Patent GB-1467059-A (GB)
Title: Stabilized amplifier
IPC: H03F1/30
Filed: 1974-05-14
Abstract: 1467059 Transistor amplifiers RCA CORPORATION 14 May 1974 [24 May 1973] 21246/74 Heading H3T The invention relates to an amplifier of the type comprising four transistors or compound transistors arranged in bridge formation. In the embodiment the compound transistors are 41, 42, 43 and 44 in each of which the commoned collectors are the equivalent emitter, the commoned emitters are the collector and the base is that of the common emitter input transistor. According to the invention a phase-splitter 201-204 and 251-253 provides a high impedance and sole source of quiescent bias and drive and a differential amplifier 50 provides negative feedback of at least the quiescent potential across the load. The embodiment is an integrated circuit in which the NPN transistors are vertical structures and the PNP of the lateral type. Input terminal T1 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 phase-splitters 20, 25 which drive composite output transistors 41, 42 and 43, 44 respectively. 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 phasesplitters 20, 25 and also in a second differential amplifier 60 to provide an overall negative feedback signal applied to the input amplifier 10. A series RC network is provided external to the integrated circuit, between terminals T8, T9 to stabilize the feedback loop, while further stabilization is provided by an external capacitor connected between terminal T7 and earth. The composite output transistors 41-44 are provided with quiescent base currents from the collectors of transistors 202, 204, 253, 252 respectively which act as current sources so that the gains of the output transistors depend on the #'s, which are very little temperature. sensitive: the arrangement is self-compensating for high or low #'s, since this is inversely proportional to the temperature developed by internal dissipation in each transistor. The phase-splitters 20, 25 are so biased by bias network 30 as to produce quiescent base currents in output transistors 41-44 which vary inversely with the mean # of these output transistors, which facilitates reduction of cross-over distortion while maintaining the internal dissipation a minimum. Feedback amplifiers.-The negative feedback is purely differential, there being no common mode feedback to interfere with the composite output transistors 40-44 seeking to adjust their dissipation levels to be substantially equal. Amplifier 50 could comprise an emitter coupled pair with a constant current source as common emitter load, a current mirror amplifier providing collector loads, and with emitter degeneration resistors to increase the usuable signal amplitude. The preferred arrangement shown requires a pair of matched resistors 504, 505 and a current mirror amplifier 501, 502: when terminals T2, T3 are at the same potential the collector voltage of transistor 502, like that of transistor 501, and connection to the diodeconnected transistors 201, 251 does not disturb the state of the phase-splitters 20, 25. Amplifier 60 is similar, but modified to provide the higher quiescent bias required by transistor 102 of input amplifier 10: the modifications include equal emitter circuit resistors 603, 604, while due to feedback over emitter-follower 605 the collector to emitter voltages of transistors 601, 602 is 2V BE . The arrangement of the limiting amplifier 15, further limiter diodes and temperature control network 70 is described in Specification No. 1,467,057 and the bias network 30 in co-pending application 21237/74.