Patent GB-1467058-A (GB)
Title: Amplifier and bias circuitry therefor
IPC: H03F3/30
Filed: 1974-05-14
Abstract: 1467058 Transistor amplifying circuits RCA CORPORATION 14 May 1974 [24 May 1973] 21235/74 Heading H3T A class B push-pull amplifier, comprises first and second output transistors 101, 102, Fig. 1, having a forward current gain #, and means for supplying a quiescent bias current to the bases comprising an auxiliary transistor 103 thermally coupled with the first and having substantially the same #, a regulated current supply 105 being connected in the emittercollector circuit of the auxiliary transistor 103 and a substantially fixed gain current inverting amplifier 107 responsive to the base current of the auxiliary transistor 103 supplying quiescent base current to the first transistor 101. By this means the quiescent output current of the transistor 101 is made independent of #. Transistor 102 is similarly biased by means of transistor 104, a second current regulator 106 and current amplifier 108. Alternatively, a single auxiliary transistor may be used, with two separate current amplifiers, to bias the two amplifier transistors (Fig. 2, not shown). The Specification illustrates a number of suitable known current mirror amplifiers (Figs. 3a-3c, not shown). In Fig. 4, compound transistors 101, 102, are biased via a "currentmirror" amplifier comprising 401, 403, 405, 407 and 406, 408 the input bias current for which is derived from the base of transistor 213 which is provided with controlled collector current by means of resistor 417 and shunt regulator transistor 415. The combined currents of transistors 213, 415 are supplied by a "currentmirror" arrangement comprising transistors 416, 419 and resistors 418, 420, 421. A further embodiment, in which the biasing arrangement also provides driving signal, comprises a pair of compound transistors 101, 102 forming a pushpull output stage. Input signals from source 502 are coupled to input amplifier 505, the output from which drives a phase-splitter in the form of a "current-mirror" stage comprising transistors 516, 517, 518. Overall negative feedback is applied from output terminal T5 to the second input of amplifier 505. The voltage drop across fixed resistor 527 is substantially constant, being the difference between that across avalanche diode 524 and that, twice the base-emitter voltage, across transistor 522. Consequently the collector current of transistor 522 is constant, calling for a base current in transistor 521 proportional to <SP>1</SP>/ # , # 2  where #, # 2  are the current gains of transistors 521, 522 respectively. Due to "current-mirror" action of transistors 513, 514 this current, which flows through diode-connected transistors 511, 512, 513 produces an equal current through transistors 514, 515: as the base current of transistor 515 is <SP>1</SP>/ #  of its collector current this can be neglected. Hence the emitter to earth potential of transistor 515 is equal to twice the collector to earth potential of each of the diodeconnected transistors 511, 512, 513 and this potential drop must be shared equally by transistors 516, 517 in the phase-splitter. Thus the bias currents of composite transistors 101, 102 are both proportional to <SP>1</SP>/ # , # 2 , so that the corresponding quiescent output currents are independent of #.