Operational amplifier

Abstract

An operational amplifier according to an embodiment of the invention includes: a control signal input terminal receiving a digital control signal from an external device; first and second transistors as a differential pair of a differential amplifier circuit; a constant current circuit supplying a predetermined current to the differential pair; a first resistor provided between the constant current circuit and the first transistor and involving a first potential difference; and a second resistor provided between the constant current circuit and the second transistor and involving a second potential difference, the operational amplifier changing a resistance value ratio between the first resistor and the second resistor in accordance with the control signal input to the control signal input terminal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an operational amplifier according to a first embodiment of the present invention and its peripheral circuit;

FIG. 2 is a circuit diagram of the operational amplifier of the first embodiment;

FIG. 3 is a circuit diagram of an operational amplifier according to a second embodiment of the present invention;

FIG. 4 is a circuit diagram of an operational amplifier according to a third embodiment of the present invention; and

FIG. 5 is a circuit diagram of a conventional operational amplifier.

Claims

1. An operational amplifier, comprising: a control signal input terminal receiving a digital control signal from an external device;first and second transistors as a differential pair of a differential amplifier circuit;a constant current circuit supplying a predetermined current to the differential pair;a first resistor provided between the constant current circuit and the first transistor and involving a first potential difference; anda second resistor provided between the constant current circuit and the second transistor and involving a second potential difference,the operational amplifier changing a resistance value ratio between the first resistor and the second resistor in accordance with the control signal input to the control signal input terminal.

2. The operational amplifier according to claim 1, wherein the first resistor and the second resistor are connected in series between the first and second transistors, andthe first resistor is a resistor connected between the constant current circuit supplying a current to the differential pair and the first transistor, and the second resistor is a resistor connected between the constant current circuit and the second transistor.

3. The operational amplifier according to claim 1, wherein the constant current circuit includes a plurality of constant current circuits, and the first and second resistors include a plurality of resistor, the plurality of constant current circuits are individually connected to nodes where the plurality of resistor is divided into a predetermined resistance ratio, and at least one of the plurality of constant current circuits supplies the predetermined current to the differential pair based on the control signal.

4. The operational amplifier according to claim 1, wherein the first and second potential differences occur across the first resistor and the second resistor in accordance with a current value output from the constant current circuit and resistance values of the first resistor and the second resistor.

5. The operational amplifier according to claim 1, wherein the operational amplifier changes a resistance value ratio between the first resistor and the second resistor in accordance with the control signal sent as serial data.

6. The operational amplifier according to claim 1, wherein the operational amplifier changes a resistance value ratio between the first resistor and the second resistor in accordance with the control signal sent as parallel data.

7. An operational amplifier, comprising: a control signal input terminal receiving a digital control signal from an external device;first and second transistors as a differential pair of a differential amplifier circuit;a first resistor involving a first potential difference in accordance with an amount of current flowing through the first transistor; anda second resistor involving a second potential difference in accordance with an amount of current flowing through the second transistor,the operational amplifier changing a resistance value ratio between the first resistor and the second resistor in accordance with the control signal input to the control signal input terminal.

8. The operational amplifier according to claim 7, wherein the first resistor includes resistors connected in series between the first transistor and the first potential, and the second resistor includes resistors connected in series between the second transistor and the first potential.

9. The operational amplifier according to claim 7, further comprising: a plurality of switch circuits connected to nodes where resistors are divided into the first resistor and the second resistor at a predetermined resistance ratio,wherein the plurality of switch circuits are turned on/off based on the control signal to change a resistance value ratio between the first resistor and the second resistor.

10. An operational amplifier, comprising: first and second transistors as a differential pair of a differential amplifier circuit; anda control unit connected between one terminal of the first transistor and one terminal of the second transistor with a potential of the one terminal of the first transistor set as a first potential and a potential of the one terminal of the second transistor set as a second potential.

11. The operational amplifier according to claim 10, wherein the control unit includes a plurality of resistor connected between the one terminal of the first transistor and the one terminal of the second transistor, and a constant current circuit supplying a current to the first and second transistors, and the constant current circuit supplies a current to the differential pair from a node where resistors are divided into the plurality of resistor at a predetermined resistance ratio in accordance with the control signal.

12. The operational amplifier according to claim 11, wherein the first potential is set based on a resistance value of the first resistor connected between a node connected with the constant current circuit and the one terminal of the first transistor and a current flowing through the first transistor, the second potential is set based on a resistance value of the second resistor connected between the node connected with the constant current circuit and the one terminal of the second transistor and a current flowing through the second transistor.

13. The operational amplifier according to claim 10, wherein the control unit includes a first resistor connected with the one terminal of the first transistor, a second resistor connected with the one terminal of the second transistor, and a plurality of switch circuits connected to nodes where resistors are divided into the first resistor and the second resistor at a predetermined resistance ratio, the plurality of switch circuits are turned on/off in accordance with the control signal to change a resistance value ratio between the first resistor and the second resistor.

14. The operational amplifier according to claim 13, wherein the first potential is set based on a resistance value of the first resistor and a current flowing through the first transistor, and the second potential is set based on a resistance value of the second resistor and a current flowing through the second transistor.