Patent ID: 6531919
Filing Date: 2003-03-11
Classification: H03F

Abstract:
An operational amplifier input stage with a phase inversion prevention circuit, comprising:an input stage comprising: a first differential pair comprising first and second transistors, each of which has a control input and first and second current circuit terminals, the control inputs of said first and second transistors connected to receive a differential input signal which has a common mode voltage Vcm, said first current circuit terminals connected together at a first common mode node, said first pair arranged to conduct respective currents which vary with said differential input signal at first and second output terminals, respectively, when a tail current Itail1 is provided at said first common mode node, each of said first and second transistors having a respective intrinsic diode present between its control input and second current circuit terminal, and a phase inversion prevention circuit, comprising: a phase inversion detection circuit which detects when either of said intrinsic diodes is near a forward-biased condition, a second differential pair comprising third and fourth transistors, said third and fourth transistors being bipolar transistors having their bases connected to receive said differential input signal and their emitters connected together at a second common mode node, said second differential pair arranged to conduct respective currents which vary with said differential input signal at said first and second output terminals, respectively, when a tail current Itail2 is provided at said second common mode node, and a switching network arranged to connect said second common mode node to Itail2 when said phase inversion detection circuit detects that either of said intrinsic diodes is near a forward-biased condition and to connect said first common mode node to Itail1 otherwise.