| 摘要 |
<p>1,257,992. Valve amplifying circuits; automatic gain control; colour television receivers. R.C.A. CORPORATION. 9 May, 1969 [13 May, 1968], No. 23724/69. Headings H3Q, H3W and H4F. A colour television receiver uses a limiter to reduce noise in the automatic gain control, sync. separator and chrominance channels. I.F. amplifier 12 feeds via transformer 32 a diode video detector 14 which produces an output referenced positively relative to earth, in which sync. pulses are negative-going (Fig. 2(a), not shown). To this end the diode cathode is given a positive bias over resistors 35, 82, 81, the anode being positively biased by means of resistors 36, 31, 37; the cathode is decoupled to earth by capacitor 35. The anode is coupled to the grid of a first video amplifier 45 via intercarrier sound trap circuit 34, 41, 42 and top-peaking inductor 43. The valve 45 acts as a cathode follower for D.C. and low frequencies, to cathode circuit being completed by delay line 16 and terminating resistor 51, at higher frequencies however boot-strapping is provided by capacitor 61 and inductor 60. Delay line 16 feeds a luminance circuit comprising the further amplifier 17. A further amplifier 70 is fed with higher frequencies from the cathode of video amplifier 45 via capacitor 80; for lower frequencies and D.C. it is coupled to the detector via resistor 31. A toppeaking circuit comprising inductor 77 and shunt damping resistor 76 is provided in the grid circuit. Amplifier 70 feeds chrominance circuits 23, sync. separator circuits 24 and a gated A.G.C. circuit 27, noise pulses being limited by means of a diode 75 provided with a delay bias comprising the voltage drop across a further diode 73 connected in the cathode circuit of the valve 70.</p> |
