Patent GB-864515-A (GB)
Title: Improvements in or relating to transistor amplifier circuit arrangements
IPC: H03G9/14
Filed: 1957-04-04
Abstract: 864,515. Transistor automatic gain control circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 4, 1957 [April 7, 1956; Dec. 5, 1956(2)], No. 11018/57. Class 40(6) A transistor amplifier has an automatic gain control voltage applied to a control electrode of at least one transistor and a voltage sensitive resistor connected between a resistor in the collector circuit of the transistor and a tuned circuit of the amplifier so that an increase in the control voltage varies the voltage applied to the voltage sensitive resistor and thus increases the pass-band width of the amplifier and decreases the impedance of the tuned circuit. Fig. 3 shows a three stage IF amplifier followed by a detector 63. The detector provides an automatic gain control voltage which is applied through resistor 70 to the base of transistor 32 to vary its input impedance and the gain of the amplifier. The collector circuit of the transistor includes a resistor 50 and a voltage sensitive resistor in the form of the rectifier 71 connected to the input tuned circuit 55 of the transistor. As the control current to transistor 32 increases the voltage at the right-hand terminal of the rectifier rises so decreasing its impedance. This has the effect of increasing the band width of the amplifier and further reducing its gain. If the circuit of transistors 31 and 32 are similar the bias on the rectifier 71 does not vary with temperature. Further, by suitably choosing the values of the resistors of the circuits, a delay in the control action may be obtained and furthermore it may be arranged that the voltage drop across the collector resistance of transistor 32 in the no-signal condition is so large that the gain of the transistor is reduced to produce silent tuning. The maximum band width of the amplifier may be limited by arranging that one of the tuned circuits of the amplifier is not controlled or by utilising the effect that the selectivity of an input circuit to a transistor increases with signal strength owing to increase of input resistance. The connector of the lefthand plate of the rectifier 71 could alternatively be made to the lower end of a resistor 51 in the collector circuit of 33 in series with a winding coupled to circuit 55. Alternatively the rectifier could be connected to a tap on circuit 57 so as to provide controlled negative feedback. In an alternative circuit (Fig. 1, not shown) the rectifier is connected across a tuned tertiary winding of a transformer connected in the collector circuit of one transistor, the rectifier being biased from a voltage developed across a resistance in the emitter circuit of the transistor. In a further circuit (Fig. 2, not shown) the rectifier is connected between the collector resistance and the tertiary winding of a tuned collector transformer, a fixed bias being applied through the winding to the transistor from a potentiometer connected across the amplifier collector current supply. The gain of the controlled transistor may be varied by inserting a resistor in its collector circuit of such a value that the resulting variation in collector voltage caused by the change in collector current effects the desired control of gain.