| 摘要 |
PURPOSE:To obtain a low-pass filter whose cut-off frequency is varied with an external control voltage by controlling a differential input fed to each base of a 1st and 2nd transistor (TR) constituting a differential amplifier so as to very the cut-off frequency. CONSTITUTION:The cut-off frequency variable low-pass filter consists of NPN TRs Q1-Q4, resistors R1-R4 and a capacitor C. Then an input terminall is connected to a base of the TR Q4 via an input resistor R2 and the resistor R4 is connected between the emitter of the TR Q4 and a ground terminal 4. Through the constitution above, the current flowing to the resistor R1 is multiplied by alpha (0<=alpha<=1) by a voltage V impressed to the bases of the TRs Q1, Q2. As a result, the relation of equation is obtained, where Vi is a voltage at an input terminal 1 and Vo is a base voltage of the TR Q4. Thus, the factor alphais varied with the change of the voltage V to make the cut-off frequency variable.
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