| 摘要 |
PURPOSE:To obtain an amplifier controlled remotely with excellent high frequency characteristic and variable transmission characteristic and an attenuator at low cost with excellent mass-productivity by controlling a current supplied to a 1st variable resistive element so as to vary the resistance of the 1st variable resistive element accurately and using a 2nd variable resistive element having substantially the same value for an amplifier. CONSTITUTION:A terminal voltage Vi of the 1st variable resistive element 30 is impressed to a plus terminal of an operation amplifier 5. Then a prescribed reference voltage Vr including zero volt is impressed to a minus input of the operational amplifier 5. Since the amplification factor of the operational amplifier 5 is actually infinite, the relation of Vi=Vr is obtained. Then, from the relation of R(Y)=Vi/Ic, the resistance R(Y) of the 1st and 2nd variable resistive elements 30, 40 is controlled optionally by an output current Ic of a constant current means 27. Thus, even if the characteristic of the 1st and 2nd variable resistive elements 30, 40 in pairs very ununiform between different pairs, since the characteristic of both elements as a pair is substantially identical, both elements shows a resistance with respect to the current Ic.
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