A voltage-controlled current source (VCCS) is provided. The VCCS controls
an output current according to a controlling voltage. The VCCS includes
an operational amplifier (OP-amplifier), a transistor, a resistor and a
current mirror. The present invention utilizes the characteristics of the
OP-amplifier to compensate for the voltage difference between the gate
and the source of the transistor so that the resulting terminal voltage
on the resistor is equal to the input control voltage. Therefore, the
VCCS of the present invention can reduce the factors including process
drift, fluctuation in the DC voltage source or the output current that
can affect the terminal voltage difference of the resistor and hence the
accuracy of the output current.