VHF AND UHF AUTOMATIC GAIN CONTROL DERIVED FROM A SINGLE CONTROL VOLTAGE

摘要

1391180 Radio receiving circuits; automatic gain control RCA CORPORATION 12 April 1972 [14 April 1971] 16817/72 Headings H3Q and H3T A television receiver includes separate V.H.F. and U.H.F. tuners, each with its own R.F. amplifier stage; an A.G.C. signal is applied to the R.F. amplifier in the V.H.F. tuner and also to an amplifier having a high input inpedance which is partly a function of the current conducted by the amplifier, a control voltage derived from this amplifier being used to control the gain of the R.F. amplifier in the U.H.F. tuner. In addition, means are coupled to the output terminal of the amplifier which are responsive to the control voltage derived therefrom to limit it in dependence on the conduction of the amplifier. The V.H.F. R.F. amplifier comprises a double gate F.E.T. 35, Fig. 2, with the signal input applied to one gate and an A.G.C. potential to the other gate via resistor 30. The input and output filters 37, 36 respectively, are tuned by means of varactor diodes 38, 39 respectively, and the R.F. amplifier is followed by a mixer comprising junction transistors 45, 46 in cascade connection. Local oscillations are derived from oscillator 55, tuned by varactor diode 56, and which is disabled when the U.H.F. tuner is in use. The A.G.C. voltage is also applied to an amplifier comprising junction transistors 31, 32 in Darlington connection, a control voltage for the U.H.F. tuner being derived from their collectors over a potentiometer comprising resistors 60, 61. This control voltage is limited to a minimum value by means of diode 66, the anode of which derives a fixed potential by means of resistors 57, 58 from Zener diode 44. U.H.F. tuner.-The U.H.F. tuner comprises an R.F. amplifier in the form of transistor 75, Fig. 3, in common base connection with input and output filters 73, 79 tuned by means of varactor diodes 74, 80 respectively. Filter 79 comprises transmission-line elements. It feeds into a mixer diode 83 which also receives local oscillations from oscillator 84, tuned by means of varactor diode 85. Mixer 83 is coupled by capacitor 86 and inductor 113 to the emitter of junction transistor 94, which comprises a common-base connected preliminary I.F. amplifier. The output from transistor 94 is derived over a filter network comprising inductors 95, 96, 103 and capacitors 98, 99, 103, a ferrite bead 98 on the collector lead of transistor 94 preventing parasitic oscillations; the output is fed to the base of transistor 45 (Fig. 2). The A.G.C. voltage derived across resistor 61 (Fig. 2), is applied to the base of transistor 75 via an emitter-follower 110, the gain of transistor 75 being decreased by an increase in collector current therein. The output from emitter follower 110 is also applied to bias a diode 116 which forms a variable attenuator shunting the output from mixer 83; when conductive, diode 116 also provides a bias reducing the gain of I.F. amplifier 94.