Patent GB-1186881-A (GB)
Title: Improvements in Automatic Gain Control Circuits
Inventor: DARVELL JOHN LOUIS
IPC: H04N5/52
Filed: 1967-01-27
Abstract: 1,186,881. Transistor automatic gain control circuits. THORN ELECTRICAL INDUSTRIES Ltd. 25 Jan., 1968 [27 Jan., 1967], No. 4258/67. Heading H3T. In an A.G.C. circuit for controlling the gain of a first and a second transistor, of opposite conductivity type by changing their emitter currents, first clamp means are provided for preventing change in the emitter current of the first transistor after the magnitude of the A.G.C. signal exceeds a first predetermined magnitude and second clamp means for preventing change in the emitter current of the second transistor until the A.G.C. signal exceeds a second predetermined magnitude. In a first embodiment, a television receiver includes an R.F. stage comprising PNP transistor 15, Fig. 1, and an I.F. stage comprising NPN transistor 10, positive-going gain and contrast control signals being applied via terminal 17 to a D.C. amplifier comprising emitter-follower 18 coupled to amplifier 19. Amplifier 19 has two series-connected load resistors R1, R2, the voltage across both being applied to the base of transistor 15 and that across resistor R2 to the emitter of transistor 10. As the control voltage increases the current in transistor 10 is increased, reducing the gain; at low levels of control signal, however, the emitter current of transistor 15 remains substantially constant since it flows through a high-emitter circuit resistance Rp. As the emitter of transistor 15 becomes more negative a point is reached at which diode D1 becomes conductive, whereby resistor Rp is shunted with a much lower value resistor R Q  and beyond this point the emitter current of transistor 15 increases, reducing its gain. The base of transistor is coupled to the common point 23 of resistors RR, R s  which are in series with resistor R Q , and once the current through resistor R Q  from transistor 15 increases, the potential of point 23 falls, so that the base potential of transistor 10 then follows its emitter potential and no further change occurs in the current in this transistor. In a second embodiment, also a television receiver, PNP transistor 15<SP>1</SP>, Fig. 3, comprises an R.F. stage and NPN transistor 10<SP>1</SP> an I.F. stage. A contrast control is obtained by means of potentiometer 26, the output of which is added to an A.G.C. voltage from detector 25 and the combination is amplified by a two-stage amplifier comprising transistors 18<SP>1</SP>, 19<SP>1</SP>. At low A.G.C. signal levels transistor 18<SP>1</SP> is non- conductive, rendering transistor 19<SP>1</SP> also non- conductive, whereby diode D2 is forward biased and diode D3 reverse biased: the base of transistor 10<SP>1</SP> is then so biased that its emitter current ensures maximum gain. When the A.G.C. signal increases, i.e. becomes more negative, transistors 18<SP>1</SP>, 19<SP>1</SP> begin to conduct and a current flows through the collector load R c  of transistor 19<SP>1</SP> and also through the emitter load, which comprises resistors Rd, R e  and diode stack DS. The voltage across the emitter load is passed to the base of transistor 10<SP>1</SP> causing its emitter current to increase and the gain to be reduced: the voltage across collector load R c , however, is not transmitted to the base of transistor 15<SP>1</SP> as diode D3 is reverse biased. The ratio of the A.G.C. signals at the collector and emitter loads of transistor 19<SP>1</SP> is initially R c /R d  + R e  but as the control voltage increases this tends to R c /R d  due to the impedance of diode stack DS falling. Further increase in the A.G.C. signal causes diode D3 to conduct and the gain of transistor 15<SP>1</SP> to be controlled: diode D2 ceases to conduct, however, and no further change occurs in the emitter current of transistor 10<SP>1</SP>. The circuit is stabilized against changes in supply voltage by means of asymmetric voltage-dependent resistors 27 connected across potentiometer 26 and 28 forming part of the potentiometer determining the base current of transistor 10<SP>1</SP> when diode D2 is reverse biased.