A simple way to extend the common-mode input-voltage range of the MOS differential pair

In this paper, we describe a simple technique involving indirect negative feedback that extends the useable common-mode input-voltage range of the MOS differential pair by a saturation voltage. In this method, we use a replica differential pair to sense when the bias transistor supplying the tail current falls out of saturation. We then set the bias voltage so that the sum of the two differential-pair output currents is equal to the bias current. We present experimental results from a version of the differential pair that was fabricated in a 0.5 μm CMOS process along with a comparison with an identical differential pair with a fixed bias with respect to their common-mode input ranges.