Patent GB-1467057-A (GB)
Title: Amplifier with over-current protection
IPC: H03G11/00
Filed: 1974-05-14
Abstract: 1467057 Transistor amplifying circuits RCA CORPORATION 14 May 1974 [24 May 1973] 21234/74 Heading H3T An amplifier with protection against overdrive comprises a transistor of first conductivity type with its base coupled to the input terminal and collector connected to the base of a second transistor of second conductivity type, which has its emitter connected to the output terminal and its collector connected to the emitter of the first transistor. A current sensing element couples the collector of the second transistor and/or the emitter of the first to a common terminal and operates means for limiting the signal excursions at the input terminal. The embodiments described comprise integrated circuits in which the NPN transistors are vertical structures and the PNP of the lateral type. Input terminal T1, Fig. 1, is coupled to an input amplifier 10 comprising an emitter coupled pair, which feeds a Darlington pair common collector stage 11, 12 followed by a common emitter stage 13. The amplified signal is applied to a peak-to-peak limiting amplifier 15, the output of which is fed to two phasesplitters 20, 21 which drive composite output transistors 41, 42 and 43, 44 respectively, which are in bridge formation. Each composite output transistor, e.g. 41, comprises a PNP input transistor 411 driving a number of parallel-connected NPN output transistors 412-415. The antiphase output voltages at terminals T2, T3 are combined in a differential amplifier 50 to provide a local negative feedback signal to the inputs of phase-splitters 20, 25 and also in a second differential amplifier 60 to provide an overall negative feedback signal applied to input amplifier 10. A series RC network is provided external to the integrated circuit, between terminals T8, T9 to stabilize the feedback loop. Output transistor 412 is provided with collector circuit resistor 82 so that the base of transistor 411 is clamped by diode 83 to the temperature-dependent potential across resistor 81 whenever the emitter current of composite transistor 41 approaches an excessive value. Composite output transistor 43 is similarly protected by diode 85. Transistors may replace diodes 83, 85, their collectors being connected to the positive supply, or alternatively to the corresponding bases. Peak to peak limiting amplifier 15.-The limiting level is set by potential V(T) at the emitter of transistor 700. Resistor 152 being of small value, the collector voltage of transistor 151 is substantially the base-emitter voltage V BE  and the drop across resistors 153, 154 is (V(T)-V BE ) : the resistors are so chosen that the current through them with transistor non-conductive is twice the desired collector current of transistor 13, which operates in class A. The quiescent potential on line 160 is V BE , due to diode-connected transistors 201, 251 so the quiescent voltage across diode 157 and resistor 156 in series is also (V(T)-V BE ) and the resistance of the series combination is so chosen that the current through it is equal to the quiescent current in transistor 13. When transistor 13 is driven by signal so positive as to pass twice its quiescent current, current is diverted from transistor 151: the base potential of transistors 151, 155 then falls and they are cut-off: the total current flow through resistor 156, equal to the quiescent current in transistor 13, consequently flows via line 160 into the input of phase-splitters 20, 25. When the signal is such as to bias transistor 13 non-conductive the collector current of transistor 151 is twice the quiescent current of transistor 13 and due to the common base bias transistor 155 passes a similar current: half of this comes via resistor 156 and the other, equal to the quiescent current in transistor 13, from the bases of phase-splitters 20, 25 via line 60. Thus the maximum and minimum flow of current to phase splitters 20, 25 is determined. Regulation of limiting level.-The limiting level control voltage V(T) at the emitter of transistor 700 is controlled by regulator circuit 70 which also provides across avalanche diode 70 a regulated potential for the input amplifier 10 and for a bias circuit 30 for the phase-splitters 20, 25. Transistors 702, 703 are in diodeconnected Darlington configuration and the collector to emitter voltage of transistor 702 is 2V EB : consequently the voltage across resistor 706 is V EB , so determining the current from the emitter of transistor 705 biasing avalanche diode 701. The base bias of transistor 700 is derived over resistor 707 from avalanche diode 701; the voltage across resistor 707 is negligible until the temperature increases and a threshold is reached, whereupon transistor 710, which is fixed biased from avalanche diode 701 by means of resistors 711, 712, conducts and the base potential of transistor 700 falls, lowering the limiting level. The overall feedback loop then acts to reduce the quiescent collector current of transistor 13 and so its transconductance; in addition the limiting level of limiter circuit 15 is reduced. When V(T) falls below 2V BE , diode 157 becomes non-conductive, preventing current being withdrawn from the bases of transistors 202, 252 to prevent unbalancing of the bridge amplifier 40 by transistors 13, 151, 171 when the emitter of transistor 700 no longer presents a low impedance. This is necessary to prevent feed-through of collector current variations of amplifier 13 to the phase-splitters 20, 25 when limiter circuit 15 is supposed to reduce the output signal currents to zero. Common-emitter connected transistor 171 supplies its collector current to the emitter of transistor 700, which combines with currents drawn through resistors 153, 156 to cancel signal current variations in the emitter current of transistor 700 at all levels of V(T) and of signal from transistor 13. The potential across resistor 706 is proportional to the current in transistor 700 and hence to V(T), so that as that is reduced with increase of temperature the bias on diodes 81, 84 reduces, reducing the limiting level of diodes 83, 84. If the supply voltage across terminals T5, T6 falls below 7À5 volts V(T) becomes zero and the amplifier is cut-off: current through resistor 721 biases transistor 722 conductive to earth the base of transistor 700. For higher voltages avalanche diode 724 is conductive to bias transistor 725 conductive, so that transistor 722 is cut-off. Terminal T4 permits the base of transistor 725 to be clamped to earth or lower potential, e.g. for muting. Alternative non-inverting limiting amplifier.- Transistor 951, Fig. 2, is provided with negative feedback, over emitter-follower 952 and resistor 953, so that its collector to emitter potential is substantially 2V EB , the drop across resistor 953 being negligible: thus the potential across resistor 954 is (V(T)-2V BE ) and determines the current in it. The resistor has such a value that the current is equal to the quiescent current in amplifier 13. Transistor 955 and resistor 956 are identical with transistor 951 and resistor 953, respectively, so the collector current in transistors 951, 955 are equal. The collector potential of transistor 955 is clamped at V BE  by diode-connected transistors 201, 251 in the phase-splitters 20, 25: the anode of diode 959 thus is at 2V BE  and the drop across resistors 957, 958 is (V(T)-2V BE ), i.e. equal to that across resistor 954. Resistors 957, 958 are so chosen that the quiescent current through them is twice that through resistor 954, half flowing through transistor 955 and the remainder through transistor 13. When transistor 13 is biased non-conductive by signal the latter current is supplied to phase-splitters 20, 25 instead, being a maximum value. When transistor 13 is biased by signal to pass twice the quiescent current, the voltage across resistor 957 is such as to bias diode 959 non-conductive the maximum possible current is then withdrawn by transistor 955 from phase-splitters 20, 25, being equal to the current through transistor 951. The method of biasing the output stages and the feedback circuits are described in Specications No. 1,467,059 and Co-pending Application 21237/74