Rail-to-rail type of operational amplifier with a low offset voltage achieved by mixed compensation

摘要

A rail-to-rail type of operational amplifier is provided, which has a low offset voltage and improved bandwidth, slew rate, and phase margin. This operational amplifier includes two level-shifting input circuits for receiving two input voltages. The input voltages are further divided into four subvoltages which are then processed by a pair of differential amplifiers. The output differential currents from the differential amplifiers are further processed respectively by two current- summing circuits. The potential difference between the outputs of these two current-summing circuits is then fed to a bias circuit which, in response to the input potential difference, generates a floating bias. An output circuit takes the floating bias as input to thereby generate an output voltage which is regarded as the output of the operational amplifier. In addition to two cascaded-Miller compensation circuits, two mixed compensation circuits are used to perform a compensation on the output voltage and feed the compensated voltage back to the differential amplifiers. This allows the offset voltage to be subject to minimum fluctuations. Moreover, it allows improved bandwidth with increased unit gain as well as improved slew rate and phase margin.