1,238,950. Protecting and stabilizing transistor amplifiers. RCA CORPORATION. 26 Sept., 1969 [27 Sept., 1968], No. 46153/68. Heading H3T. An overload protection arrangement for an amplifier 11 comprises a transistor 16 on the same substrate connected across the input path, the base receiving a steady bias (e.g. from 17 and 20) such that when the temperature exceeds a given value the transistor conducts. The protecting transistor varies in temperature with the amplifier transistor and by arranging that the steady bias offsets the temperature stable component of the base-emitter threshold of the protecting transistor, protection from overload can be made dependent upon the temperature varying component so that it becomes effective at a lower output level as the temperature increases. As shown, protection from overload is effected in dependence upon current in and voltage across the amplifier as sensed by resistor 15 and potentiometers 23, 22 respectively. Fig. 4 shows a class B amplifier incorporating in each half the protection arrangement 16-23 of Fig. 2. Each amplifying transistor has its input shunted by a diode connected transistor having an area in relation to its associated amplifier to define the gain. For positive excursions of the input voltage at 56 transistor 51 cuts off and the lower half of the amplifier is driven via transistor 50. For negative excursions transistor 51 acts as a common base stage to drive the upper half. The inputs of transistors 51, 50 are shunted by a transistor-diode series 53, 52 of some 20 times the area so that the required quiescent bias may be obtained from a conveniently low valued resistor 55. This resistor also provides the necessary large current drive for high signal peaks. The circuit permits the total quiescent current to be very low.
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