Class ab folded cascode stage and method for low noise, low power, low-offset operational amplilier
First Claim
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1. An operational amplifier circuit comprising:
- (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 i. 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, ii. 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;
(c) 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 conducting a voltage representative of a common mode input voltage of the second stage, and first and second compensation capacitors coupled to the first and second inputs, respectively, of the output stage; and
(d) a bias circuit for biasing control terminals of the first, second, third, and fourth controlled current sources in response to the voltage conducted by the tail current conductor.
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Abstract
An amplifier circuit includes an pair of input transistors, the drains of which are connected to emitters of first and second cascode transistors. First and second controlled current sources are connected to the emitters of the first and second cascode transistors, respectively, and third and fourth controlled current sources are connected to the collectors thereof. A bias circuit controls the 4 controlled current sources in response to the emitter voltage of a pair of input transistors of an output stage the inputs of which are connected to the drains of the first and second cascode transistors.
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Citations
20 Claims
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1. An operational amplifier circuit comprising:
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(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 i. 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, ii. 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;
(c) 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 conducting a voltage representative of a common mode input voltage of the second stage, and first and second compensation capacitors coupled to the first and second inputs, respectively, of the output stage; and
(d) a bias circuit for biasing control terminals of the first, second, third, and fourth controlled current sources in response to the voltage conducted by the tail current conductor. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. 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:
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(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. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19)
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20. 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.
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Specification