TRANSISTOR PROTECTION CIRCUIT

摘要

An output transistor (T1) is to be protected from damage due to excessive currents. This transistor supplies a current (Io) to a load (3) and is driven by a driver transistor (T2). The base-emitter junction of a current-sensing trnsistor (T3) is connected in parallel with that of the output transistor (T1). The collector current of transistor (T3) is a measure of the current (Io) of the output transistor (T1). A current generator (5) supplying a reference current (Iref) is coupled to the collector of the current-sensing transistor (T3). If the collector current of the current-sensing transistor exceeds the reference current (Iref) the protection circuit is activated so that the driver transistor is bottomed via an emitter-follower (T4). The base current of transistor T1 is thereby limited. Thus, the output current of the output transistor is limited to a maximum value which is substantially equal to the product of the reference current (Iref) and the ratio between the emitter areas of the output transistor (T1) and the current-sensing transistor (T3).