Operational amplifier and method for operating an operational amplifier

Patent metadata

Field	Value
Publication number	US-12301188-B2
Application number	US-202217813485-A
Country	US
Kind code	B2
Filing date	Jul 19, 2022
Priority date	Jan 20, 2020
Publication date	May 13, 2025
Grant date	May 13, 2025

Abstract

The present invention relates to an operational amplifier, including: a symmetrical differential amplifier; a local common mode feedback circuit coupled to the symmetrical differential amplifier; a tail current source circuit including at least one first transistor and a second transistor and a current source resistor. The tail current source circuit is configured to adjust a control voltage of the first transistor by using the second transistor such that a predetermined reference current flows through a load path of the first transistor.

First claim

The invention claimed is:

1. Operational amplifier, comprising:
a symmetrical differential amplifier;
a local common mode feedback circuit coupled to the symmetrical differential amplifier;
a tail current source circuit comprising at least one first transistor and a second transistor and a current source resistor,
wherein the tail current source circuit is configured to adjust a control voltage of the first transistor by using the second transistor such that a predetermined reference current flows through a load path of the first transistor, wherein a load path of the transistor is a path between a sink and a source of the transistor,
wherein a control terminal of the second transistor is coupled to a sink terminal of the first transistor, and wherein a source terminal of the second transistor is coupled to the current source resistor,
wherein the control voltage of the first transistor and a voltage applied across the current source resistor are correlated with each other;
wherein a tail current of the symmetrical differential amplifier is based on a current flow through a load path of the second transistor and
wherein the current flow through the load path of the second transistor is based on a current flow through the current source resistor,
wherein the tail current source circuit comprises a third transistor between the current source resistor and the control terminal of the first transistor,
wherein a source terminal of the first transistor and one end of the current source resistor are both connected to a ground of the operational amplifier, wherein a sink terminal of the second transistor is coupled to the symmetrical differential amplifier,
wherein a sink terminal of the third transistor is coupled to the control terminal of the first transistor, a source terminal of the third transistor is coupled to the current source resistor and the source terminal of the second transistor, wherein the sink terminal of the third transistor receives a current flow originating from the same source as the reference current, and the source terminal of the second transistor, and a control terminal of the third transistor is coupled to the control terminal of the first transistor to counteract, in particular to reduce, a voltage drop across the current source resistor, wherein the local common mode feedback circuit comprises at least two resistors and the current source resistor is selected of a same type as the resistors of the local common mode feedback circuit, wherein of the same type means that the current source resistor comprises the same temperature coefficients and/or the same process variations and/or the same orientations in the geometrical configuration of the operational amplifier as the resistors of the local common mode feedback circuit.

2. Operational amplifier according to claim 1 , wherein the control voltage of the first transistor and the voltage applied across the current source resistor are selected in a predetermined relation to one another, such that the control voltage of the first transistor and the voltage applied across the current source resistor are equal or differ by a gate-source voltage of the third transistor.

3. Operational amplifier according to claim 1 , wherein the third transistor is coupled to the first transistor and the current source resistor to control a voltage drop across the current source resistor and/or to compensate temperature variations of other devices of the operational amplifier, in particular the first transistor.

4. Operational amplifier according to claim 1 , wherein the tail current of the symmetrical differential amplifier is determined by the current flow through the load path of the second transistor and is in particular equal to the same.

5. Operational amplifier according to claim 1 , wherein at least part of the current flowing through the current source resistor flows through the load path of the third transistor or wherein at least the predetermined reference current flows through the load path of the third transistor.

6. Operational amplifier according to claim 1 , wherein the symmetrical differential amplifier is replaced by a normal operational amplifier.

7. Method for operating an operational amplifier, in particular according to claim 1 , the method comprising:
providing a symmetrical differential amplifier, a common mode feedback circuit and a tail current source circuit;
coupling the local common mode feedback circuit to the symmetrical differential amplifier;
coupling the tail current source circuit to the local common mode feedback circuit, wherein the tail current source circuit comprises at least a first transistor and a second transistor and a current source resistor, wherein the tail current source circuit is configured to adjust a control voltage of the first transistor by using the second transistor, such that a predetermined reference current flows through a load path of the first transistor, wherein a load path of the transistor is a path between a sink and a source of the transistor,
wherein a control terminal of the second transistor is coupled to a sink terminal of the first transistor, and wherein a source terminal of the second transistor is coupled to the current source resistor,
wherein the control voltage of the first transistor and a voltage applied across a current source resistor are correlated with each other;
wherein a tail current of the symmetrical differential amplifier is based on a current flow through a load path of the second transistor and
wherein the current flow through the load path of the second transistor is based on a current flow through the current source resistor,
wherein the tail current source circuit comprises a third transistor between the current source resistor and the control terminal of the first transistor,
wherein a source terminal of the first transistor and one end of the current source resistor are both connected to a ground of the operational amplifier,
wherein a sink terminal of the second transistor is coupled to the symmetrical differential amplifier,
wherein a sink terminal of the third transistor is coupled to the control terminal of the first transistor, a source terminal of the third transistor is coupled to the current source resistor and the source terminal of the second transistor, wherein the sink terminal of the third transistor receives a current flow originating from the same source as the reference current and a control terminal of the third transistor is coupled to the control terminal of the first transistor to allow reducing a voltage drop across the current source resistor
wherein the local common mode feedback circuit comprises at least two resistors and the current source resistor is selected of a same type as the resistors of the local common mode feedback circuit, wherein of the same type means that the current source resistor comprises the same temperature coefficients and/or the same process variations and/or the same orientations in the geometrical configuration of the operational amplifier as the resistors of the local common mode feedback circuit.

8. Method according to claim 7 , wherein an operating point of the third transistor is selected such that a temperature dependency of the current source resistor is compensated during operation of the operational amplifier, in that a current flowing through the third transistor originates from the same current source as the reference current, wherein the same variations can occur in both currents.

9. Method according to claim 7 , wherein the current flow through the devices of the tail current source circuit is regulated such that variations, in particular temperature and/or process variations of the local common mode feedback circuit a

Assignees

• Hahn Schickard Ges Fuer Angewandte Forschung E V

Inventors

• Taschwer Armin
• Maurer Michael
• Kuderer Markus

Classifications

• H03F2203/45462

  the CSC comprising a cascode circuit · CPC title

• H03F2203/45286

  the temperature dependence of a differential amplifier being controlled · CPC title

• H03F3/45183

  Long tailed pairs (H03F3/4521, H03F3/45237 take precedence) · CPC title

• H03F1/30 Primary

  Modifications of amplifiers to reduce influence of variations of temperature or supply voltage {or other physical parameters (in differential amplifiers H03F3/45479)} · CPC title

• G05F3/262

  using field-effect transistors only · CPC title