Matra Bae Dynamics (uk) Limited

British Aerospace Public Limited Company

Geoffrey Smith

Jiapal S Brar

A switching circuit (3) having a low component count and suitable for use with a nuclear event detector circuit (2) provides a means for isolating a power source (1) from a load (2, 4). A bipolar transistor (5) provides a temperature insensitive, low voltage drop supply to the load (2, 4) and a MOSFET (7) provides base current to the bipolar transistor during standby mode; the MOSFET being turned on by the supply (1). When the supply voltage drops below the threshold voltage of the MOSFET (7), the base bias to the bipolar transistor (5) is removed. The bipolar transistor (5) cannot therefore conduct in the reverse sense and the circuit provides true isolation for the power supply (1) and the load (2, 4).

Power switching circuits

A pulse timer circuit comprising a monostable multivibrator which includes two complementary transistors Q5, Q6, has the advantage of good tolerance to temperature fluctuation owing to the provision of a Schottky diode D4 connected between the emitter and base of one of the transistors Q5. The Schottky diode, D4, which has a temperature coefficient matched to that of the transistor Q5, ensures that the output pulse width from the monostable is unaffected by fluctuations in ambient temperature.

Pulse timer circuit

A threshold detector circuit (2) suitable for use with a nuclear event detector for sensing an ionising radiation pulse has a high tolerance to ambient temperature fluctuations. The threshold detector (2) comprises an amplifier stage (3) which includes a pair of complementary transistors (Q2, Q3) and a comparator stage (6) which includes a third transistor (Q4) and a voltage reference (7). Temperature compensation is achieved by the addition of a feedback loop (5) in the amplifier stage (3) including a fourth transistor (Q1) whose temperature coefficient is closely matched to that of the third transistor (Q4).

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Geoffrey Smith