Class ab folded cascode stage and method for low noise, low power, low-offset operational amplilier

A method of operating an amplifier circuit including (1) an input stage including first and second input transistors coupled to a first tail current source and to first and second load devices, respectively, and (2) a folded cascode circuit including first and second cascode transistors each having a first electrode, a second electrode, and a control electrode, the first electrode of the first cascode transistor being coupled by a first conductor to the first input transistor and the first load device, the first electrode of the second cascode transistor being coupled by a second conductor to the second input transistor and the second load device, the control electrodes being coupled to a first bias voltage, the method comprising:

• (a) providing a first controlled current source coupled between the first electrode of the first cascode transistor and a first reference voltage, a second controlled current source coupled between the first electrode of the second cascode transistor and the first reference voltage, and also providing a third controlled current source coupled between the second electrode of the first cascode transistor and a second reference voltage, and a fourth controlled current source coupled between the second electrode of the second cascode transistor and the second reference voltage;
• (b) providing a second stage having first and second inputs coupled to the second electrodes of the first and second cascode transistors, respectively, an output conductor for conducting an output signal, and a tail current conductor coupled to a second tail current source, the tail current conductor;
• and
• (c) biasing control terminals of the first, second, third, and fourth controlled current sources by means of a bias circuit in response to a signal conducted by the tail current conductor.

An amplifier circuit including

• (a) an input stage including first and second input transistors coupled to a first tail current source and to first and second load devices, respectively;
• (b) a folded cascode circuit including first and second cascode transistors each having a first electrode, a second electrode, and a control electrode, the first electrode of the first cascode transistor being coupled by a first conductor to the first input transistor and the first load device, the first electrode of the second cascode transistor being coupled by a second conductor to the second input transistor and the second load device, the control electrodes being coupled to a first bias voltage;
• (c) means for providing a first controlled current source coupled between the first electrode of the first cascode transistor and a first reference voltage, a second controlled current source coupled between the first electrode of the second cascode transistor and the first reference voltage, a third controlled current source coupled between the second electrode of the first cascode transistor and a second reference voltage, and a fourth controlled current source coupled between the second electrode of the second cascode transistor and the second reference voltage;
• (d) output means having first and second inputs coupled to the second electrodes of the first and second cascode transistors, respectively, an output conductor for conducting an output signal, and a tail current conductor coupled to a second tail current source, the tail current conductor;
• and
• (e) means for biasing control terminals of the first, second, third, and fourth controlled current sources in response to a signal conducted by the tail current conductor.