JP2006287295A - Light receiving amplifier circuit and display device provided with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light receiving amplifier circuit capable of reducing the effect of noise in the case of measuring the intensity of an incident light. <P>SOLUTION: In the light receiving amplifier circuit provided with a transistor TR<SB>21</SB>, and a current mirror circuit comprising transistors TR<SB>22</SB>, TR<SB>23</SB>and resistors R<SB>21</SB>, R<SB>22</SB>, wherein the emitter of the TR<SB>21</SB>is connected to a terminal to which a light receiving element D is connectable, and a constant voltage is applied to the base of the TR<SB>21</SB>, a collector current of the TR<SB>21</SB>controls a mirror current of the current mirror circuit. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light receiving amplification circuit for adjusting the light amount of a light emitting element and a display device including the same.

  In a transmission control type display device such as a liquid crystal display device, a backlight is required. Therefore, a technology is used in which light emitting diodes of each color of red (R), green (G), and blue (B) are two-dimensionally arranged and light of each color is mixed to be used as a white backlight. (Non-Patent Document 1, etc.). Thus, by arranging the light emitting diodes of R, G, and B colors in a matrix and adjusting the light emission intensity for each color, the color of the image can be finely adjusted. Therefore, there is an advantage that higher color reproducibility can be realized than a display device using a white phosphor.

  On the other hand, there are individual differences in the light emission intensity of the light emitting diodes, and the temperature dependence of the light emission intensity for each color of the light emitting diodes is different, which may cause color unevenness and color shift in the screen. Therefore, feedback control is performed to suppress color unevenness and color misregistration in the screen by adjusting the light emission intensity of the light emitting diode according to the measured value using a light receiving amplification circuit that measures the light emission intensity for each color. Yes.

As shown in FIG. 4, the conventional photoreceiver / amplifier circuit 100 includes an emitter follower circuit including resistors R ₁₁ and R ₁₃ , a transistor Tr _11, and an integrating circuit 10. The light receiving amplification circuit 100 is used in combination with a light receiving element D ₁ for photoelectric conversion. One photoreceiver / amplifier circuit 100 is provided in association with each color of RGB in each region obtained by dividing the light emitting unit of the display device. As light incident on the light receiving element D ₁ becomes strong, the current flowing through the light-receiving element D ₁ is increased. The emitter of the transistor Tr ₁₁ has a voltage substantially equal to the base voltage divided by the resistances of the fixed resistor R ₁₁ and the light receiving element D ₁ . This voltage is temporally averaged by the integrating circuit 10 and output. By using this output signal as a feedback signal for adjusting the light emission intensity of the light emitting diode, color unevenness and color shift are suppressed in the screen.

"LED backlight changes the color of TV", Nikkei Electronics, December 20, 2004, p57-62

To accurately measure the luminous intensity of the display device, the light-receiving element D ₁ it should be installed in the vicinity of the light emitting region. Meanwhile, in order to install the light receiving amplifier circuit 100 in the vicinity of the light-receiving element D _1, it is necessary to pull the power wiring of the light receiving amplifier circuit 100 not to the signal line only in the vicinity of the light-receiving element D _1. Therefore, the light receiving element D ₁ is often installed in the vicinity of the light emitting region, and the light receiving amplification circuit 100 is often arranged at a position away from the light receiving element D ₁ .

At this time, when the light receiving element D ₁ is connected to the base of the transistor Tr ₁₁ , the input impedance is increased, and noise is easily superimposed from the wiring to the light receiving amplification circuit 100 disposed at a position away from the light receiving element D ₁ .

Further, by providing a wiring of the power supply to the light receiving amplifier circuit 100, in order to install the light receiving amplifier circuit 100 in the vicinity of the light-receiving element D ₁ is arranged in association with the light receiving amplifier circuit 100 to each of the plurality of light emitting regions If necessary, it is difficult to integrate the transistors Tr ₁₁ and the resistors R ₁₁ and R ₁₃ included in the plurality of light receiving and amplifying circuits 100. Therefore, there is a problem that the manufacturing cost of the light receiving amplifier circuit 100 increases.

