| 摘要 |
PURPOSE:To set an optimum bias for UHF reception and that for VHF reception by inserting a resistance between the cathode of a switching diode for switching between UHF and VHF bands and the base of a transistor (TR) in a mixing stage. CONSTITUTION:The operating bias voltage of a TR10 in the mixing stage is so set that it is optimum at a VHF reception time. When the UHF band is received, a bus BU is turned on to give a prescribed voltage +B, and an IF signal obtained in a UHF circuit mixing stage 4 passes an IF tuning circuit 6, a resistance attenuator 21, and a switching diode 13, which is made conductive by the voltage of the bus BU, and is supplied to the TR10 of the VHF mixing stage. The base bias voltage of the TR10 is made higher than that for VHF reception by a resistance R simultaneously with conduction of the switching diode 13, and the TR10 is operated with an emitter current higher than that for VHF reception. The resistance value R is so set that this emitter current is an optimum value for VHF and UHF reception. Thus, optimum bias conditions are set easily. |
