References

Category:Autodesk the voltage supply to the output is high, and by a high-impedance current source, the negative charge at the capacitor is prevented from increasing to the voltage supply by a high resistance, so the voltage of the output is kept constant. In a second step, a current source of a low impedance is connected to the capacitor, so that the accumulated charge is drained to the ground. As a result, the output voltage becomes high. Therefore, the input voltage is higher than the output voltage and an error voltage in the high region is produced. The error voltage can be increased and decreased by adjusting the resistance value of the high-impedance current source.
With this constitution, the input voltage is always constant, so the error voltage is always produced by the change of the accumulated charge. The error voltage is proportional to the accumulated charge. The difference of the error voltages in the first step and the second step is proportional to the quantity of the accumulated charge, so the error voltage is proportional to the input voltage. This makes it possible to decrease the input voltage to the same degree. By varying the ratio of the two steps, the output voltage can be changed by the quantity of the accumulated charge.
The characteristic of the output voltage is similar to that of a multiplying circuit. The error voltage of the output is proportional to the input voltage and the characteristic of the output voltage is nonlinear. The error voltage of the output may be 0.1 V.
The circuit shown in FIG. 7 is a circuit that is widely used. The circuit is constituted by an operational amplifier which is constituted by a differential amplifier of a current source and a voltage source of a small current capacity and three resistors which determine the accuracy of the operational amplifier.
The operational amplifier is made in the form of a high-gain circuit, and a reference potential is defined by the reference voltage VREF.
As is well known, the input impedance of the operational amplifier is very high. Thus, the input impedance of the operational amplifier is typically 10 M.OMEGA..
The output impedance of the operational amplifier is typically 3 k.OMEGA..
Since the input impedance and the output impedance of the operational amplifier are very high, it is possible to ignore the internal resistance of the operational amplifier. As a result, the operational amplifier can be operated without being affected by the feedback resistor Rf, the constant current source and the resistors R1 and R2 which are connected to the operational amplifier.

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