GB1490315A - Active breakdown circuits for increasing the operating range of circuit elements - Google Patents

Abstract

1490315 Transistor amplifiers SIGNETICS CORP 17 Oct 1975 42767/75 Heading H3T To allow extension of the operating range of supply voltage V to circuit elements, for example comprising generalized circuit 116, a transistor 103 is connected in the supply feed, with a bias 108 which keeps the base-emitter junction forward biased and the base-collector junction reverse biased, and with a rectifier such as diode 111 to prevent reverse base current, so that under elevated supply voltage V breakdown of transistor 103 occurs under the openbase condition, the collector-emitter voltage drop thus reducing the effective supply voltage applied to the elements of the circuit 116. The circuit 116 may comprise a transistor with a diode string from a bias line to bias the bases of both the protected and protecting transistors (Figs. 3A-3C, not shown). Alternatively a complementary series pair of transistors with common bases and common emitters, forming a single-ended push-pull amplifier, may be driven from the supply line via a protecting transistor of the same type as the member of the pair whose collector it feeds; in this case the bias is taken from the supply line via a constant current source of high-valued resistor, leading to a diode string (Fig. 4B, not shown). In a third embodiment (Fig. 5C, not shown) an amplifying transistor carries a protecting transistor fed from a supply line via a constant current source, the protecting transistor also acting to increase the output impedance in a cascode circuit. In the absence of diode 111 excess supply voltage would with the onset of avalanche breakdown cause the a of the protecting transistor 106 to exceed unity, the # to become negative with reverse base current, any resistance in the base bias line then tending to increase the collector current, leading to runaway.