JP2012089738A - Optical sensor and electronic equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical sensor with a small circuit scale capable of securing a high gain.SOLUTION: An optical sensor 1 comprises a photodiode PD1, a dummy photodiode PD2, diodes D1, D2 converting output currents of the photodiode PD1 and the dummy photodiode PD2 into a voltage, and a differential amplifier for differential computing the converted voltage. The dummy photodiode PD2 is arranged at a position, at which a leak current of the photodiode PD1 is received, and is shielded from lights. The leak current flows to the dummy photodiode PD2, and an electric potential of the diode D2 is increased, and therefore, an electric potential difference between the photodiode PD1 side and the dummy photodiode PD side can be restrained, so as to restrain malfunction of the optical sensor 1. Also, when lights are incident on the photodiode PD1, a high gain can be obtained by the diode D1. Additionally, a characteristic can be improved without compensating the current, and therefore, a circuit scale does not increase.

Description

  The present invention relates to an optical sensor having a light receiving circuit for optically performing object detection and physical quantity detection, and an electronic apparatus including the optical sensor.

  The necessary number of photosensors such as photon traps used for object detection are arranged in the apparatus toward the detection target. For example, in a copying machine, many tens of optical sensors are used to detect paper that moves in various places in the machine. Therefore, in order to suppress an increase in the price of the apparatus, a cheaper and higher performance optical sensor is required.

  As a solution, techniques disclosed in Patent Documents 1 and 2 can be cited. The optical sensors disclosed in Patent Documents 1 and 2 use two photodiodes in the light receiving circuit, and convert the light reception current from each photodiode into a voltage by logarithmically compressing with the diode, and then the difference. The gain is secured by dynamic comparison. In this optical sensor, it is considered that the circuit scale can be reduced by performing current-voltage conversion with a diode instead of a resistor.

  As for the improvement in performance, it is conceivable to use a dummy photodiode as disclosed in Patent Documents 3 to 5 as techniques for suppressing noise.

  Patent Document 3 discloses that a PN junction made of polysilicon is formed in a dummy photodiode region adjacent to a photodiode to absorb light that can be a noise component.

  Patent Document 4 discloses that the electric charge overflowing from the pixel portion is captured and released by an adjacent dummy photodiode to suppress a decrease in the S / N ratio.

  Furthermore, Patent Document 5 discloses that noise immunity is increased by inserting an amplifier circuit for amplifying a noise signal between the input portion of the differential amplifier and the dummy photodiode.

JP 2006-294682 A (released on October 26, 2006) JP 57-142523 A (published September 3, 1982) JP 2006-190857 A (published July 20, 2006) JP 2009-182127 A (released on August 13, 2009) JP 9-92874 A (published April 4, 1997)

  In the optical sensors disclosed in Patent Documents 1 and 2, it is assumed that light enters both photodiodes and the impedance of the diodes is several MΩ or less. For this reason, if one of the photodiodes is used as a dummy photodiode as disclosed in Patent Documents 3 to 5, on the contrary, the gain becomes too high, such as several tens of MΩ. Causes deterioration.

  The present invention has been made in view of the above problems, and an object of the present invention is to realize an optical sensor with a small circuit scale and a high gain.

  In order to solve the above-described problems, an optical sensor according to the present invention provides a photodiode, a dummy photodiode disposed at a position for receiving the leakage current of the photodiode and shielded from light, and an output current of the photodiode. A first diode that converts the output current of the dummy photodiode into a voltage, and a differential amplifier that performs a differential operation on the voltage converted by the first diode and the second diode. It is characterized by having.

  In order to reduce the impedance of the diode connected to the dummy photodiode without increasing the circuit scale, it is only necessary to use a dummy photodiode that is arranged at a position to receive a leakage current and is shielded from light. Patent Documents 3 and 4 disclose that a dummy photodiode is provided between ICs and photodiodes other than the photodiode in order to prevent leakage current. On the other hand, the present invention utilizes the fact that a minute current flows in the dummy photodiode adjacent to the photodiode upon receiving a leakage current from the photodiode. Patent Documents 3 and 4 do not disclose the use of a leak current flowing through a dummy photodiode.