  It is an object of the present invention to provide a light receiving amplification circuit for adjusting the light amount of a light emitting element and a display device including the same, which have solved at least one of the problems of the prior art.

  The present invention is a light receiving amplifier circuit that amplifies and outputs an output of a light receiving element that converts incident light into an electric signal, and includes a transistor and a current mirror circuit, and the emitter of the transistor is connected to the light receiving element. The mirror current of the current mirror circuit is controlled by the collector current of the transistor by being connected to a possible terminal and applying a constant voltage to the base of the transistor.

  Since the emitter of the transistor has a low input impedance, the superimposition of noise from the light receiving element to the wiring can be reduced by enabling the light receiving element to be connected to the emitter of the transistor. Therefore, the light receiving amplification circuit can be disposed at a position away from the light receiving element. In addition, the photoreceiver / amplifier circuit can be integrated, and the manufacturing cost can be reduced.

  A mirror that flows through the current mirror circuit by comparing a voltage value corresponding to a DC component of a mirror current flowing through the current mirror circuit with a predetermined reference voltage in the differential amplifier circuit; It is also preferable to perform feedback control that compensates for the DC component of the current.

  This makes it possible to configure a circuit that is resistant to white noise that generates DC component noise. For example, the influence of noise can be reduced when only the wavelength region to be measured is selectively measured from the light including the wavelength region other than the wavelength region to be measured. Therefore, the amount of light in the wavelength region to be measured can be measured with high accuracy.

  The light receiving amplification circuit of the present invention can be applied to a display device. The light receiving amplification circuit according to the present invention, a light emitting element, a light receiving element that receives light from the light emitting element and converts it into an electrical signal, and a control circuit that controls the light emission intensity of the light emitting element, the light receiving amplification The light receiving element is connected to an emitter of the transistor of the circuit, and the control circuit controls light emission intensity of the light emitting element based on a signal corresponding to a mirror current of the current mirror circuit in the light receiving amplifier circuit. A display device can be realized.

  According to the present invention, it is possible to reduce the influence of noise when measuring the intensity of incident light. In addition, it is possible to dispose the light receiving element and the light receiving amplification circuit at positions separated from each other. Furthermore, the direct current component due to white noise can be compensated, and the intensity of light in the wavelength region to be measured can be accurately measured.

  As shown in FIG. 1, the display device 200 according to the embodiment of the present invention includes a light emitting unit 20, a light receiving unit 22, a driving unit 24, and a control unit 21.

  For example, the light emitting unit 20 has a configuration in which a plurality of pixels are arranged in a matrix. Each pixel is provided with a liquid crystal transmission layer whose transmittance can be controlled independently. Generally, the light emitting unit 20 is divided into a plurality of divided regions, and backlights each including three red (R), green (G), and blue (B) light emitting diodes are arranged in each divided region. . An image can be displayed on the screen of the light emitting unit 20 by causing the backlight to emit light and controlling the transmittance of the liquid crystal transmission layer of each pixel.

  In order to avoid complicated explanation, FIG. 1 exemplifies a configuration including only one red (R), green (G), and blue (B) light emitting region.

  The light receiving unit 22 is used for feedback control of light emission intensities of red (R), green (G), and blue (B) light emitting diodes constituting the backlight. The light receiving unit 22 includes a plurality of light receiving elements D. The light receiving element D is provided in association with each color of red (R), green (G), and blue (B) in each divided region of the light emitting unit 20. Each light receiving element D is preferably provided with an optical filter that transmits only the wavelength region of the corresponding color. Each light receiving element D detects the light emission intensity of the light emitting diode for each color of red (R), green (G), and blue (B) in each divided region of the light emitting unit 20 and outputs it to the control unit 21.

