# US 7,411,452 B2 · Low DC power rail-to-rail buffer amplifier for liquid crystal display application

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- Number: US 7,411,452 B2
- Document ID: US7411452B2
- Status: Granted
- Kind: B2
- Application number: 11/559,395
- Filed: 2006-11-13
- Published: 2007-06-07
- Granted: 2008-08-12
- Earliest claimed: 2005-12-07
- Assignees: New Vision Micro Inc.
- Inventors: Chih-Wen Lu; Peter H. Xiao
- Web page: https://patentgrep.com/patent/US7411452B2

## CPC

- H03F 3/45: Differential amplifiers (differential sense amplifiers G11C7/062)
- H03F: AMPLIFIERS

## Abstract

A low DC power rail-to-rail buffer amplifier is suitable for liquid crystal display application. The buffer amplifier comprises: a dc bias stage, a pair of complementary differential amplifiers, a second stage amplifier, and output stage. The dc bias stage is used for the dc bias of the differential amplifiers and the second stage to obtain a rail-to-rail operation. This buffer amplifier can be operated in the power supply ranges of 5.5 V to the breakdown voltage. Simulated results show that the circuit draws only 5.3 μA static current.

## Claims

### Claim 1

A buffer amplifier, comprising:
a DC bias stage;
two complementary differential amplifiers;
a second stage amplifier coupled to the pair of complementary differential amplifiers and the DC bias stage; and
an output stage coupled to the second stage amplifier;
wherein the second stage amplifier comprises
a first set of bias transistors, M11 and M12;
a second set of bias transistors, M13 and M14;
the two sets of bias transistors forming a bi-circuit:
wherein the M11 and M12 bias transistors are coupled to a transistor M15, which is coupled to a transistor M17;
wherein the M13 and M14 bias transistors are coupled to transistor M16, which is coupled to a transistor M18;
wherein in a stable state, the bias transistors are biased from the bias circuit and the current from the differential amplifiers do not flow into the bias transistors, the M16 M17 transistors are in the triode region and output transistors of the output stage consume no power.

### Claim 2 (depends on 1)

The buffer amplifier of claim 1, wherein the differential amplifiers each comprises
a differential pair that is biased by a constant current source, and
a current mirror that actively loads the differential pair.

### Claim 3 (depends on 2)

The buffer amplifier of claim 2, wherein the differential pairs of the differential amplifiers have substantially the same bias current.

### Claim 4 (depends on 1)

The buffer amplifier of claim 1, wherein the second stage amplifier operates under a rail-to-rail input from the DC bias stage.

### Claim 5 (depends on 1)

The buffer amplifier of claim 1, wherein currents from the differential amplifiers do not enter the second stage amplifier when the buffer amplifier is in a stable state.

### Claim 6 (depends on 1)

The buffer amplifier of claim 1, wherein the output stage is coupled to a colunm line of a liquid crystal display screen.

### Claim 7 (depends on 1)

The buffer amplifier of claim 1, wherein when input voltage is increased, gate voltages of transistors M15-M18 increase, thereby causing M16 to remain in the triode region an M17 to enter the saturation region.

### Claim 8 (depends on 7)

The buffer amplifier of claim 7, wherein the output stage comprises a M19 and a M20 transistor,
wherein a decrease in drain voltage of the M17 transistor turns on the M19 transistor, which charges an output node of the output stage;
wherein the M20 transistor is in the cut off range when the M19 transistor charges the output node.

### Claim 9 (depends on 8)

The buffer amplifier of claim 8, wherein when the voltage difference between input and output is almost zero, the M19 transistor ceases charging the output node.

### Claim 10 (depends on 9)

The buffer amplifier of claim 9, wherein when the input voltage decreases, the gate voltage of the M20 transistor is increased and discharges an output load until the output voltage is almost equal to the input voltage.

### Claim 11 (depends on 1)

An LCD driver incorporating the buffer amplifier of claim 1.

### Claim 12 (depends on 11)

An LCD incorporating the LCD driver of claim 11.