Multiple stage operational amplifier for portable apparatus with low supply voltage, has transconductor input, intermediate stage and output stage, with Miller capacitances to intermediate stage

摘要

The operational amplifier comprises an input transconductor stage (110) with differential inputs (112-,112+) and an output (114). There is an output stage (150), and an intermediate stage (120,130) between input and output stages. There is a first Miller capacitor (162) connected between the input and output of intermediate stage. A second Miller capacitance (160) is connected between the intermediate stage input and the output of the following stage. The bipolar transistor of each stage has a base forming an input of that stage, and a collector forming an output of the stage. The amplifier circuit also comprises a current mirror (200) with a driver branch (202) and a slave branch (204), the driver branch connected between the output terminal (156) via a copy capacitor (230), and the driven branch is connected to the output of intermediate stage (120) equipped with the first Miller capacitor. The ratio of copy currents between the two branches of current mirror (200) and the value of copy capacitance are chosen so to deduct a current substantially equal to the current traversing the second Miller capacitor (160). The ratio of values of the second Miller capacitance and the copy capacitance is substantially equal to the ratio of copy currents. The ratio of copy currents is substantially equal to unity, and the value of the copy capacitance is substantially equal to that of the second Miller capacitance.