  The control unit 21 receives signals of the emission intensity for each color in each divided region from each light receiving element D of the light receiving unit 22, and controls the light emission of the backlight in each divided region of the light emitting unit 20 based on those signals. . The drive unit 24 includes a light emission control circuit 30 and a light reception amplification circuit 40 for each color in each divided region. The control unit 21 can be integrated as an IC or the like.

  The light emission control circuit 30 includes a pulse width modulation circuit (PWM circuit) 32, a switch 34, a transistor 36, and a constant current circuit 38. The PWM circuit 32 receives the output signal from the photoreceiver / amplifier circuit 40, and generates and outputs a light emission control signal having a pulse width corresponding to the intensity of the output signal. Specifically, the PWM circuit 32 causes the control unit 21 to narrow the pulse width of the light emission control signal to be output as the intensity of the output signal from the light reception amplification circuit 40 becomes higher than a predetermined value, and output from the light reception amplification circuit 40. The pulse width of the emission control signal to be output is increased as the signal intensity is lower than a predetermined value. The switch 34 switches a signal applied to the base of the transistor 36 that controls light emission of the light emitting diode. By switching the switch 34, either the PWM circuit 32 or the external terminal EXT is selected. The transistor 36 drives a current flowing through the light emitting diode of the light emitting unit 20. When the PWM circuit 32 is selected by the switch 34, a light emission control signal having a pulse width corresponding to the output signal from the light receiving amplifier circuit 40 is applied to the base of the transistor 36. Then, the constant current set by the constant current circuit 38 is supplied to the light emitting diode only during the period when the light emission control signal is at the high level (H). It should be noted that the current value of the constant current circuit 38 can be adjusted by the voltage applied to the external terminal CNT of the drive unit 24 in order to initially adjust the emission intensity and emission wavelength for each color in each divided region of the light emitting unit 20. Is preferred.

  As described above, when the light emission intensity of the light emitting diode is increased by the light emission control circuit 30, it is weakened. When the light emission intensity of the light emitting diode is weakened, the light emission intensity is increased, and the light emission intensity of the light emitting diode is defined by a predetermined value. Feedback control is performed so as to maintain the above.

The light receiving amplification circuit 40 includes an amplification circuit 42 and an integration circuit 44. FIG. 2 shows a basic configuration of the amplifier circuit 42 in the present embodiment. The amplifier circuit 42 includes transistors Tr ₂₁ , Tr ₂₂ , Tr ₂₃ , Tr ₂₄ , resistors R ₂₁ , R ₂₂ , R ₂₃ , a constant voltage source Q ₁ , and a constant current source Q ₂ .

The emitter of the transistor Tr ₂₁ is provided with a terminal so that the light receiving element D of the light receiving unit 22 can be connected. The light receiving element D is connected to this terminal. A constant voltage source Q ₁ is connected to the base of the transistor Tr ₂₁ . As the constant voltage source Q ₁ , a bias voltage provided in the IC of the drive unit 24 can be used. To the light receiving element D, (V ₁ −V _BE ) obtained by subtracting the base-emitter voltage V _BE of the transistor Tr ₂₁ from the voltage V ₁ of the constant voltage source Q ₁ is applied as a reverse bias.

Transistors Tr ₂₂ and Tr ₂₃ and resistors R ₂₁ and R ₂₂ constitute a current mirror circuit. The current flowing through the light receiving element D changes according to the amount of light received by the light receiving element D. The current of the light receiving element D is transmitted from the transistor Tr ₂₁ to the collector current (mirror current) of the transistor Tr ₂₃ through the transistor Tr ₂₂ . As a result, the current of the light-receiving element D is converted into a voltage between the terminals of the resistor R ₂₃ is connected to the current mirror circuit. Voltage across the terminals of the resistor R ₂₃ is connected via a buffer circuit composed of a constant current source Q ₂ and the transistor Tr _24, is output to the integrating circuit 44.

