circuit shown in fig5 npn transistors 381 382 38m and 391 392 39m of current mirror circuits 511 512 51m can be constructed by the vertical type transistors which occupy very small areas on integrated circuit chips fig7 shows a circuit diagram for a further improvement of the thermal shutoff circuit shown in fig3 the circuit of fig7 gives the supply of the bias current a thermal hysteresis characteristic in the circuit of fig7 constant current source 33 is comprised of npn transistor 40 two resistors 41 42 and a base bias source such as a battery 43 npn transistor 40 is connected at its collector to the base of shutoff control transistor 18 and at its emitter to reference potential terminal 12 through a series circuit of resistors 41 42 the base of npn transistor 40 is connected to the positive terminal of battery 43 of which the negative terminal is connected to reference potential terminal 12 the emitter of shutoff control transistor 18 is connected to the connection node of resistors 41 42 constant current source 33 shown in fig7 supplies a constant current i33a expressed as the following equation when shutoff control transistor 18 is deactivated equ9 where v43 is the voltage of battery 43 vbe 40 is the base to emitter voltage of transistor 40 and r41 r42 are resistances of resistors 41 42 the operation of the circuit of fig7 will be described with reference to fig8 which shows a graph of the bias current supplied to the external circuit through output terminal out vs temperature t when temperature t rises to the prescribed temperature t1 current i2 begins to flow through transistor 32 and constant current source 33 when current i2 reaches the value of current i33a as set by.