GM-controlled current-isolated indirect-feedback instrumentation amplifier

• US 6,559,720 B1
• Filed: 10/26/2001
• Issued: 05/06/2003
• Est. Priority Date: 10/26/2001
• Status: Expired due to Term

• Current Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Shahi, Behzad, Huijsing, Johan Hendrik
• Primary Examiner(s): Choe, Henry
• Application Number:
  US10/014,124
• Time in Patent Office: 557 Days
• Field of Search: 330/253, 330/258, 330/292, 330/259, 330/260, 330/261
• US Class Current: 330/253
• CPC Class Codes: H03F 2200/261 Amplifier which being suitable for instrumentation applications
• CPC Class Codes: H03F 2203/45352 the AAC comprising a combination of a plurality of transistors, e.g. Darlington coupled transistors
• CPC Class Codes: H03F 2203/45702 the LC comprising two resistors
• CPC Class Codes: H03F 3/45197 Pl types H03F3/45224, H03F3/45251 take precedence
• CPC Class Codes: H03F 3/4565 Controlling the common source circuit of the differential amplifier

Abstract

Constant GM models and GM-controlled current-isolated indirect-feedback instrumentation amplifiers. The constant GM models provide a transconductance that is proportional to the inverse of a resistance in the constant GM model circuits over a wide range of common mode and differential mode inputs. Current-isolated indirect-feedback instrumentation amplifiers using constant GM models to track the common mode and differential inputs of, and provide the tail currents for, the differential input pair in the transconductance amplifiers greatly enhance the instrumentation amplifier performance in numerous ways.

Claims

1. A single constant GM model comprising:

first and second transistors, each having first and second terminals and a control terminal, the current though the first and second terminals of each transistor being responsive to the voltage between the respective control terminal and the first terminal, the first transistor being N times the size of the second transistor;

a resistor;

a first controllable current source;

a second current source; and

,

an amplifier having a differential input coupled to the second terminals of the first and second transistors and an output coupled to the first controllable current source;

the first controllable current source being coupled to the first terminal of the second transistor, and through the resistor to the first terminal of the first transistor;

the control terminals of the first and second transistors being coupled to a common mode input;

the amplifier amplifying the difference in current from the second terminals of the first and second transistors to control the first controllable current source;

the first controllable current source being coupled to the second current source to mirror the current of the first controllable current source to the second current source to provide a constant GM model output current.

2. A single constant GM model comprising:

first and second transistors, each having first and second terminals and a control terminal, the current though the first and second terminals of each transistor being responsive to the voltage between the respective control terminal and the first terminal, the first transistor being N times the size of the second transistor;

a resistor;

a first controllable current source;

a second current source; and

,

an amplifier having a differential input coupled to the second terminals of the first and second transistors and an output coupled to the first controllable current source;

the first controllable current source being coupled to the first terminal of the second transistor, and through the resistor to the first terminal of the first transistor;

the control terminals of the first and second transistors being coupled to a differential input;

the amplifier amplifying the difference in current from the second terminals of the first and second transistors to control the first controllable current source;

the first controllable current source being coupled to the second current source to mirror the current of the first controllable current source to the second current source to provide a constant GM model output current.

3. A differential constant GM model comprising:

first, second, third and fourth transistors, each having first and second terminals and a control terminal, the current though the first and second terminals of each transistor being responsive to the voltage between the respective control terminal and the first terminal, the first and third transistors being N times the size of the second and fourth transistors;

a first resistor;

a first controllable current source;

a second current source; and

,

a feedback differential amplifier having first and second inputs, the first input being coupled to the second terminals of the first and third transistors, the second input being coupled to the second terminals of the second and fourth transistors, the amplifier having an output coupled to the first controllable current source;

the first controllable current source being coupled to the first terminals of the second and fourth transistors, and through the first resistor to the first terminals of the first and third transistors;

the control terminals of the first and second transistors being coupled to one input of the differential constant GM model, and the control terminals of the third and fourth transistors being coupled to another input of the differential constant GM model;

the amplifier amplifying the difference in current from the second terminals of the first and third transistors, and the second and fourth transistors, to control the first controllable current source;

the first controllable current source being coupled to the second current source to mirror the current of the first controllable current source to the second current source to provide a differential constant GM model output current.

