Patent GB-1473999-A (GB)
Title: Amplifiers
IPC: H03F3/45
Filed: 1975-02-10
Abstract: 1473999 Transistor amplifiers WESTERN ELECTRIC CO Inc 10 Feb 1975 [11 Feb 1974] 5638/75 Heading H3T An amplifier, e.g. an integrated operational amplifier, comprises a differential input stage 11 with inverting 22 and non-inverting 23 input terminals, connected via further stages to an output stage 15, and a biasing circuit 18 providing to at least two stages biasing voltages which follow the voltage changes applied at the non-inverting input. In one embodiment of the biasing circuit 18 current sources 50, 51 are provided by transistors (108, 112 respectively), Fig. 2 (not shown), with their bases connected to biasing terminals (121, 122 respectively) which also provide bias voltages for other stages in the amplifier. Voltage follower amplifier 56 is provided by transistors (102, 103) functioning as emitter followers, so that the voltages at terminals 52, 54, 58 and 60 follow the potential changes applied at input 23. Additional transistors (116-119) are provided to enable the biasing circuit to respond to rapid changes of potential, and maintain the quiescent current low. Between the input stage 11 and the output stage 15, the further stages of the amplifier comprise a balanced common base stage 13 and an emitter follower stage 14. A range extender stage 16 couples signals from the input stage 11 directly to the output stage 15 in parallel with stages 13, 14, with the object of improving the slow rate, the small signal gain of circuit 16 being much lower than that of stages 13, 14. A feed-forward capacitor 17 further enhances slew rate. Differential input stage 11 has an active load 12 fed by a current source 37 and having 6d B gain which isolates stage 11 from subsequent stages. Resistors (not shown) may be added between collector and base of the active load transistors 35 and 36, to obtain additional gain. Resistors 67, 68 and 69, with capacitor 76 determine the frequency response of stage 14, and ensure that it is unchanged for high current signals. Common base input transistors 71, 73 of push-pull output stage 15 may have two emitters connected to separate input terminals (201, 202), Fig.s 3 and 4B (not shown), so that the input from range extender stage 16 may be isolated from that of stage 14. PNP output transistor 72 is preferably vertically diffused by ion implantation. Transistor 72 may be replaced by two transistors (217, 218) coupled to a diode connected transistor (219) in a current doubling network. In another embodiment of the amplifier each current source 24 &c. consists of a transistor (310 &c.) and a resistor (313 &c.), Figs. 4A and 4B (not shown). A resistor (321) and capacitor (322) in series increase the signal coupled to common base stage 13, particularly to improve transient response. A capacitor (326) passes high signal transients, and another capacitor (327) provides negative feedback for stabilization. The use of biasing voltages driven by the applied signal allows high common mode rejection range without high collector-base bias voltages, limits the voltage across circuit capacitances, and prevents transistors reaching saturation during transients.