# Patent

## Title
Operational amplifier circuit having digitally controllable output stage

## Abstract

An operational amplifier circuit includes a control stage and an output stage. The control stage includes an analog control stage for generating a first control signal according to an input signal and a digital control stage for generating a second control signal according to the input signal. The output stage includes a sourcing circuitry coupled to the analog control stage and a load for sourcing a first current to the load according to the first control signal, and a sinking circuitry coupled to the digital control stage and the load for sinking a second current from the load according to the second control signal.

## Background
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to an operational amplifier (op amp), and more particularly, to an operational amplifier circuit having a digitally controllable output stage.
2. Description of the Prior Art
The operational amplifier (op amp) is an essential circuit building block of universal importance. A reason for the popularity of the op amp is its versatility; a great amount of operations can be done with op amps. Normally, an operational amplifier circuit is used to drive a load according to an analog input signal. However, if a digital input signal is adopted, a digital-to-analog converter (DAC) must be set in the signal input end of the operational amplifier circuit to convert the digital input signal into an analog input signal. The operational amplifier circuit can then drive the load according to the analog input signal.
For instance, the liquid crystal display (LCD) source driver is a kind of device that needs to utilize an operational amplifier circuit including a DAC to drive a LCD display unit according to a digital input signal. FIG. 1 shows a schematic diagram of a conventional operational amplifier circuit 100 utilized in a LCD source driver. The operational amplifier circuit 100 shown in FIG. 1 includes a DAC 110, an analog control stage 120, and an output stage (which comprises five transistors T1 ̃T5). The LCD display unit driven by the operational amplifier circuit 100 is represented by a block “load 190” in FIG. 1.
The DAC 110 converts a digital input signal SIN of the LCD source driver into an analog input signal S′IN. The analog control stage 120 then generates a control signal VC according to the analog input signal S′IN. With the five transistors T1 ̃T5 of the output stage, currents are sourced to and sank from the load 190 according to the control signal VC, the load 190 is therefore driven by the operational amplifier circuit 100 according to the digital input signal SIN.
In the conventional operational amplifier circuit 100, gates of the second and fourth transistors T2 and T4 are coupled to a constant bias voltage VBIAS, hence the two transistors can be thought of as constant current sinks for providing bias currents. After the operational amplifier circuit 100 is properly installed and supply voltages are provided to the operational amplifier circuit 100, power will be continually consumed by the second and fourth transistors T2 and T4. In practice, the operational amplifier circuit 100 is not always under operation, even if supply voltages are provided to the operational amplifier circuit 100 properly. For example, if no digital code is inputted into the operational amplifier circuit 100 as the digital input signal SIN, the operational amplifier circuit 100 will not be under operation. However, when the operational amplifier circuit 100 is not under operation, power is still continually consumed by the second and fourth transistors T2 and T4. In other words, under the structure of the conventional operational amplifier circuit 100, some power is unnecessarily wasted.
SUMMARY OF INVENTION
One of many objectives of the claimed invention is to provide an operational amplifier circuit that reduces unnecessary power consumption.
According to the claimed invention, an operational amplifier circuit is disclosed. The operational amplifier circuit comprises a control stage and an output stage. The control stage includes an analog control stage for generating a first control signal according to an input signal and a digital control stage for generating a second control signal according to the input signal. The output stage includes a sourcing circuitry coupled to the analog control stage and a load for sourcing a first current to the load according to the first control signal, and a sinking circuitry coupled to the digital control stage and the load for sinking a second current from the load according to the second control signal.
According to the claimed invention, another operational amplifier circuit is disclosed. The operational amplifier circuit comprises a control stage and an output stage. The control stage includes an analog control stage for generating a first control signal according to an input signal and a digital control stage for generating a second control signal according to the input signal. The output stage includes a sinking circuitry coupled to the analog control stage and a load for sinking a first current from the load according to the first control signal, and a sourcing circuitry coupled to the digital control stage and the load for sourcing a second current to the load according to the second control signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a schematic diagram of a conventional operational amplifier circuit.
FIG. 2 shows a schematic diagram of an operational amplifier circuit according to the present invention.
FIG. 3 shows a schematic diagram of another operational amplifier circuit according to the present invention.



