High bandwidth fast settling time operational amplifier and method therefor

摘要

A wide bandwidth fast settling operational amplifier (71) comprises a first stage (72) and a second stage (73). The first stage attenuates a differential input signal applied to the operational amplifier (71). The second stage (73) provides all the gain of the operational amplifier (71). The first stage is a wide bandwidth stage having a differential input transistor pair (74,75) coupled in a voltage follower configuration. The differential input transistor pair (74,75) are degenerated by resistors (76,77) to reduce voltage gain and to lower an impedance coupled to the second stage (73). The first stage (72) is biased via a current source. The first stage (72) provides a reference voltage to the second stage that corresponds to and varies with an input common mode voltage. The reference voltage is used to bias a cascode stage in the second stage (73) to increase common mode range.