AUTOMATIC GAIN CONTROL CIRCUIT

1,188,752. Automatic gain control. MOTOROLA Inc. 6 Aug., 1968 [28 Sept., 1967], No. 37519/68. Heading H3T. In a signal receiver the A.G.C. controlled current in one stage is used to control the gain of a later stage in reverse sense aboveapredetermined signal level, so that the gain of the later stage then remains constant. As applied to a television receiver an A.G.C. circuit of the gated type derives a delayed control potential from sync. signals in circuit 50 which is applied over line 52 to the base of I.F. amplifier 58, via resistor 54; the arrangement is such that gain is reduced by increasing the collector current. The change in potential across emitter circuit resistor 60 so caused is applied via resistor 64 to the base of preceding I.F. amplifier 68 so that its gain is likewise reduced. The gain control potential on line 52 is also applied over a further delay circuit 74 and resistor 76 to reduce the gain of R.F. amplifier 80 by increase of collector current. The gain is initially reduced in the I.F. stages; when the gain in the R.F. stage 80 begins to be reduced the increase in voltage drop across resistor 82 in the collector circuit is applied over resistor 100 to the base of the I.F. stage 58. The gain control potential derived from line 52 is thereby opposed, so that the current in transistor 58 remains constant, thereby maintaining the gain of this stage and of transistor 68 constant. When the R.F. stage 80 becomes biased back, no further change in the potential derived via resistor 100 occurs, and the gain of the I.F. stages reduces once again.