  By using this dummy photodiode, it is possible not only to secure a gain of about several MΩ at the time of light input, but also to suppress malfunctions such as noise caused by the second diode at the time of light input and deterioration of frequency characteristics due to the gain being too high. Further, in order to suppress the occurrence of propagation delay when the light is turned on / off, current compensation of about several nA may be performed at both input portions by the photodiode and the dummy photodiode.

  Here, it is assumed that the photodiode current is, for example, about 10 nA, and the dummy photodiode current is optically detected under the condition of about 1 nA. If it is 10 nA, a gain of about 2.6 MΩ can be obtained by the first diode. Also, as the 1 nA dummy photodiode current flows, as shown in FIG. 3, the voltage of the second diode rises from 0 bias to about 0.47 V, and the potential difference from the photodiode side is suppressed to several tens mV. It is done. As a result, potential imbalance is eliminated, and malfunction of the optical sensor due to the gain being too high can be suppressed. In addition, when current compensation is performed on both input sections, the voltage at the time of no input is raised by the amount of current compensation.

  The photosensor includes a first MOS transistor and a second MOS transistor through which a constant current flows, anodes of the photodiode and the dummy photodiode are grounded, and a cathode of the photodiode and the first diode is a gate of the first MOS transistor. The anode of the first diode is connected to the drain of the first MOS transistor, the cathode of the dummy photodiode and the second diode are connected to the gate of the second MOS transistor, and the anode of the second diode is It is preferable to be connected to the drain of the second MOS transistor.

  In the circuit disclosed in Patent Document 3, since the base current flows when the diode is connected to the bipolar transistor, the diode does not become zero bias and gain is low.

  On the other hand, as described above, by connecting the first and second diodes to the first and second MOS transistors, respectively, zero bias can be realized when no photodiode current is input. As a result, a negative feedback circuit is configured, so that the drain voltage of the first MOS transistor monitors only the voltage fluctuation in which the photodiode current is voltage-converted by the first diode. As a result, photoelectric conversion can be properly realized, which is beneficial.

  In addition, when current compensation is performed for both input sections, the voltage does not become zero bias, but is increased by the amount of current compensation, so it is necessary to suppress the current compensation to about several nA.

  The optical sensor including the first and second MOS transistors preferably includes a resistor connected between the source of the second MOS transistor and the ground.

  When the above photodiode current is assumed to be about 10 nA, when the dummy photodiode current is further reduced to 100 pA or less, the input potential difference of the connected differential arithmetic unit becomes too large, and light is almost not input. It is conceivable that a detection signal is sometimes output. In this case, by introducing a resistor as described above (here, having a resistance value of about several tens of kΩ), the input voltage on the dummy photodiode side can be increased when there is no input and raised. . As a result, erroneous detection can be suppressed, which is beneficial.

  Patent Document 5 discloses that an amplifier circuit for amplifying a noise signal is inserted between an input section to a differential amplifier and a dummy photodiode. On the other hand, introducing a resistor as described above changes the capability of a current-voltage converter using a diode, and does not introduce an amplifier for amplifying a noise signal. In the circuit disclosed in Patent Document 5, since a resistor is used for the current-voltage converter, if the resistor is introduced as described above, the potential fluctuation increases, and the operation range as the optical sensor is significantly narrowed. As a result, the characteristics deteriorate.

  In the optical sensor including the first and second MOS transistors, it is preferable that the value of the constant current flowing through the second MOS transistor is larger than the value of the constant current flowing through the first MOS transistor.

  By increasing the value of the constant current flowing in the first MOS transistor as described above instead of the above resistance, the potential between the gate and the source is increased, and the input voltage on the dummy photodiode side is increased when there is no input, It becomes possible to raise it. This eliminates the need for consideration of resistance variation, which is beneficial.

  The present invention changes the capability of a current-voltage converter using a diode by increasing the current, and is different from the one in which the noise signal amplification amplifier circuit disclosed in Patent Document 5 is inserted. In the circuit disclosed in Patent Document 5, a resistor is used for the current-voltage converter. When the constant current is increased as described above, the potential fluctuation increases, so the operation range as an optical sensor is significantly narrowed. Characteristics deteriorate.

