| 摘要 |
PURPOSE:To improve the sensitivity of weak electric field by using a comparator output to control charging and discharging currents, which generate a voltage proportional to the level of a video synchronizing signal, in an AGC circuit of peak-to-peak value type in a VIF block. CONSTITUTION:An IF signal input 1 from a tuner passes an IF amplifier 2 and a detector 4 and appears as a video detection output 25. This output 25 is amplified and is inputted to the base of a transistor TR6 of a differential amplifier. If the leading edge of the synchronizing signal of a video signal VB becomes lower than a base voltage VC of the TR6, discharging is stopped immediaely though a terminal voltage VA of a capacitor 18 is dropped. In the video period, the voltage VA rises because the video signal VB is higher than the voltage VC. In case of a weak electric field, if the voltage VA rises to exceed a voltage VD determined by the intersection between resistances 17 and 28, comparators 30 and 32 detect this state to make a TR29 conductive, and resistances 27 and 9 are connected in parallel, and a charging current i10 is increased, and the IF amplifier can have a maximum gain.
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