Device for balancing the rise and fall slew-rates of an operational amplifier
An amplifier includes a pair of transistors connected in a differential stage, and a bias current source connected to a common node of the differential stage. A slew-rate compensation circuit is configured to derive from the common node a dynamic compensation current during a phase in which the voltage of the common node varies.
1. An amplifier comprising:
a pair of transistors coupled in a differential stage including a common node;
a bias current source coupled to the common node of the differential stage; and
a slew-rate compensation circuit configured to derive from the common node a dynamic compensation current during a phase in which a voltage of the common node varies, the slew-rate compensation circuit comprising
a capacitor configured to replicate voltage variations of the common node on the capacitor and to generate the dynamic compensation current from a current flowing in the capacitor,
a follower transistor having a control terminal coupled to a control terminal of a first transistor of the differential pair and a first conduction terminal coupled to a terminal of the capacitor, and
a current mirror having an input coupled to the second conduction terminal of the follower transistor and an output coupled to the common node.
2. The amplifier of claim 1 , wherein the slew-rate compensation circuit comprises a current source coupled to the first conduction terminal of the follower transistor, and configured to produce a constant current greater than a peak compensation current.
3. An amplifier comprising:
a pair of transistors coupled at a common node;
a bias current source coupled to the common node;
a capacitor configured to replicate voltage variations of the common node on the capacitor and to generate a compensation current from a current flowing in the capacitor;
a follower transistor having a control terminal coupled to a control terminal of a first transistor of the pair and a first conduction terminal coupled to the capacitor; and
a current mirror having an input coupled to a second conduction terminal of the follower transistor and an output coupled to the common node.
4. The amplifier of claim 3 , further comprising a current source coupled to the first conduction terminal of the follower transistor, and configured to produce a constant current greater than a peak compensation current.
5. A method of making an amplifier, the method comprising:
coupling a pair of transistors in a differential stage at a common node;
coupling a bias current source to the common node of the differential stage; and
coupling a slew-rate compensation circuit to the common node to generate a dynamic compensation current during a phase in which a voltage of the common node varies, the slew rate compensation circuit comprising a capacitor, a follower transistor having a control terminal coupled to a control terminal of a first transistor of the pair and a first conduction terminal coupled to the capacitor, and a current mirror having an input coupled to a second conduction terminal of the follower transistor and an output coupled to the common node.
6. The method of claim 5 , wherein the capacitor is configured to replicate voltage variations of the common node on the capacitor and to generate the dynamic compensation current from a current flowing in the capacitor.
7. The method of claim 6 , wherein the slew-rate compensation circuit comprises a current source coupled to the first conduction terminal of the follower transistor, and configured to produce a constant current greater than a peak compensation current.
8. A method of operating an amplifier comprising a pair of transistors coupled in a differential stage at a common node, a bias current source coupled to the common node of the differential stage, and a slew rate compensation circuit comprising a capacitor, a follower transistor having a control terminal coupled to a control terminal of a first transistor of the pair and a first conduction terminal coupled to the capacitor, and a current mirror having an input coupled to a second conduction terminal of the follower transistor and an output coupled to the common node, the method comprising:
balancing slew rates in the amplifier by generating a dynamic compensation current during a phase in which a voltage of the common node varies using the slew rate compensation circuit.
9. The method of claim 8 , the method comprises replicating voltage variations of the common node on the capacitor to generate the dynamic compensation current from a current flowing in the capacitor.
10. The method of claim 9 , wherein the slew-rate compensation circuit comprises a current source coupled to the first conduction terminal of the follower transistor; and the method further comprises producing a constant current, with the current source, greater than a peak compensation current.