Eliminating complex conjugate poles in two-stage operational amplifiers with current buffer Miller compensation

This paper describes conditions and simple circuit modifications that eliminate undesired complex conjugate poles in two-stage operational amplifiers that are based on current buffer Miller compensation. The complex conjugate poles have been the main problem of this frequency compensation topology since its first introduction more than 30 years ago. It is shown that a resistor added between the second stage and the amplifier output creates a zero that improves stability and reduces the circuit to a second-order system. This greatly simplifies the design of an otherwise complex topology and at the same time retains the valued benefits of current buffer based frequency compensation. In addition, for the case where a short channel transistor is used for the current buffer, conditions that create sufficient phase margin for the inner loop of the amplifier without the nulling resistor are formulated.