PURPOSE:To prevent the generation of noises by obtaining the voltage proportional to the product of the level of an input digital signal and the value of a variable reference power supply at the output side of an amplifier circuit and at the same time delivering the sum of said voltage and the variable offset voltage proportional to the value of said power supply. CONSTITUTION:Variable current sources 21 and 22 serve as variable reference power supplies produce currents IRV and I'rV which have polarities different from each other and levels proportional to each other. These two currents have the relation as shown by an equation I, where N means the resolution of a converter. The currents I'RV given from the source 22 and has the relation with the current IRV as shown by the equation I is supplied to the reverse input terminal of an amplifier circuit 25. Therefore the voltage obtained by multiply ing the difference between the current I'RV and the product of the current IRV and a component DELTA changed according to an input digital signal by the resis tance value Rf of a resistor 27, i.e., the voltage (IRV.DELTA-I'RV)Rf is obtained at an output terminal 28 of the circuit 25.
展开▼
