1.  Field of the Invention
The present invention is in general related to an integrated circuit.  More specifically, the present invention is related to a four-quadrant multiplier, which is composed of CMOS transistors and suited to applications of low-voltage operation. 
2.  Description of the Prior Art
Owing to the rapid development of very large integrated circuits (VLSIs), concentration of transistors in a finite chip area continues to increase.  Therefore, the voltage source powering integrated circuits has been reduced from 5 V to 3 V, and even to 1 V. Toward this trend, in a digital circuit, one way that reduces the feature size of components therein can be taken to reach this requirement.  However, in an analog circuit, it may need to re-design the whole circuit again. 
In bipolar transistor technology, Gilbert cells are usually used to build a four-quadrant multiplier.  But in MOS transistor technology, four approaches have emerged.  The first approach, similar to the case of bipolar technology, is to use Gilbert cells to build a four-quadrant multiplier.  The second approach is to use the properties of MOS transistors operated in the saturation region to design multipliers.  The saturation region is also called an active region where the drain current value is proportional to the square of the gate-drain voltage value.  The third approach is to use the properties of MOS transistors operated in the linear region to design multipliers.  The linear region is also called the triode region where the drain current value is linearly proportional to the gate-source voltage while the drain-source voltage is a constant.  The fourth or last approach is to use the properties of MOS transistors operated in the subthreshold region to design multipliers,
Multipliers are essential components that serve as building blocks for a large amount of applications, such as adaptive filters, frequency doublers, and modulators.  The operation of a four-quadrant multiplier involves receiving a first input signal V1, a second input signal V2, a third input signal V3, and a fourth input signal V4, then outputting a product signal, which is the result of a multiplying operation of the voltage differences V1-V2 and V3-V4.  However, in present practice, using a four-quadrant multiplier with low-voltage operation still presents some problems.