| 摘要 |
PURPOSE:To obtain a TV tuner which never causes the loss of the signals or lowering of the noise exponent by giving the conductive and nonconductive control to the switching diode via the switching voltage. CONSTITUTION:The switching voltage is applied to power source Bu for the UHF tuner at the UHF reception time and then to serial-connected switching diode Ds for conduction. As a result, the intermediate frequency signal given from the UHF tuner is supplied to the base of transistor Tr for the VHF mixing/UHF intermediate frequency amplifying purpose through the coupling capacity. While at the VHF reception time, the anode side of Ds is grounded via breeder resistances R12 and R13 in choke coil RFC and UHF local oscillation circuit OU. On the other hand, voltage V2 divided by resistances R8 and R9 is applied to the cathode side to make Ds nonconductive. Here resistance Rc is inserted in parallel to capacity C to increase the base voltageof transistor Tr in which Ds is nonconductive and then to set the bias to the level suited to the action of the UHF intermediate frequency amplification. |