  The integrating circuit 44 averages the output voltage from the amplifying circuit 42 and outputs the result. This output signal is input to the control unit 21 as a detection signal. The control unit 21 compares this detection signal with a desired level, and creates and outputs a control signal. The output control signal is input to the PWM circuit 32 of the light emission control circuit 30. Thus, feedback control can be performed so that the light emission intensity of the light emitting diode is maintained at a predetermined intensity.

As described above, by performing feedback control based on the light emission intensity of the light emitting diode, color unevenness and color shift in the screen of the display device 200 caused by a change in the light emission efficiency due to the temperature and operation time of the light emitting unit 20 can be prevented. Can be suppressed. At this time, since the input impedance of the emitter of the transistor Tr ₂₁ is low, the influence of noise mixed in from the wiring between the light receiving element D and the light receiving amplification circuit 40 is affected by connecting the light receiving element D to the emitter of the transistor Tr _21. Can be reduced. Therefore, the light receiving amplification circuit 40 can be disposed at a position away from the light receiving element D. In addition, a plurality of light receiving amplifier circuits 40 can be integrated together.

  As described above, adjustment of the light amount of the light emitting unit 20 is generally performed by pulse width modulation. In such a case, for example, the light receiving element D for detecting the amount of light in the red (R) wavelength region is provided with an optical filter for transmitting the red (R) wavelength region. By being controlled by pulse width modulation, light in the red (R) wavelength region to be detected is received as an optical pulse. However, it is impossible to completely block light in the wavelength region of green (G) or blue (B) other than red (R) with an optical filter. In the backlight, light in each wavelength region of red (R), green (G), and blue (B) is repeatedly scattered and refracted, and the wavelength and the pulse phase are in a state of random white noise. This white noise influences as noise of a DC component in the light receiving element D, like disturbance light incident from the outside. Under such circumstances, even if an optical filter is used, it is affected by an unnecessary wavelength region.

Therefore, it is preferable to use an improved light receiving amplification circuit. FIG. 3 is a diagram showing a configuration of the light receiving amplification circuit 50 improved in the present embodiment. Receiving amplifier circuit 50, in addition to the basic configuration of the light receiving amplifier circuit 40, the transistors _{Tr 31, Tr 32, Tr 33} , Tr 34, Tr 35, Tr 36, resistors _{R 31, R 32, R 33} , capacitors C ₃₁ The constant voltage source Q ₃ and the constant current source Q ₄ are included.

A current proportional to the amount of light received by the light receiving element D is converted into a voltage between the terminals of the resistor R ₂₃ via the transistors Tr ₂₁ , Tr ₂₂ , Tr ₂₃ . The light receiving amplifier circuit 50, the transistor Tr ₃₁ also constitute a transistor Tr ₂₂ and the current mirror circuit. Therefore, a current (mirror current) corresponding to the collector current of the transistor Tr ₂₂ , that is, the current flowing through the light receiving element D, flows through the transistor Tr ₃₁ . Current flowing through the transistor Tr ₃₁ flows in the series circuit and the resistor R ₃₁ and resistor R ₃₂ and capacitor C _31. Here, it is assumed that the resistor R ₂₃ has a resistance value substantially equal to the resistor R ₃₁ . The transistors Tr ₃₃ and Tr ₃₄ constitute a differential amplifier. A reference voltage V ₂ is applied from the constant voltage source Q ₃ to the base of the transistor Tr ₃₄ . A charging voltage V ₃ charged with a time constant of a series circuit of a resistor R ₃₂ and a capacitor C ₃₁ is applied to the base of the transistor Tr ₃₃ .

When a direct current component due to white noise whose wavelength and pulse phase are random is received and a current corresponding to the direct current component starts to flow through the transistor Tr ₃₄ , the current is fed back via the transistor Tr ₃₂ . As a result, an output signal from which the DC component due to white noise has been canceled is output to the integrating circuit 44.

