| 摘要 |
PURPOSE:To make it possible to select pluralities of frequency characteristics with simple circuit constitution by providing a group of RC elements differing in circuit constant and a switch method that selects a random RC element properly. CONSTITUTION:As one of terminal A-C control signals is inputted from gate input terminal 7 of multiplexer IC1 and IC2, one of channels conducts corresponding to the input signal. For example, when terminals A-C are all applied with a signal of level L, the channel between terminal T0 and COM conducts. Therefore, resistances R10 and R20 and connected in series between signal input terminal 1 and input terminal (+) of operational amplifier A, and consequently active LPF is formed of amplifier A, resistances R10 and R20, and capacitors C1 and C2. For this purpose, when a rectangular wave signal is inputted to terminal 1 and channels are selected one after another with control signals, active LPF with a proper cut-off frequency is formed and a sine wave appears at output terminal 3. Thus, it is not necessary to increase operational amplifiers in number according to the number of filters. |