DETAILED DESCRIPTION
Please refer to FIG. 2, which shows an operational amplifier circuit 200 according to a first embodiment of the present invention. In this embodiment, the operational amplifier circuit 200 is set inside a LCD source driver for driving a LCD display unit according to a digital input signal SIN. The LCD display unit is represent by a block “load 290” in FIG. 2. In other embodiments, however, the operational amplifier circuit of the present invention can also be set in other kind of device for driving a load according to a digital input signal or an analog input signal.
The operational amplifier circuit 200 shown in FIG. 2 comprises a control stage 210 and an output stage 260. The control stage 210 includes a DAC 220, a code comparator 230, and an analog control stage 240. The DAC 220 converts the digital input signal SIN of the LCD source driver into an analog input signal S′IN. The analog control stage 240 generates a first control signal V1 according to the analog input signal S′IN received from the DAC 220. The first control signal V1 is then utilized to control the value of a first current I1 sourced to the load 290. The code comparator 230 serves as a digital control stage of the control stage 210. It compares a reference code with codes corresponding to the input signal SIN and generates a second control signal V2 accordingly. The second control signal V2 is then utilized to control the value of a second current I2 sank from the load 290.
The output stage 260 comprises three transistors T1 ̃T3. The first transistor T1 serves as a sourcing circuitry of the output stage 260, which sources the first current I1 to the load 290 according to the first control signal V1. The second and third transistors T2 and T3 form a sinking circuitry of the output stage 260 for sinking the second current I2 from the load 290 according to the second control signal V2. In this embodiment, the second current I2 sank by the sinking circuitry comprises a bias current I3 provided by the second transistor T2 and an adjustable current I4 provided by the third transistor T3. Since the gate of the second transistor T2 is coupled to a bias voltage VBIAS, it serves as a first current sink for sinking the constant bias current I3 from the load 290. Since the gate of the third transistor T3 is coupled to the code comparator 230 for receiving the second control signal V2, it serves as a second current sink for sinking the adjustable current I4 from the load 290 according to the second control signal V2. In this embodiment, the third transistor T3 is a NMOS transistor having a control terminal coupled to the code comparator 230 for receiving the second control signal V2, a first terminal coupled to the load 290 for sinking the adjustable current I4 from the load 290, and a second terminal coupled to a low bias voltage VSSA. In other embodiments, the third transistor T3 can also be a PMOS transistor or other kind of transistor.
By comparing the reference code with codes of the input signal SIN, the code comparator 230 can determine whether the level of the output voltage VOUT should be abated accordingly. When it is determined that the level of the output voltage VOUT should be abated, the code comparator 230 turns the third transistor T3 on for a short period to increase the discharging speed of the output stage 260. When it is determined that the level of the output voltage VOUT should be raised or sustained, the code comparator 230 turns the third transistor T3 off. With the idea of utilizing the code comparator 230 to control the second current sink formed by the third transistor T3, a bias current path (such as the bias current path formed by the fourth transistor T4 of FIG. 1) is excluded. Hence, with the digital control scheme, power will not be continually consumed by the third transistor T3. When it is determined that the level of the output voltage VOUT should be raised or sustained, or when the operational amplifier circuit 200 is not under operation (ex. no code is inputted into the code comparator 230 as the digital input signal SIN), power will not be consumed by the third transistor T3. Therefore, unnecessary power consumption is reduced.
In the above-mentioned embodiment, a digital control stage is used to control a sinking circuitry of an operational amplifier circuit. In other embodiments, a digital control stage can also be used to control a sourcing circuitry of an operational amplifier circuit. FIG. 3 shows an operational amplifier circuit 300 according to a second embodiment of the present invention. Similar to the operational amplifier circuit 200 of the first embodiment, the operational amplifier circuit 300 of the second embodiment is also set inside a LCD source driver for driving a LCD display unit according to a digital input signal SIN. The LCD display unit is represent by a block “load 390” in FIG. 3. Please note that in other embodiments, the operational amplifier circuit of the present invention can also be set in other kind of device for driving a load according to a digital input signal or an analog input signal.
The operational amplifier circuit 300 shown in FIG. 3 comprises a control stage 310 and an output stage 360. The control stage 310 includes a DAC 320, a code comparator 330, and an analog control stage 340. The DAC 320 converts the digital input signal SIN of the LCD source driver into an analog input signal S′IN. The analog control stage 340 generates a first control signal V1 according to the analog input signal S′IN received from the DAC 220. The first control signal V1 is then utilized to control the value of a first current I1 sank from the load 390. The code comparator 330 serves as a digital control stage of the control stage 310. It compares a reference code with codes corresponding to the input signal SIN and generates a second control signal V2 accordingly. The second control signal V2 is then utilized to control the value of a second current I2 sourced to the load 390.
The output stage 360 comprises three transistors T1 ̃T3. The first transistor T1 serves as a sinking circuitry of the output stage 360, which sinks the first current I1 from the load 390 according to the first control signal V1. The second and third transistors T2 and T3 form a sourcing circuitry of the output stage 360 for sourcing the second current I2 to the load 390 according to the second control signal V2. In this embodiment, the second current I2 sourced from the sourcing circuitry includes a bias current I3 provided by the second transistor T2 and an adjustable current I4 provided by the third transistor T3. Since the gate of the second transistor T2 is coupled to a bias voltage VBIAS, it serves as a first current source for sourcing the constant bias current I3 to the load 390. Since the gate of the third transistor T3 is coupled to the code comparator 330 for receiving the second control signal V2, it serves as a second current source for sourcing the adjustable current I4 to the load 390 according to the second control signal V2. In this embodiment, the third transistor T3 is a PMOS transistor having a control terminal coupled to the code comparator 330 for receiving the second control signal V2, a first terminal coupled to the load 390 for sourcing the adjustable current I4 to the load 390, and a second terminal coupled to a high bias voltage VDDA. In other embodiments, the third transistor T3 can also be a NMOS transistor or other kind of transistor.
By comparing the reference code with codes of the input signal SIN, the code comparator 330 can determine whether the level of the output voltage VOUT should be raised. When it is determined that the level of the output voltage VOUT should be raised, the code comparator 330 turns the third transistor T3 on for a short period to increase the charging speed of the output stage 360. When it is determined that the level of the output voltage VOUT should be abated or sustained, the code comparator 330 turns the third transistor T3 off. With the idea of utilizing the code comparator 330 to control the second current source formed by the third transistor T3, a bias current path is excluded. Hence, with the digital control scheme, power will not be continually consumed by the third transistor T3. When it is determined that the level of the output voltage VOUT should be abated or sustained, or when the operational amplifier circuit 300 is not under operation (ex. no code is inputted into the code comparator 330 as the digital input signal SIN), power will not be consumed by the third transistor T3. Therefore, unnecessary power consumption is reduced.
In contrast to conventional operational amplifier circuits, the operational amplifier circuits disclosed by the embodiments of the present invention have digitally controllable output stages. Fewer bias current paths are required; hence, the operational amplifier circuits disclosed by the embodiments of the present invention will consume less power.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. An operational amplifier circuit comprising:
a control stage comprising:
an analog control stage for generating a first control signal according to an input signal; and
a digital control stage for generating a second control signal according to the input signal; and