  An optical sensor comprising the first and second MOS transistors, comprising: a first capacitor connected in parallel with the first diode; and a second capacitor connected in parallel with the second diode; Preferably, the second capacitor is made of polysilicon-diffusion capacitance. Polysilicon corresponds to a gate portion used in a MOS transistor.

  The frequency characteristics of the diode that varies from zero bias according to light input varies greatly depending on the photoelectric flow rate. Therefore, the capacitance values of the first and second capacitors fluctuate depending on the potential of the first and second diodes by arranging the first and second capacitors made of polysilicon-diffusion capacitance in parallel with the first and second diodes, respectively. To do. This is beneficial because it can cancel out fluctuations in frequency characteristics.

  In addition, since the capacitance can be formed by polysilicon and P diffusion or N diffusion with respect to the nitride film capacitance, etc., it can be formed in the same process as the MOS transistor forming process, thereby reducing the process of manufacturing the optical sensor IC. Be beneficial.

  The optical sensor preferably includes a capacitor connected in parallel with the dummy photodiode.

  The dummy photodiode that surrounds the periphery of the photodiode requires a large area in order to match the capacitance value of the photodiode. For this reason, by installing a capacitor in parallel with the dummy photodiode, it becomes possible to match the capacitance value with the photodiode, and it is possible to improve the resistance such as noise characteristics, which is beneficial.

  For the formation of the capacitor, it is necessary to form a diffusion for separation around the capacitor in order to insulate from other elements. Therefore, in order to reduce the total area including the separation portion, it is desirable to form a capacitor with another element in a simple form such as a quadrangle so as to reduce the separation area. In addition, it is desirable to form a capacitor with a polysilicon-diffusion capacitor because a leak current does not flow with respect to the PN junction capacitor forming the photodiode and a malfunction due to reverse bias does not occur.

  An electronic apparatus according to the present invention includes any one of the above optical sensors. As a result, it is possible to arrange an optical sensor having a smaller circuit scale and capable of ensuring a high gain. Therefore, the volume occupied by the optical sensor in the electronic device can be reduced, and the function of the electronic device can be improved, which is beneficial.

  By being configured as described above, the present invention has an effect that it is possible to realize an optical sensor with a small circuit scale and a high gain. Further, according to the present invention, it is possible to easily suppress the increase in size and increase the functionality of an electronic device including an optical sensor, and the effect is particularly remarkable in an electronic device including a large number of the optical sensors.

It is a circuit diagram which shows the structure of the light receiving circuit of the optical sensor which concerns on Embodiment 1 of this invention. (A) is a top view which shows the structure of the photodiode and dummy photodiode which the optical sensor of each embodiment containing the optical sensor of FIG. 1 has in common, (b) is the AA arrow of (a). FIG. It is a graph which shows the characteristic of the diode of the light receiving circuit in the optical sensor of each embodiment containing the optical sensor of FIG. (A) is the graph which shows the resistance value-diode potential heterogeneity characteristic of the light receiving circuit in the optical sensor of each embodiment containing the optical sensor of FIG. 1, (b) is the graph which expanded the graph of (a). (A) is a graph which shows the capacitance value-diode potential heterogeneity characteristic of the light-receiving circuit in the optical sensor of each embodiment containing the optical sensor of FIG. 1, (b) is the graph which expanded the graph of (a). It is a circuit diagram which shows the structure of the light receiving circuit of the optical sensor which concerns on Embodiment 2 of this invention. It is a circuit diagram which shows the structure of the light receiving circuit of the optical sensor which concerns on Embodiment 3 of this invention. It is a circuit diagram which shows the structure of the light receiving circuit of the optical sensor which concerns on Embodiment 4 of this invention. It is a front view which shows the internal structure of the copying machine which concerns on Embodiment 5 of this invention.

[Embodiment 1]
The first embodiment of the present invention will be described with reference to FIGS. 1 to 5 as follows.