  As described above, the influence of noise in light reception can be reduced. Therefore, it is possible to measure the amount of light in the wavelength region to be measured with high accuracy.

  The application range of the present invention is not limited to a backlight using a light emitting diode. For example, it can be applied to optical communication using a light emitting diode.

In the case of optical communication, it is necessary to make the light receiving sensitivity very high. In particular, optical noise such as sunlight has an extremely large influence. In an environment where the direct current component, which is optical noise, is large, detection of a required signal pulse component is hindered, and it is important to reduce the influence of noise. When the photoreceiver / amplifier circuit according to the present invention is applied to a display device, the mirror ratio that determines the current ratio of the transistors Tr ₂₂ and Tr ₂₃ and Tr ₃₁ and the transistor Tr ₃₅ and transistors Tr ₃₆ and Tr ₃₂ of the current mirror circuit is 1. It may be about 1 to 1 and can exhibit an effect of reducing noise. In the case of optical communication, the DC component that causes noise is often stronger than the AC component signal to be detected, and the current ratio of the transistors Tr ₃₆ and Tr ₃₂ is larger than the transistor Tr ₃₅ of the current mirror circuit. It is preferable to do. As a result, the amount of compensation for the DC component that becomes noise increases, and noise can be reduced. By setting the compensation amount large, saturation of the light receiving and amplifying circuit can be prevented by a direct current component that is optical noise.

It is a figure which shows the structure of the display apparatus in embodiment of this invention. It is a figure which shows the basic composition of the light reception amplifier circuit in embodiment of this invention. It is a figure which shows the modification of the light reception amplifier circuit in embodiment of this invention. It is a figure which shows the structure of the conventional light reception amplifier circuit.

Explanation of symbols

10 integration circuit, 20 light emitting unit, 21 control unit, 22 light receiving unit, 24 drive unit, 30 light emission control circuit,
34 switch, 36 transistor, 38 constant current circuit, 40 light receiving amplifier circuit, 42 amplifier circuit, 44 integrating circuit, 50 light receiving amplifier circuit, 100 light receiving amplifier circuit, 200 display device, C ₃₁ capacitor, Q ₁ constant voltage source, Q ₂ constant current source, Q ₃ constant voltage source, Q ₄ constant current _{source, R 11, R 12, R} 13, R 21, R 22, R 23, R 31, R 32, R 33 resistors, Tr _11, Tr _21, Tr ₂₂ , Tr ₂₃ , Tr ₂₄ , Tr ₃₁ , Tr ₃₂ , Tr ₃₃ , Tr ₃₄ , Tr ₃₅ , Tr ₃₆ transistors.

Claims (3)

A light receiving amplification circuit that amplifies and outputs an output of a light receiving element that converts incident light into an electrical signal,
A transistor and a current mirror circuit;
The emitter of the transistor is connected to a terminal to which the light receiving element can be connected, and a constant voltage is applied to the base of the transistor, thereby controlling the mirror current of the current mirror circuit by the collector current of the transistor. A light receiving amplifier circuit characterized by the above. The light receiving amplifier circuit according to claim 1,
In addition, a differential amplifier circuit is provided.
Feedback control for compensating the DC component of the mirror current flowing through the current mirror circuit by comparing a voltage value corresponding to the DC component of the mirror current flowing through the current mirror circuit and a predetermined reference voltage in the differential amplifier circuit. A light receiving amplifier circuit characterized in that The light receiving amplifier circuit according to claim 1 or 2,
A light emitting element, a light receiving element that receives light from the light emitting element and converts it into an electrical signal, and a control circuit that controls the light emission intensity of the light emitting element,
The light receiving element is connected to an emitter of the transistor of the light receiving amplification circuit;
The display device, wherein the control circuit controls light emission intensity of the light emitting element based on a signal corresponding to a mirror current of the current mirror circuit in the light receiving amplification circuit.

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