Voltage controlled current sources have been widely used because of a relatively simplified design and ease of debugging. One implementation includes operational amplifiers, but may have a disadvantage of reduced accuracy because lower offset of the input terminals of the operational amplifier may lead to a larger output error. A conventional method to overcome this problem is to utilize high power MOSFETs or BJTs to form an input differential pair of the operational amplifier, and matched layout to decrease random offsets. However, an input offset of several mV will may exist even though layout is well matched for implementations employing high power MOSFETs. Further, such high power MOSFETs may not be available for some applications, such as light emitting diode (LED) drivers, due to the output error caused by the input offset. In addition, the input offset can be influenced by temperature, illumination, radiation and other effects, possibly reducing voltage controlled current source applications. Also, implementations employing BJTs may have disadvantages related to conventional CMOS process restrictions, larger volume, and influences by temperature and other external factors.
USPTO Backgrounds
USPTO Backgrounds