Operational amplifier with class AB output

Patent metadata

Field	Value
Publication number	US-9871494-B2
Application number	US-201615229756-A
Country	US
Kind code	B2
Filing date	Aug 5, 2016
Priority date	Aug 7, 2015
Publication date	Jan 16, 2018
Grant date	Jan 16, 2018

Abstract

An operational amplifier includes an output stage, an input stage, a first auxiliary amplifier, and a second auxiliary amplifier. The output stage includes a first output transistor and a second output transistor. The input stage is configured to drive the output stage. The first auxiliary amplifier is coupled to an output of the input stage and to an input of the first output transistor. The first auxiliary amplifier is configured to bias the first output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the first output transistor. The second auxiliary amplifier is coupled to the output of the input stage and to an input of the second output transistor. The second auxiliary amplifier is configured to bias the second output transistor for class AB operation, and to isolate the input stage from a bias voltage applied to the second output transistor.

First claim

What is claimed is:

1. An operational amplifier, comprising:
an output stage comprising a first output transistor and a second output transistor;
an input stage configured to drive the output stage;
a first auxiliary amplifier coupled to an output of the input stage and an input of the first output transistor, the first auxiliary amplifier configured to bias the first output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the first output transistor, the first auxiliary amplifier comprising a first input transistor and a second input transistor coupled as a differential pair, wherein an input terminal of the first input transistor is coupled to the output of the input stage, and an input terminal of the second input transistor is connected to a predetermined DC voltage; and
a second auxiliary amplifier coupled to the output of the input stage and an input of the second output transistor, the second auxiliary amplifier configured to bias the second output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the second output transistor.

2. The operational amplifier of claim 1 , wherein the first auxiliary amplifier further comprises a third input transistor configured as a diode, wherein the third input transistor is coupled to a reference voltage and coupled, via a resistor, to an output of the first auxiliary amplifier, to set a quiescent output current of the first output transistor.

3. The operational amplifier of claim 2 , wherein the first auxiliary amplifier provides gain as a function of a transconductance of the first input transistor, a transconductance of the second input transistor, and a sum of resistance of the resistor and reciprocal of transconductance of the third input transistor.

4. The operational amplifier of claim 1 , wherein the second auxiliary amplifier comprises a first input transistor and a second input transistor coupled as a differential pair, wherein an input terminal of the first input transistor is coupled to the output of the input stage, and an input terminal of the second input transistor is connected to a predetermined DC voltage.

5. The operational amplifier of claim 4 , wherein the second auxiliary amplifier further comprises a third input transistor configured as a diode, wherein the third input transistor is coupled to a power supply voltage and coupled, via a resistor, to an output of the second auxiliary amplifier, to set a quiescent output current of the second output transistor.

6. The operational amplifier of claim 5 , wherein the first auxiliary amplifier provides gain as a function of transconductance of the first input transistor and the second input transistor, and a sum of resistance of the resistor and reciprocal of transconductance of the third input transistor.

7. The operational amplifier of claim 1 , wherein transconductance of the output stage is approximately a sum of:
a product of gain of the first auxiliary amplifier and transconductance of the first output transistor; and
a product of gain of the second auxiliary amplifier and transconductance of the second output transistor.

8. The operational amplifier of claim 7 , wherein the gain of the first auxiliary amplifier is less than ten and the gain of the second auxiliary amplifier is less than ten.

9. The operational amplifier of claim 1 , wherein headroom of the first stage is approximately one-half of the power supply voltage that provides supply voltage to the operational amplifier.

10. A multi-stage operational amplifier, comprising:
a first stage configured to amplify an input voltage;
a second stage configured to drive an output terminal of the multi-stage operational amplifier, wherein the second stage is coupled to an output of the first stage;
a first auxiliary amplifier coupled to the output of the input stage and to a first input of the second stage, the first auxiliary amplifier configured to:
generate a first bias voltage and apply the first bias voltage to the second stage for class AB operation; and
isolate the first stage from the first bias voltage; and

a second auxiliary amplifier coupled to the output of the input stage and to a second input of the second stage, the second auxiliary amplifier configured to:
generate a second bias voltage and apply the second bias voltage to the second stage for class AB operation; and
isolate the first stage from the second bias voltage,

wherein each of the first auxiliary amplifier and the second auxiliary amplifier comprises a first input transistor and a second input transistor coupled as a differential pair, and wherein in each of the first auxiliary amplifier and the second auxiliary amplifier an input terminal of the first input transistor is coupled to the output of the first stage, and an input terminal of the second input transistor is connected to a predetermined DC voltage.

11. The multi-stage operational amplifier of claim 10 , wherein:
the first auxiliary amplifier further comprises a third input transistor configured as a diode, wherein the third input transistor is coupled to a reference voltage and coupled, via a resistor, to an output of the first auxiliary amplifier; and
the second auxiliary amplifier further comprises a fourth input transistor configured as a diode, wherein the fourth input transistor is coupled to a power supply voltage and coupled, via a resistor, to an output of the second auxiliary amplifier.

12. The multi-stage operational amplifier of claim 11 , wherein a gain of each of the first auxiliary amplifier and the second auxiliary amplifier is a function of transconductance of the first input transistor and the second input transistor and a sum of resistance of the resistor and reciprocal of transconductance of one of the third input transistor or the fourth input transistor.

13. The multi-stage operational amplifier of claim 10 , wherein transconductance of the second stage is approximately a sum of:
a product of gain of the first auxiliary amplifier and transconductance of a low-side output transistor; and
a product of gain of the second auxiliary amplifier and transconductance of a high-side output transistor.

14. The multi-stage operational amplifier of claim 13 , wherein the gain of the first auxiliary amplifier is less than ten and the gain of the second auxiliary amplifier is less than ten.

15. The multi-stage operational amplifier of claim 10 , wherein the first auxiliary amplifier and the second auxiliary amplifier provide headroom in the first stage of approximately one-half of the power supply voltage that provides supply voltage to the multi-stage operational amplifier.

16. A class AB output operational amplifier, comprising:
an output stage comprising a high-side output transistor and a low-side output transistor;
an input stage configured to drive the output stage;
a first auxiliary amplifier coupled to an output of the input stage and an input of the low-side output transistor, the first auxiliary amplifier configured to bias the low-side output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the low-side output transistor, the first auxiliary amplifier comprising:
a first transistor and a second transistor coupled as a differential pair, wherein an input terminal of the first transistor is coupled to the output of the input stage, and an input terminal of the second transistor is connected to a predetermined DC voltage; and

a second auxiliary amplifier coupled to the output of the input stage and an input of the high-side outpu

Assignees

• Texas Instruments Inc

Inventors

• Gupta Amit Kumar
• Cao Peng

Classifications

• H03F3/211 Primary

  using a combination of several amplifiers (H03F3/60 takes precedence) · CPC title

• H03F3/3022

  CMOS common source output SEPP amplifiers (H03F3/3008 takes precedence) · CPC title

• H03F2203/45288

  Differential amplifier with circuit arrangements to enhance the transconductance · CPC title

• H03F3/45192

  Folded cascode stages · CPC title