORPORATION OF AMERICA. April 28, 1950 [May 28, 1949], No. 10525/50. Class 40 (iv). A variable impedance device comprises a transistor having impedance elements connected in circuit with the respective base, emitter and collector electrodes, an input signal being connected across at least a portion of one of the impedance elements, and means being provided to vary the current flowing through at least one of the electrodes to control the amplitude of the output signal from one of the electrodes. Fig. 1 shows a circuit arrangement comprising a transistor having a semi-conductor body 10 of N-type germanium, and emitter and collector electrodes 11, 12 biased positively and negatively respectively with respect to base electrode 13. Base electrode 13 is grounded through resistances 14 and 15, the input signal being applied across resistance 15. The output signal may be taken from terminals 21 between base electrode and ground, or from terminals 28 between collector and ground. It is shown that if the collector current is varied by altering the collector bias by means of tap 25, or by varying resistance 26 in the collector circuit, the impedance between the base electrode and ground is varied over a range of positive and negative values, thereby giving control of the amplification or attenuation of the signal. Alternative arrangements are described in which the input is applied across part of the impedance in the collector and emitter circuits, and the output may be taken from the collector, base or emitter electrodes. The impedance between any of these electrodes and ground may be made positive, infinite, negative or zero, and control of this impedance may be affected by varying the collector or emitter bias, or the value of the impedances in the collector, emitter or base electrode circuits. The control facilities may be applied separately or simultaneously, whereby the amplitude of the output signal is a function of four variables. The device may be used in servo systems or as part of an analogue computer. Condenser 20 in Fig. 1 is provided to assist stability. The semi-conductor body may also consist of boron, silicon, tellurium or selenium. A constant-current source may be used in place of the emitter bias sourse to maintain the emitter current constant.