Operational amplifier
An operational amplifier includes a differential input stage that amplifies a differential input signal to generate an intermediate signal; an amplification stage including an output transistor that is connected between an output terminal and a fixed voltage line, and is driven according to the intermediate signal; and an assist circuit, wherein the assist circuit includes: a first transistor connected in parallel with the output transistor; and a drive circuit that drives the first transistor according to a gate voltage of the output transistor.
1 . An operational amplifier comprising:
a differential input stage that amplifies a differential input signal to generate an intermediate signal;
an amplification stage including an output transistor that is connected between an output terminal and a fixed voltage line, and is driven according to the intermediate signal; and
an assist circuit,
wherein the assist circuit includes:
a first transistor connected in parallel with the output transistor; and
a drive circuit that drives the first transistor according to a gate voltage of the output transistor,
wherein the drive circuit includes a first current source and a second current source,
wherein one end of the first current source and one end of the second current source are connected to a gate of the first transistor, and the other end of the second current source is connected to the fixed voltage line.
2 . The operational amplifier of claim 1 , wherein the output transistor is a low-side transistor of an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and the fixed voltage line is a ground line,
wherein the first current source sources a first current according to the gate voltage of the output transistor to the gate of the first transistor, and the second current source sinks a second current from the gate of the first transistor.
3 . The operational amplifier of claim 2 , wherein the first current source includes:
a second transistor that includes a source connected to the ground line and is biased to flow a constant current;
a third transistor that includes a gate connected to a gate of the low-side transistor and a source connected to a drain of the second transistor; and
a current mirror circuit that returns a current flowing through the third transistor to source the current to the gate of the first transistor.
4 . The operational amplifier of claim 2 , wherein the second current source includes a fourth transistor that includes a source connected to the ground line and a drain connected to the gate of the first transistor, and is biased to flow a constant current.
5 . The operational amplifier of claim 2 , wherein the drive circuit further includes a fifth transistor that includes a gate connected to a gate of the low-side transistor and a source connected to the gate of the first transistor.
6 . The operational amplifier of claim 5 , wherein the drive circuit further includes a sixth transistor that includes:
a gate and a drain connected to a drain of the fifth transistor; and
a source connected to a power supply line.
7 . The operational amplifier of claim 1 , wherein the output transistor is a P-type high-side transistor, and the fixed voltage line is a power supply line, and
wherein the first current source sinks a first current according to the gate voltage of the output transistor from the gate of the first transistor, and the second current source sources a second current to the gate of the first transistor.
8 . The operational amplifier of claim 7 , wherein the first current source includes:
a second transistor that includes a source connected to the power supply line and is biased to flow a constant current;
a third transistor that includes a gate connected to a gate of the high-side transistor and a source connected to a drain of the second transistor; and
a current mirror circuit that returns a current flowing through the third transistor to sink the current from the gate of the first transistor.
9 . The operational amplifier of claim 7 , wherein the second current source includes a fourth transistor that includes a source connected to the power supply line and a drain connected to the gate of the first transistor, and is biased to flow a constant current.
10 . The operational amplifier of claim 7 , wherein the drive circuit further includes a fifth transistor that includes a gate connected to a gate of the high-side transistor and a source connected to the gate of the first transistor.
11 . The operational amplifier of claim 10 , wherein the drive circuit further includes a sixth transistor that includes:
a gate and a drain connected to a drain of the fifth transistor; and
a source connected to the ground line.
12 . An operational amplifier comprising:
a differential input stage that amplifies a differential input signal to generate an intermediate signal;
an amplification stage including a low-side transistor that is connected between an output terminal and a ground line, and is driven according to the intermediate signal; and
an assist circuit,
wherein the assist circuit includes:
a first transistor connected in parallel with the low-side transistor;
a second transistor that includes a source connected to the ground line and is biased to flow a constant current;
a third transistor that includes a gate connected to a gate of the low-side transistor and a source connected to a drain of the second transistor;
a current mirror circuit that returns a current flowing through the third transistor to source the current to a gate of the first transistor; and
a fourth transistor that includes a source connected to the ground line and a drain connected to the gate of the first transistor, and is biased to flow a constant current.
13 . The operational amplifier of claim 12 , wherein the assist circuit further includes:
a fifth transistor that includes a gate connected to the gate of the low-side transistor and a source connected to the gate of the first transistor; and
a sixth transistor that includes:
a gate and a drain connected to a drain of the fifth transistor; and
a source connected to a power supply line.
14 . An operational amplifier comprising:
a differential input stage that amplifies a differential input signal to generate an intermediate signal;
an amplification stage including a high-side transistor that is connected between an output terminal and a power supply line, and is driven according to the intermediate signal; and
an assist circuit,
wherein the assist circuit includes:
a first transistor connected in parallel with the high-side transistor;
a second transistor that includes a source connected to the power supply line and is biased to flow a constant current;
a third transistor that includes a gate connected to a gate of the high-side transistor and a source connected to a drain of the second transistor;
a current mirror circuit that returns a current flowing through the third transistor to sink the current from a gate of the first transistor; and
a fourth transistor that includes a source connected to the power supply line and a drain connected to the gate of the first transistor, and is biased to flow a constant current.
15 . The operational amplifier of claim 14 , wherein the assist circuit further includes:
a fifth transistor that includes a gate connected to the gate of the high-side transistor and a source connected to the gate of the first transistor; and
a sixth transistor that includes:
a gate and a drain connected to a drain of the fifth transistor; and
a source connected to a ground line.
16 . The operational amplifier of claim 1 , which is integrated on one semiconductor substrate.