| 摘要 |
PROBLEM TO BE SOLVED: To obtain a low noise output over the entire filter frequency from a narrow band to a broadband with immunity to an impedance change when the band of the filter is varied. SOLUTION: The filter frequency bandwidth of an input signal of a crystal vibrator 3a of the bandwidth variable filter 1 is freely varied in response to a varied impedance and controlling a current to a PIN diode 3b by a current control section 5 varies this impedance. A buffer circuit 4 consists of a pre-stage FET 7 and post-stage transistors(TRs) 8 in Darlington connection subjected to source follower and emitter follower respectively. Thus, even in the case that the filter frequency band is wide, the input impedance is high, a generated noise voltage is decreased to reduce the noise and a prescribed gain can be obtained.
|
