GB965421A - Improvements relating to transistor circuits - Google Patents

Abstract

965,421. Protecting transistor circuits. TELEFUNKEN A.G. Nov. 7, 1960 [Nov. 6, 1959], No. 38116/60. Heading H3T. In a circuit comprising a number of transistors 1, 2, 3 (Fig. 3) connected in parallel with their collectors and emitters respectively connected together for a direct current, individual diodes 4, 5 and 6 are connected in each base current path and poled in - a direction to pass base current so that in the event of the collector base current path of one transistor breaking down the diode becomes non-conducting, so preventing damage to the other transistor. In the amplifier circuit shown in Fig. 3 comprising an input transformer 7 and output transformer 8, the diodes are of silicon type and the biasing of the transistors being effected by a network 11, 15. In Fig. 7 the diodes 4, 5 and 6 are connected in series with normally conducting silicon diodes 16, 17 and 18 which together with resistors 19, 20 and 21 provide individual bias for each of the transistors. The resistors 19, 20 and 21 are connected effectively to the collectors through a resistor 26 to provide negative feedback though by connecting them directly to the direct current source the feedback may be omitted (Fig. 4, not shown). In order to protect the transistors against excessive emitter collector voltage due, for example, to open circuiting of the load circuit or induced voltages in the output line a further transistor 27 is connected between emitters and collectors through a resistor 26 and is biased normally to the non-conducting by diodes 28 and 29. If the desired maximum emitter collector voltage is exceeded diode 28 breaks down in its reverse direction in Zener fashion causing transistor 27 to conduct so that the base emitter paths of transistors 1, 2, 3 are effectively short circuited through resistor 26 and the transistor 27. In addition, the change of voltage at the collector of transistor 27 reduces the bias applied through resistors 19, 20 and 21 and provides further protection. Furthermore if the H.T. supply is applied in the reverse direction the transistors are protected by diodes 28 and 29. It is stated in the Specification that the protective arrangement may be applied to push pull circuits.