Comparison of input offset voltage of differential amplifiers using bipolar transistors and field-effect transistors

A field-effect transistor (whether junction type or MOS type) has very high input impedance. For those who desire to achieve a higher input impedance, it is often asked `Why aren´t FET pairs used as input stages and bipolar transistors used as output stages, since compatible FET and bipolar transistor monolithic structures have been developed?´ This correspondence is a study of this question.