LEAKAGE CURRENT ELIMINATION

1,094,542. Transistor switching circuits. UNITED AIRCRAFT CORPORATION. May 17, 1965 [May 27, 1964], No. 20779/65. Heading H3T. [Also in Divisions G3 and H2] To stabilize the current supplied by a transistor switching circuit to a load three measures are adopted: (a) the current is passed through a measuring resistor to generate a voltage which controls the current supply to the switching transistors, (b) leakage current which flows through the switching transistors that are nominally turned off is prevented from passing through the load or the measuring resistor 1, and (c) the base currents of the transistors that are turned on are stabilized so that their saturation currents will be constant. For this purpose there are employed silicon planar diodes having low reverse currents, these being biased off, when required, by voltages equal to ten times the uncertainty in the bias voltages due to temperature changes and drift in the transistor characteristics. In Fig. 1 the switching transistors 19-22 are supplied by a power supply 2 through current-regulators 11, 18 stabilized by Zener diodes 10, 15 and the current through the load 34 flows to earth through measuring resistor 35. The voltage across this resistor is compared with the voltage across Zener diode 17 in a differential D.C. amplifier 36 and the difference used to control the regulating transistor 18. The input square-wave signal at 62 is differentiated at 63, 64 and controls an emitter-coupled on bi-stable circuit 68, 69 connected to the switching transistors 19, 20. The load current also flows through transistors 21, 22 which are controlled by a further bistable circuit 79, 80 which is operated by the differentiated signal through a balanced-capacitance pulse transformer 59. The transistors 79, 80 are powered through a transformer 5 which provides a D.C. supply independent of the supply to the rest of the circuit except for its centre tap being joined to the line 87. Operation.-When the square-wave at 62 goes positive, the differentiated pulse turns on transistors 69, 79 and turns off 68, 80. The collector currents of these transistors supply base currents through the switching transistors so that 19, 22 are turned on while 20, 21 are turned off. The switching current then flows through 19, diode 23, load 34, transistor 22, diode 29 and measuring resistor 35. Current also flows through resistor 14, diode 40 and resistor 48, clamping the collector of 20 at 9 volts, thus reverse biasing the silicon pl