an output stage comprising:
a sourcing circuitry coupled to the analog control stage and a load, for sourcing a first current to the load according to the first control signal; and
a sinking circuitry coupled to the digital control stage and the load, for sinking a second current from the load according to the second control signal, comprising:
a first current sink coupled to the load for sinking a bias current from the load; and
a second current sink coupled to the digital control stage and the load for sinking an adjustable current from the load according to the second control signal, wherein the second current comprises the bias current and the adjustable current.

2. The operational amplifier circuit of claim 1, wherein the digital control stage comprises a code comparator for comparing a reference code with codes corresponding to the input signal and generating the second control signal accordingly.

3. The operational amplifier circuit of claim 1, wherein the input signal is a digital input signal, the control stage further comprises a digital to analog converter (DAC) coupled to the analog control stage for converting the digital input signal into an analog input signal, and the analog control stage generates the first control signal according to the analog input signal received from the DAC.

4. The operational amplifier circuit of claim 1, wherein the second current sink comprises a transistor having a control terminal coupled to the digital control stage for receiving the second control signal, a first terminal coupled to the load for sinking the adjustable current from the load, and a second terminal coupled to a low bias voltage.

5. The operational amplifier circuit of claim 1, wherein the operational amplifier circuit is set inside a liquid crystal display (LCD) source driver for driving the load according to the input signal, and the load is a LCD display unit.