GB896547A - Constant power dissipating transistor circuit - Google Patents

Abstract

896,547. Transistor comparator circuits. GISHOLT MACHINE CO. Oct. 26, 1960 [June 30, 1960], No. 36718/60. Class 40(6). [Also in Group XXXVI] The current through and voltage across a solid state amplifying device is maintained constant in the stand-by condition so that its temperature rise is constant. A voltage comparator comprising transistors 1 and 2 has a voltage to be measured applied from a source 5 to the base of transistor 1 so that the transistor is normally conducting and a reference voltage switchable to the base of a normally non-conducting transistor 6 from a sweep voltage generator 8. The emitters of the two transistors are supplied from a transistor 12 2 arranged as a constant current source so that in operation when switch 9 is operated an increasing voltage is applied to the base of transistor 2 and when this voltage exceeds that from 5 the current is diverted from transistor 1 to transistor 2 to produce an output at lead 16. In order to prevent variations of the power dissipated in transistor 1 in the stand-by condition and the resulting temperature difference between the transistors the collector to emitter voltage is maintained constant as well as the collector current. This is effected by an emitter follower transistor 29 which varies the potential at the lower electrode of a zener diode 27 with changes in the emitter potential of transistor 1 and thus with changes in the base input voltage while the upper electrode of the zener diode is connected to the base of a further emitter follower transistor 19 connected to the collector of transistor 1 so that the emitter collector voltage is maintained at a value substantially equal to the constant voltage of the zener diode.