[Configuration of optical sensor]
FIG. 1 shows a configuration of a light receiving circuit of the optical sensor 1 of the present embodiment.

  The optical sensor 1 includes a light emitting circuit (not shown) and a light receiving circuit shown in FIG. The light receiving circuit of the optical sensor 1 includes a photodiode PD1, a dummy photodiode PD2, diodes D1 and D2, pch MOS transistors Tr1 and Tr2 (hereinafter simply referred to as pch transistors Tr1 and Tr2), nch MOS transistors Tr3 and Tr4 (hereinafter simply nch). Transistors R3 and R4), resistors R1 to R3, capacitors C1 and C2, and constant current sources CS1 to CS4.

  The photodiode PD1 generates a photocurrent corresponding to the intensity of incident light as an output current. Although the dummy photodiode PD2 will be described in detail later, in order to reduce the impedance of the diode D2 connected to the dummy photodiode PD2, the dummy photodiode PD2 does not depend on the direct incident light, but the photocurrent is generated by a minute leak current from the photodiode PD1. It is shielded from light so as to generate an output current.

  The anode of the photodiode PD1 is connected to the ground to which the ground potential GND is applied (grounded), and the cathode is connected to the gate of the nch transistor Tr3 (first MOS transistor). Similarly, the cathode of the diode D1 (first diode) is also connected to the gate of the nch transistor Tr3. The source of the nch transistor Tr3 is grounded, and the drain is connected to the anode of the diode D1. A capacitor C1 (first capacitor) is connected between the anode and cathode of the diode D1. The constant current source CS3 is connected between the power supply line to which the reference potential Vref is applied and the drain of the nch transistor Tr3, and causes the constant current I1 to flow through the drain and the anode of the diode D1.

  The diode D1 converts the photocurrent of the photodiode PD1 into a voltage by logarithmically compressing it. The nch transistor Tr3 amplifies the voltage converted by the diode D1.

  On the other hand, the anode of the dummy photodiode PD2 is grounded, and the cathode is connected to the gate of the nch transistor Tr4 (second MOS transistor). Similarly, the cathode of the diode D2 (second diode) is also connected to the gate of the nch transistor Tr4. The source of the nch transistor Tr4 is grounded, and the drain is connected to the anode of the diode D2. A capacitor C2 (second capacitor) is connected between the anode and cathode of the diode D2. The constant current source CS4 is connected between the power supply line and the drain of the nch transistor Tr4, and supplies a constant current I2 (= I1) to the drain and the anode of the diode D2.

  The diode D2 converts the photocurrent of the dummy photodiode PD2 into a voltage by logarithmically compressing it. The nch transistor Tr4 amplifies the voltage converted by the diode D2.

  The pch transistors Tr1 and Tr2, resistors R1 to R3, and constant current sources CS1 and CS2 form a differential amplifier. This differential amplifier amplifies the difference between the output voltage of the nch transistor Tr3 and the output voltage of the nch transistor Tr4 and outputs a differential output voltage.

  In the pch transistor Tr1, the gate is connected to the drain of the nch transistor Tr3, the drain is grounded via the resistor R1, and the source is connected to the power supply line via the constant current source CS1. On the other hand, the pch transistor Tr2 has a gate connected to the drain of the nch transistor Tr4, a drain connected to the ground via the resistor R2, and a source connected to the power supply line via the constant current source CS2. A resistor R3 is connected between a connection point between the constant current source CS1 and the source of the pch transistor Tr1 and a connection point between the constant current source CS2 and the source of the pch transistor Tr2.

  In the above-described differential amplifier, the gates of the pch transistors Tr1 and Tr2 are the differential input sections. Further, a positive output voltage Vout1 is output as a differential output voltage from between the resistor R1 and the drain of the pch transistor Tr1. On the other hand, a negative output voltage Vout2 is output as a differential output voltage from between the resistor R2 and the drain of the pch transistor Tr2.

