Patent GB-1139605-A (GB)
Title: Improved amplifier
IPC: H03F3/45
Filed: 1966-06-23
Abstract: 1,139,605. Transistor amplifying circuits. APPLIED DYNAMICS, Inc. 23 June, 1966, [9 July, 1965], No. 28036/66. Heading H3T. The invention relates to a direct-coupled amplifier with first and second input circuits, the first of which is fed from a summing junction to which is applied the signal input and also feedback from the amplifier output; the summing junction also provides an input to a stabilizer amplifier system which applies a balancing signal to the second input circuit of the directcoupled amplifier. According to the invention means are provided to switch the stabilizer amplifier system between a normal stable state and an unstable condition, as determined by a selected signal level occurring at a selected point in the direct-coupled amplifier. The arrangement is intended to cut short the period for which an operational amplifier is rendered inoperative by an excessive signal input, due to the charging up of associated capacitances. Input terminal 13 of D.C. amplifier 20, Fig. 1 is fed, via frequency-shaping network R1, C1, C2, and input limiting diodes X1-4, form summing junction 11, to which can be applied a current input i 3  and also voltage inputs via resistors R1, 2; in addition overall feedback is applied to the terminal via impedance R-F, from output terminal 30. The signal at terminal 30 is also applied to a stabilizer amplifier 21 which applies a correcting voltage to the second input 23 of D.C. amplifier 20. An overload signal is derived from D.C. amplifier 20 via line 24 and is applied to stabilizer 21, to gas-discharge tube overload indicator DS1 and to an overload output line 24, for application to other apparatus. D.C. amplifier 20 comprises an input amplifier and an output amplifier, while stabilizer amplifier 21 is of the "chopper" type. D.C. input amplifcer.-The input stage comprises a differential amplifier formed by transistors Q2, 3, Fig. 2, with individual emitter resistors R4, 5 and coupled by means of common emitter circuit resistors R6, 7: these transistors are fed via field-effect transistors Q1, 4 respectively. The input is applied from terminal 13 to the gate of F.E.T. Q1 via roll-off network R1, C1 and D.C. blocking capacitor C2: the stabilizer circuit output is applied to the gate of F.E.T. Q4, as is also a D.C. balancing voltage via resistors R12, 13 and variable resistor R-B, the circuit including a time-constant capacitor C4. A stabilizing capacitor C3 is connected between the emitter of transistor Q3 and gate of F.E.T. Q1. An output from the collector of transistor Q3 is fed via emitter-follower Q5 to two cascaded Darlington pairs comprising transistors Q6, 7 and Q8, 9 respectively: each pair includes a local negative feedback network comprising resistors R16, 17 and capacitors C6, 7 and resistors R23, 24 and capacitors C9, 10 respectively. The collector of transistor Q9 feeds a voltage to the output amplifier via terminal 26; it is also coupled via emitter-follower Q10 to the base of transistor Q11, while the emitter is coupled to the base of transistor Q 12. If the base of transistor Q9 exceeds a predetermined positive potential transistor Q11 is cut off, while if it falls below a predetermined value transistor Q12 is cut off; in either case the overload signal potential on terminal 24 rises to that of the positive supply line. Power supply to differential amplifier.-The amplifier input stage is provided with a stabilized + 11À8v. supply from the main + 170v.- 170v. supply via a potentiometer network comprising resistor R1B, Zener diode VR1 and diodes X1B-X3B which provide temperature compensation. A -39v. supply which varies in the same manner and so minimises offset errors is obtained from the diode chain via phase-reversing transistor Q1B and output emitter-follower Q2B. D.C. output amplifier.-The voltage on line 26 (Fig. 3) is fed to cascaded emitter-followers 2Q2, 2Q3; the base bias circuit of transistor 2Q2 includes the emitter-collector path of transistor 2Q1 which acts as a constant current generator. The voltage on line 28 drives transistor 2Q5 in the output stage, feeding output terminals 30; this is of the single-ended pushpull type, also comprising transistor 2Q4 which is driven via diodes 2X1-3 from the voltage developed across resistor 2R12 in the collector circuit of transistor 2Q5. Both transistors are biased to a low standing current and negative feedback is applied to the output stage by means of resistor 2R10. Diodes 2X5, 2X5A in the emitter circuits of the respective transistors clamp their bases and prevent them being driven too far negative. An excessive positive drive, on the other hand, causes a positive potential to be developed across the respective emitter circuit resistor 2R18, 2R15 which causes a corresponding auxiliary transistor 2Q7, 2Q6 respectively to become conductive and clamp the base potential of the corresponding output transistor; time-delay networks 2R17, 2C4 and 2R14, 2C3 respectively are provided so that instantaneous peaks are not limited. Chopper type D.C. stabilizing amplifier.-The voltage at terminal 13 (Fig. 4) is passed via a low-pass filter 3R1, 2 and 3C1 to an interruptor comprising shunt photo-resistor 3PR1; a connection is made at this point between "power" ground and the "signal" ground of the computer via a low-pass filter comprising capacitor 3C3 and resistor SR3. The "chopped" signal is passed to an A.C. amplifier comprising field-effect transistor 3Q1 followed by commonemitter amplifier 3Q2, stabilized by negative feedback over resistor 3R8. The signal then passes to two cascaded Darlington pairs 3Q3, 3Q4 and 3Q5, 3Q6 respectively; part of the output of transistor 3Q6 is applied to a feedback limiter comprising diodes 3X1, 3X2 so as to limit the collector voltage excursion on overload. Transistor 3Q7 comprises a Zener diode type limiter 60 cps. is fed from terminal MD to the base of transistor 3Q1A to drive in turn transistor 3Q2A which switches on and off neon lamp DS-1A, providing chopped illumination for the photo - resistor 3PR1. Transistor 3Q1A also drives a synchronous detector comprising transistor 3Q8 to which the output of transistor 3Q6 is fed, to produce a rectified output which is fed via resistor 3R21 to the stabilizer output terminal 23. Smoothing is provided by capacitor C4 (Fig. 2) in parallel with which is normally the much larger capacitor 3C13, connected to ground via the collector-emitter path of transistor 3Q9. When overload occurs the positive potential on overload line 24 renders transistor 3Q10 conductive, and this in turn causes the base bias of transistor 3Q9 to fall and render it non-conductive. Capacitor 3C13 is thus removed from circuit, the resulting reduction in time-constant rendering the amplifier D.C. loop oscillatory: this rapidly reduces to small values the voltages to which the overload may have charged any of several long time-constant circuits, including that involving capacitor C2 (Fig. 2) and each long time-constant circuit associated with the stabilizer channel (Fig. 4). The brief oscillatory period is terminated when transistor 3Q9 re-connects capacitor 3C13, whereupon the amplifier is ready for re-use.