| 摘要 |
A local oscillator circuit for a television tuner or the like includes a transistor and an oscillatory circuit connected thereto. Selector means are connected to the oscillatory circuit to cause the oscillatory circuit to operate at low and high band frequencies respectively. Separate biasing means are provided to bias the transistor at first and second voltage levels respectively, and bias control means are connected to the selector means to connect the appropriate biasing means to the transistor when the selector means adapts the oscillatory circuit to operate at a high band or a low band frequency. The bias control means comprises a switching diode connected between ground and the dividing point of high and low frequency band coils of the oscillatory circuit. A negative voltage source is connected through a high resistance to an end of the low band coil. The selector means comprises a switch controlled by the channel selector shaft of the tuner and is disposed between the end of the low band coil and a power source to control the switching diode such that the transistor bias voltage is regulated in accordance with the mode of operation of the circuit.
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