[Photodiode and dummy photodiode structure]
FIG. 2A shows the photodiode PD1 and the dummy photodiode PD2 that are commonly included in the optical sensors 1 to 4 (see FIGS. 1, 6, 7, and 8) of each embodiment including the present embodiment. A sectional structure is shown, and Drawing 2 (b) shows an AA line arrow sectional structure of Drawing 2 (a). FIG. 3 shows the characteristics (voltage-current characteristics) of the diodes D1 and D2.

  As shown in FIGS. 2A and 2B, the photodiode PD1 and the dummy photodiode PD2 are formed on the P substrate 11, and the dummy photodiode PD2 is arranged so as to surround the photodiode PD1. ing. The upper portion (light receiving surface) of the dummy photodiode PD2 is covered by the light shielding layer 12, but the upper portion of the photodiode PD1 is not covered by the light shielding layer 12, and is formed so that light can enter.

  In the above structure, carriers CA are generated on the P substrate 11 by the incidence of light on the photodiode PD1. On the other hand, the dummy photodiode PD2 detects a sneak current (leakage current) from the photodiode PD1 side by the carrier CA even when the upper portion is shielded from light by the light shielding layer 12.

  In the optical sensor 1 configured as described above, since the forward current input portion of the diode D1 is connected only to the photodiode PD1, both ends of the diode D1 are 0 when no photodiode current is input. It becomes a bias. Here, the zero bias is a potential difference of about 0V to several mV. Thus, by performing current-voltage conversion with the diode D1, not only can an appropriate gain be ensured in a small area, but also malfunction of the optical sensor 1 due to noise or the like can be suppressed, which is beneficial. Further, since the characteristics can be improved without performing current compensation, it is not necessary to generate a current for compensation, and the circuit scale is not further increased.

  Here, it is assumed that light detection is performed under the condition that the photodiode current is about 10 nA and the dummy photodiode current is about 1 nA. When the photodiode current is 10 nA, a gain of about 2.6 MΩ is obtained by the diode D1. Further, when a 1 nA dummy photodiode current flows through the dummy photodiode PD2, the potential of the diode voltage rises from 0 bias to about 0.47 V as shown in FIG. Thereby, the potential difference between the dummy photodiode PD2 side and the photodiode PD1 side is suppressed to several tens mV, so that the potential imbalance is eliminated. As a result, malfunction of the optical sensor 1 can be suppressed.

  In addition, since the dummy photodiode PD2 surrounds the periphery of the photodiode PD1, leakage current to the peripheral circuit can be prevented.

  In order to suppress the occurrence of propagation delay when the light is turned on / off, current compensation of about several nA may be performed at both input portions by the photodiode PD1 and the dummy photodiode PD2. In addition, when current compensation is performed on both input sections, the voltage at the time of no input is raised by the amount of current compensation.

  Needless to say, the above-described effects can be obtained in the same manner for the optical sensors 2 to 4 described later.

  Needless to say, the structures of the photodiode PD1 and the dummy photodiode PD2 may be any shapes and arrangements that allow the dummy photodiode PD2 to receive a leakage current from the photodiode PD1. Therefore, in the above example, the dummy photodiode PD2 has a structure surrounding the photodiode PD1, but the present invention is not limited to this structure. For example, a part of the dummy photodiode PD2 may be opened to form a U shape or an L shape.

[Configuration of negative feedback circuit]
In the optical sensor 1, the cathode of the photodiode PD1 and the cathode of the diode D1 are connected to the gate of the nch transistor Tr3, and the anode of the diode D1 is connected to the drain of the nch transistor Tr3, thereby forming a negative feedback circuit. The On the other hand, the negative photodiode is configured by connecting the cathode of the dummy photodiode PD2 and the cathode of the diode D2 to the gate of the nch transistor Tr4 and connecting the anode of the diode D2 to the drain of the nch transistor Tr4. Thereby, zero bias can be realized when no photodiode current is input.

  Further, in order to keep the gate-drain voltage of the nch transistor Tr3 constant, a constant current I1 is supplied from the current source CS3 to the drain. Similarly, a constant current I2 is supplied from the current source CS4 to the drain in order to keep the gate-drain voltage of the nch transistor Tr4 constant.

