Patent GB-1246571-A (GB)
Title: Improvements in or relating to differential amplifiers
IPC: H03F3/347
Filed: 1969-01-09
Abstract: 1,246,571. Transistor amplifier circuits. INTERNATIONAL BUSINESS MACHINES CORP. 9 Jan., 1969 [12 Jan., 1968], No. 1355/69. Heading H3T. The transistors T3a, T3b, of one stage of a differential amplifier have a current source, comprising for example transistors T1, T2 and T4a, which equalizes their base-emitter voltages and reduces their collector currents below what they would otherwise be, the amplifier also including temperature compensation R10, R12, R77-R80, zero offset facility R80, R81, and common mode balance R15-R20. The transistors T1, T2 have their emitters coupled through a potentiometer R81 whose wiper receives a positive voltage. Their collectors are connected to the wipers of respective potential divider networks R7, R13 between emitter and base of the input stage transistors T3a, T3b. The collectors of T3a and T3b drive the bases of second stage transistors T5a, T5b which are emitter-coupled to a common current drain comprising a resistor R43, a transistor T10 and a further resistor R35. This transistor T10 has its base biased from the same point on a voltage divider D1, D2, R6 as the bases of both the transistors T1 and T2; and its collector is connected to the base of T4a which is emitter-coupled to a further transistor T4b by a common resistor R34. The transistor T4a being one of the current source transistors for T3a and T3b, common mode rejection is obtained as follows. If the collector voltage of T3a and T3b rise due to a common mode input component, then the currents in the second stage transistors T5a and T5b will rise; hence their common emitter potential will rise, and the current in transistor T10 will rise; this raises the potential of T4a base and its current rises; as this current is drawn from T3a and T3b their collector currents tend to rise and to offset the initial voltage rise of their collectors; feedback is also effected by the reduced conduction of T1 and T2 due to current in their emitter circuits being by-passed through a resistor R83 connected also to T4a collector, their reduced collector currents (flowing in portions of the potential dividers in parallel with T3a and T3b emitter-base junctions) causing T3a and T3b to supply additional current. Corrective balancing of erroneous differential signals arising from common mode signals is made not only by switching an appropriate one of the resistors R15-R20 between Earth and the emitter of T3b; but also by a connection (Fig. 5, not shown) between the collector of T4a and the wiper of a potentiometer (R162) connected between the ends of collector resistors R38, R40 remote from the collectors (of T3a and T3b), another potentiometer similarly connected receiving the positive supply at its wiper. In addition to thermistors R10, R12 in the base circuits of T3a and T3b, a thermistor T79 is also provided in the emitter circuits of T1 and T2. The remainder of the differential amplifier (Fig. 2b, not shown) after the second stage (T5a, T5b) includes an emitter coupled pair (T7, T8) with a constant current source (T6); a single-ended stage (T9, T12) with a current control transistor (T11) biased from the same point as the base of T4b; and an emitter-follower transistor (T13) driving a class AB stage using complementary pairs (T14, T15 and T16, T17). Feedback from the output is taken at 40 to the emitter of input transistor T3a through a resistor R31 and parallel capacitor C8; this resistor R31 is matched with the resistor R30 connecting the emitter of T3b to the switched resistors (R15-R20) of the common mode balance correction circuit.