Patent GB-1094542-A (GB)
Title: Leakage current elimination in switching circuits
IPC: H03K17/60
Filed: 1965-05-17
Abstract: 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 planar diode 24 by 1 volt. Similarly, current flows through diode 42 and resistor 46 to forward bias diode 27 by 1 volt (switch 33 being in position shown). Thus the leakage collector currents of 20 and 21 flow to earth through resistor 48 and through diode 27 and resistor 46 respectively, diodes 28, 30, 39 and 41 being similarly reverse biased. When the square-wave at 62 goes negative, the transistors and diodes change into their opposite conduction states. Since the power supply to transistors 79, 80 is floating, the base drive currents of transistors 21, 22 pass back along line 87 and do not flow through the measuring resistor 35. The collector resistors of transistors 68, 69, 79, 80 are unequal and are so chosen in relation to Zener diodes 74, 85 as to give the correct base drive currents to the switching transistors. Modifccations.-(1) Diodes 27, 30, 41, 42 and resistors 43, 44, 45, 46 can be eliminated if the diodes 31, 32 and resistors 37, 38 are included, the arrows leading to the points where the anodes of diodes 27 and 30 were connected. The bi-stable circuit 79, 80 will now need to supply more current, and the measuring resistor should be increased in value; the circuit is also more susceptible to unequal base/emitter potentials across transistors 21, 22 and unequal forward drops across diodes 28, 29. The former effect can be avoided by changing switch 33 to the other position. (2) Transistors 19, 20 may have a floating base current supply as shown for transistors 21, 22. This eases the frequency-response requirements of the differential amplifier 36, but does not provide for bleeding off the leakage collector currents of transistors 21, 22. (3) By using silicon planar transistors for 21, 22 (Fig. 2, not shown), the bi-stable circuit 79, 80 and diodes 28, 29 can be eliminated, the transformer 58 supplying the bases of transistors 21, 22 in push-pull. If the mark-space ratio of the input square wave is not unity, a D.C. restoration circuit is included. This arrangement back-biases the emitter-base junctions of transistors 21, 22 instead of merely open-circuiting them and the clamping conditions are therefore reached more quickly. (4) The D.C. restoration circuit of (3) can also be used for transistors 19, 20 by supplying them through a similar isolating transformer.