Patent GB-1529068-A (GB)
Title: Differential amplifier circuit
IPC: H02H3/02
Filed: 1976-01-21
Abstract: 1529068 Transistor amplifiers RCA CORPORATION 21 Jan 1976 [29 Jan 1975] 02307/76 Heading H3T A differential amplifier for low supply voltage, e.g. an integrated operational transconductance amplifier, comprises transistors 95, 97 having their emitters connected together, signal input terminals 111, 113 coupled to each base, a first current source 101 coupling the emitters to supply line 119, second and third current sources 129, 131 respectively coupling the collectors to supply line 127, and means 107 coupled to the collectors of 95, 97 for maintaining the collectors at a reference level between the supply potentials. A current mirror biasing network 103 receives a biasing current at 125, which is reproduced in diode 117, the diode 123 and the transistor 121 having equal baseemitter junction areas 1X. Current source 101 is a current mirror in which transistor 115 has area 4X, so that the current supplied to the coupled emitters of 95, 97 is four times the bias current at 125, and divides equally between them in the absence of an input signal. Mirror 103 is also coupled to sources 129, 131, each of area 4X, so that in the absence of a signal common base transistors 133, 135 each carry twice the current at 125, and these currents are reproduced in transistors 149, 151, the collector circuits of which are coupled by a current mirror 147,155. If a signal P.D. is applied between 111 and 113, the currents in 95,97 become unequal, and the currents in 133, 135 change by equal and opposite amounts, since the currents through 129, 131 are held constant. The currents in 133, 135 are reproduced in 149, 151 respectively, and under the influence of current mirror 147, 155, an output current equal to the difference between the currents in 149 and 151 appears at output terminal 153. The output stage 109 operates in Class A. In another embodiment in a voltage follower, Fig. 3 (not shown), the output transistors 149, 151 are omitted, and the outputs from the collectors of the common base transistors (235, 237) pass to an amplifier (249) providing negative feedback to one of the signal input terminals (233). The action of the circuit is to produce an output voltage (at 255) equal to an applied input voltage (at 231). Additional components (shown dotted in Fig. 3) may be included to protect the amplifier against the effect produced if the input signal (at 231) is such as to forward bias the collector-base junction of the corresponding input transistor (221). If this happens and the impedance of the signal source is low a large reversed collector current may flow in the input transistor (221) causing the amplifier (249) to pass suddenly from off to saturation. The additional components include a transistor (257) having its base connected to the base of (221) its emitter floating and its collector connected to the emitter of another transistor (263) whose base is connected to the bases of the common base transistors (235, 237) and whose collector is connected to an appropriate input of the amplifier (249). The additional transistors have twice the junction area of the transistors (221, 235, 237) to which they are coupled. If the collector-base junction of the transistor (257) becomes forward biased by a signal the current produced is such as to compensate in the amplifier (249) for the reversed collector current in the input transistor (221). Other additional transistors (259,261) may be included to protect the other input transistor (223).