4. The differential constant GM model of claim 3 further comprising second through fifth resistors, the first controllable current source being coupled to the first terminals of the second and fourth transistors through the third and fifth resistors, respectively, and through the first resistor to the first terminals of the first and third transistors through the second and fourth resistors, respectively.

5. The differential constant GM model of claim 3 wherein the first controllable current source and the second current source comprise fifth and sixth transistors, respectively.

6. An instrumentation amplifier having an instrumentation amplifier differential input and an instrumentation amplifier output comprising:

first and second MOS amplifiers having differential inputs;

first and second differential transconductance models, each having two MOS differential pairs coupled to a differential input to provide a tail current to operate a MOS differential pair over a range of common mode and differential inputs at a fixed transconductance;

an output amplifier;

the instrumentation amplifier differential input being coupled to the differential input of the first MOS amplifier and the differential input of the first differential transconductance model;

the differential input of the second MOS amplifier and the differential input of the second differential transconductance model being coupled to the instrumentation amplifier output;

the first and second differential transconductance models being coupled to provide tail currents to the first and second MOS amplifiers, respectively;

the output amplifier being coupled to provide an instrumentation amplifier output responsive to the difference in outputs of the first and second MOS differential amplifiers;

each differential transconductance model comprising;

first, second, third and fourth transistors, each having first and second terminals and a control terminal, the current though the first and second terminals of each transistor being responsive to the voltage between the respective control terminal and the first terminal, the first and third transistors being N times the size of the second and fourth transistors;

a first resistor;

a first controllable current source; and

,

a feedback differential amplifier having first and second inputs, the first input being coupled to the second terminals of the first and third transistors, the second input being coupled to the second terminals of the second and fourth transistors, the amplifier having an output coupled to the first controllable current source;

the first controllable current source being coupled to the first terminals of the second and fourth transistors, and through the first resistor to the first terminals of the first and third transistors;

the control terminals of the first and second transistors being coupled to the one differential input of the differential transconductance model, and the control terminals of the third and fourth transistors being coupled to the other input of the differential transconductance model;

the amplifier amplifying the difference in current from the second terminals of the first and third transistors, and the second and fourth transistors, to control the controllable current source;

the first controllable current source being coupled to the second current source to mirror the current of the first controllable current source to the second current source to provide a differential constant GM model output current;

each differential constant GM model output current being coupled to provide the tail current for the respective first and second differential amplifiers.

7. The instrumentation amplifier of claim 6 wherein each differential constant GM model is further comprised of second through fifth resistors, the first controllable current source being coupled to the first terminals of the second and fourth transistors through the third and fifth resistors, respectively, and through the first resistor to the first terminals of the first and third transistors through the second and fourth resistors, respectively.

8. An instrumentation amplifier having an instrumentation amplifier differential input and an instrumentation amplifier output comprising:

first and second MOS amplifiers having differential inputs;

first and second single transconductance models, each having a MOS differential pair coupled to a differential input to provide a tail current to operate a MOS differential pair over a range of common mode and differential inputs at a fixed transconductance;

an output amplifier;

the instrumentation amplifier differential input being coupled to the differential input of the first MOS amplifier and the differential input of the first single transconductance model;

the differential input of the second MOS amplifier and the differential input of the second single transconductance model being coupled to the instrumentation amplifier output;

the first and second single transconductance models being coupled to provide tail currents to the first and second MOS amplifiers, respectively;

the output amplifier being coupled to provide an instrumentation amplifier output responsive to the difference in outputs of the first and second MOS differential amplifiers;

each single transconductance model comprises;

first and second transistors, each having first and second terminals and a control terminal, the current though the first and second terminals of each transistor being responsive to the voltage between the respective control terminal and the first terminal, the first transistor being N times the size of the second transistor;

a first resistor;

a first controllable current source;

a second current source; and

,

an amplifier having a differential input coupled to the second terminals of the first and second transistors and an output coupled to the first controllable current source;

the first controllable current source being coupled to the first terminal of the second transistor, and through the first resistor to the first terminal of the first transistor;

the control terminals of the first and second transistors being coupled to the differential input of the respective single transconductance model;

the amplifier amplifying the difference in current from the second terminals of the first and second transistors to control the first controllable current source;

the first controllable current source being coupled to the second current source to mirror the current of the first controllable current source to the second current source to provide a constant GM model output current;

each differential constant GM model output current being coupled to provide the tail current for the respective first and second differential amplifiers.