  As a result, the drain voltage is useful because it can properly monitor only the fluctuation of the voltage obtained by converting the photocurrent by the diodes D1 and D2.

  In addition, since the optical sensors 2 to 4 described later also have the negative feedback circuit described above, the same effects as described above can be obtained.

  Further, as described above, when current compensation is performed on both input portions, the voltage does not become zero bias, but is increased by the amount of current compensation, so that it is necessary to suppress the current compensation to about several nA.

[Capacitor configuration]
The frequency characteristic varies depending on the product of the capacitance values of the capacitors C1 and C2 and the impedances of the diodes D1 and D2. Since the impedances of the diodes D1 and D2 vary depending on the current value, the capacitance values of the capacitors C1 and C2 also vary depending on the diode potential, so that fluctuations in frequency characteristics can be suppressed. For this reason, the capacitors C1 and C2 are constituted by variable capacitance capacitors (variable capacitance diodes) utilizing changes in polysilicon-diffusion capacitance.

  In order to secure a sufficient gain, the diode potential needs to be 0.6 V or less, and the impedance (resistance) fluctuation at that time is about 0.2 V to 0.6 V as shown in FIG. 50% occurs. On the other hand, the polysilicon-diffusion capacitance varies about 50% as shown in FIG. For this reason, although the correlation differs depending on the potential, the frequency fluctuation can be suppressed low, which is beneficial.

  Further, since the capacitors C1 and C2 can be formed by polysilicon and P diffusion or N diffusion with respect to the nitride film capacitance or the like, and can be formed in the same process as the MOS transistor, the IC of the optical sensor 1 can be formed. This is beneficial because the number of manufacturing steps can be reduced.

  In addition, since the capacitors C1 and C2 are similarly configured in the optical sensors 2 to 4 described later, the same effects as described above can be obtained.

[Embodiment 2]
The second embodiment of the present invention will be described with reference to FIGS. 3 and 6 as follows.

  In the following third and fourth embodiments including this embodiment, components having functions equivalent to those of the components in the first embodiment are denoted by the same reference numerals and description thereof is omitted.

[Configuration of optical sensor]
FIG. 6 shows the configuration of the light receiving circuit of the optical sensor 2 of the present embodiment.

  As shown in FIG. 6, the optical sensor 2 has a configuration in which a resistor R <b> 4 is further added to the optical sensor 1. The resistor R4 is connected between the source of the nch transistor Tr4 and the ground.

  In the optical sensor 1 of the first embodiment, when the photodiode current is about 10 nA and the dummy photodiode current is further minute (100 pA or less), the input potential difference of the differential arithmetic unit becomes too large (from 100 mV to FIG. 3). As described above, the potential difference between the photodiode PD1 side and the dummy photodiode PD2 side is further widened as compared with the case of several tens of mV described above. For this reason, it is conceivable that the light detection signal is output even when light is almost not input, and the design likelihood of the light receiving circuit is lowered.

  Therefore, in the optical sensor 2, assuming that a constant current I2 flows to the nch transistor Tr4 by several μA, if the resistor R4 has a resistance value of several tens of kΩ, a potential of about 0.4 V is applied to the resistor R4. can get. When the dummy photodiode current is smaller as described above, the potential difference between the photodiode PD1 side and the dummy photodiode PD2 side is several tens of mV as shown in FIG. 3 by increasing the potential by the resistor R4. Can be suppressed. As a result, potential imbalance is eliminated, and malfunction of the optical sensor 2 can be suppressed.

  Of course, the resistance value of the resistor R4 is not limited to the above example, and may be appropriately set according to the value of the constant current I2.

[Embodiment 3]
Embodiment 3 of the present invention will be described below with reference to FIG.

[Configuration of optical sensor]
FIG. 7 shows the configuration of the light receiving circuit of the optical sensor 3 of the present embodiment.

  As shown in FIG. 7, the light receiving circuit of the optical sensor 3 is configured in the same manner as the light receiving circuit of the optical sensor 1. However, in the optical sensor 3, the constant current source CS4 is different from the optical sensor 1 in that a constant current I3 having a value larger than the constant current I1 that the constant current source CS3 flows is passed.

