| 摘要 |
PURPOSE:To constitute a filter circuit whose cut-off frequency is adjustable onto an IC by varying a capacitance of a capacitive component depending on a voltage gain of an amplifier and selecting the cut-off frequency decided by an input resistor and the capacitive component to a desired frequency. CONSTITUTION:An amplifier consists of transistors (TRs) Q1, Q2, a constant current circuit CS1, a reference voltage source Vref and a resistor R2 and formed onto a semiconductor substrate by the bipolar process. The voltage gain of the amplifier depends on Gv=R2/Re, where Re is an emitter resistor of the TR Q1. The voltage gain Gv is varied by the control of the constant current circuit CS1. When the voltage gain Gv is large, the cut-off frequency ft is decided by ft=1/2piGvCR1. That is, the cut-off frequency ft is selected to a desired frequency by the control of the current Ic of the constant current circuit CS1.
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