  As described above, by increasing the constant current I3 on the dummy photodiode PD2 side, the potential between the gate and the source of the nch transistor Tr4 can be increased by about 0.4V. Thereby, the potential difference between the photodiode PD1 side and the dummy photodiode PD2 side is suppressed to several tens mV. As a result, potential imbalance is eliminated, and malfunction of the optical sensor 3 can be suppressed.

  If the value of the constant current I3 is slightly larger than the value of the constant current I1, the potential between the gate and the source of the nch transistor Tr4 can be raised, but in order to raise more reliably, it should be about 2 to 7 times. Is preferred.

  Further, since the resistor R4 is not required unlike the optical sensor 2 described above, it is not necessary to consider the variation of the resistor 4, which is beneficial.

[Embodiment 4]
Embodiment 5 of the present invention will be described below with reference to FIG.

[Configuration of optical sensor]
FIG. 8 shows the configuration of the light receiving circuit of the photosensor 4 of the present embodiment.

  As shown in FIG. 8, the optical sensor 4 has a configuration in which a capacitor C <b> 3 is further added to the optical sensor 1. The capacitor C3 is connected in parallel with the dummy photodiode PD2.

  In the above configuration, the dummy photodiode PD2 surrounding the photodiode PD1 needs a large area to have a capacitance value equivalent to that of the photodiode PD1. For the formation of the capacitor, it is necessary to form a diffusion for separation around the capacitor in order to insulate from other elements. Therefore, in order to reduce the total area including the separation portion, it is desirable to form a capacitor with another element in a simple form such as a quadrangle so as to reduce the separation area. In addition, it is desirable to form a capacitor with a polysilicon-diffusion capacitor because a leakage current does not flow to the PN junction capacitor forming the photodiode PD1 and a malfunction due to reverse bias does not occur.

  Therefore, by providing the capacitor C3 in parallel with the dummy photodiode PD2, it is possible to approach the capacitance value of the photodiode PD1. Thus, by providing the capacitor C3 in parallel with the dummy photodiode PD2, it is possible to achieve matching of capacitance values between the photodiode PD1 side and the dummy photodiode PD2 side. As a result, it is possible to suppress fluctuations in response characteristics of the optical sensor 4 and improve noise characteristics, which is beneficial.

  Specifically, for example, assuming that the capacitance value of the photodiode PD1 is around 10 pF, the capacitance value of the dummy photodiode PD2 is several pF. Therefore, a capacitor C3 having a capacitance value of several pF is arranged in parallel. The capacitance values are adjusted so that the sum is 10 pF.

[Embodiment 5]
Embodiment 6 of the present invention will be described below with reference to FIG.

[Application of optical sensors to electronic devices]
The optical sensors 1 to 4 according to the first to fourth embodiments can be applied to various electronic devices that need to optically detect an object or detect various physical quantities. For example, photosensors 1 to 4 are used for copying machines, printers, portable devices, electric products using motors, and the like that use a photointerrupter that requires high sensitivity as a device that detects object speed and object motion speed. It is preferable. It is also preferable to use the optical sensors 1 to 4 for a smoke sensor, a proximity sensor, a distance measuring sensor, or the like that requires high sensitivity, or an optical sensor that cannot secure a sufficient volume.

[Configuration of copier]
Here, a copier will be described as a specific example of an electronic device using an optical sensor. FIG. 9 is a front view showing the internal configuration of the copying machine.

  As shown in FIG. 9, the copying machine 21 irradiates a sheet of paper placed on an original table 23 provided on the upper portion of a main body 22 with light from a light source lamp 24, and reflects reflected light from the original with a mirror group 25 and a lens. The photosensitive drum 27 charged through 26 is irradiated and exposed. Further, the copying machine 21 forms a toner image by attaching toner to the electrostatic latent image formed on the photosensitive drum 24 by exposure, and moves the conveyance system 31 from the manual paper feed tray 28 and the paper feed cassettes 29 and 30. Then, the toner image on the photosensitive drum 27 is transferred to the sheet supplied via the fixing device 32, and the toner image is fixed by the fixing device 32, and then discharged outside the main body 22.

  In the copying machine 21 configured as described above, optical sensors S1 to S12 are arranged to detect the position of each part and the passage of paper.

  The optical sensors S1 to S4 are arranged to detect the position of a part of the mirror group 25 that moves in the optical scanning direction of the document. The optical sensors S5 and S6 are arranged to detect the position of the lens 26 that moves together with a part of the mirror group 25. The optical sensor S7 is arranged to detect the rotational position of the photosensitive drum 27.

  The optical sensor S8 is arranged to detect the presence or absence of paper on the manual paper feed tray 28. The optical sensor S9 is arranged to detect whether or not a sheet fed from the upper sheet cassette 29 is conveyed. The optical sensor S10 is arranged to detect whether or not a sheet fed from the lower sheet cassette 30 is conveyed.

  The optical sensor S11 is arranged to detect separation of the paper from the photosensitive drum 27. The optical sensor S12 is arranged to detect the discharge of the sheet to the outside of the copying machine 21.

  As described above, the copying machine 21 has a large number of optical sensors S1 to S12. Therefore, by using the optical sensors 1 to 4 of the above-described embodiments as the optical sensors S1 to S12, it is possible to suppress the enlargement of the copying machine 21 and increase the functionality of the photosensors S1 to S12.

  In the above example, the optical sensors S1 to S12 have been described for the sake of convenience. However, in actual copying machines, a larger number of optical sensors are often used. Therefore, the above effect becomes more remarkable in such an electronic device.

[Summary of Embodiment]
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and the embodiments can be obtained by appropriately combining technical means disclosed in different embodiments. The form is also included in the technical scope of the present invention.

  The present invention is suitable for use in photocopiers, printers, portable devices, electric products using motors, etc. that use a photointerrupter that requires high sensitivity as a device for detecting object speed and object motion. Can do. Further, the present invention can be suitably used for a smoke sensor, a proximity sensor, a distance measuring sensor, or the like that requires high sensitivity, or an optical sensor that cannot secure a sufficient volume.

1-4 Photosensor 12 Light-shielding layer 21 Copying machine (electronic equipment)
C1 capacitor (first capacitor)
C2 capacitor (second capacitor)
C3 capacitors CS1 to CS4 constant current source D1 diode (first diode)
D2 diode (second diode)
I1 to I3 Constant current PD1 Photodiode PD2 Dummy photodiodes S1 to S12 Photosensors Tr1 and Tr2 pchMOS transistor Tr3 nchMOS transistor (first MOS transistor)
Tr4 nchMOS transistor (second MOS transistor)
R1-R4 resistance

Claims (7)

A photodiode;
A dummy photodiode that is disposed at a position to receive the leakage current of the photodiode and is shielded from light;
A first diode for converting the output current of the photodiode into a voltage;
A second diode for converting the output current of the dummy photodiode into a voltage;
An optical sensor comprising: a differential amplifier that differentially calculates a voltage converted by the first diode and the second diode. A first MOS transistor and a second MOS transistor through which a constant current flows;
The anodes of the photodiode and the dummy photodiode are grounded,
The cathode of the photodiode and the first diode is connected to the gate of the first MOS transistor, the anode of the first diode is connected to the drain of the first MOS transistor,
2. The dummy photodiode and the cathode of the second diode are connected to the gate of the second MOS transistor, and the anode of the second diode is connected to the drain of the second MOS transistor. Light sensor.   The optical sensor according to claim 2, further comprising a resistor connected between a source of the second MOS transistor and a ground.   3. The optical sensor according to claim 2, wherein a value of a constant current flowing through the second MOS transistor is larger than a value of a constant current flowing through the first MOS transistor. A first capacitor connected in parallel with the first diode;
A second capacitor connected in parallel with the second diode;
3. The optical sensor according to claim 2, wherein the first capacitor and the second capacitor are made of polysilicon-diffusion capacitance.   The optical sensor according to claim 1, further comprising a capacitor connected in parallel with the dummy photodiode.   An electronic apparatus comprising the optical sensor according to claim 1.

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