JPH089098A - Image sensor - Google Patents

Abstract

PURPOSE:To provide the image sensor in which dispersion hardly takes place in an output current even when dispersion is in existence in a current amplification factor (hFE) of a photo transistor (TR). CONSTITUTION:The image sensor is provided with a photo TR 3 operated in the charge storage mode as an optical sensor array, a charging circuit charging the photo TR 3, a TR 5 whose current amplification factor (hFE) is the same as a current amplification factor (hFE) of the photo TR and located to the charging circuit, and a capacitor 8 connecting to a base of the TR 5. After a scanning signal is released, a charging voltage of the capacitor 8 is discharged through a base of the TR 5 and its characteristic enters an integration time period of the photo TR after the end of discharge time corresponding to the current amplification factor (hFE), then the integration time of the photo TR is controlled depending on the current amplification factor (hFE) of the photo TR.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor adapted to optically read original information, and more particularly to a current amplification factor (hFE) of each phototransistor in an optical sensor array.
The present invention relates to an image sensor in which variations in output current are unlikely to occur even when variations occur.

[0002]

2. Description of the Related Art In recent years, image sensors have been used in optical reading devices as an input unit of facsimiles, digital copying machines and the like. Hereinafter, with reference to FIG. 3 to FIG.
An example of a conventional image sensor will be described. FIG. 3 is a configuration diagram showing a basic configuration of a conventional image sensor, and FIG.
5 is a circuit diagram showing a circuit of a sensor unit and a charging unit of the image sensor shown in FIG. 3, and FIG. 5 is a timing diagram showing waveforms of a start signal and a base potential of a PNP transistor in the circuit shown in FIG.

In FIG. 3, 101 is an optical sensor array composed of a plurality of phototransistors, 102 is an output signal line, 103 is a charging circuit, 104 is a scanning circuit, 1
Reference numeral 05 is a start signal line, and 106 is a clock signal line.

Next, the operation of the image sensor configured as described above will be described. First, when the start signal and the clock signal are applied from the start signal line 105 and the clock signal line 106, respectively, the scanning circuit 104 starts scanning, and the scanning signals are sequentially transmitted to the charging circuit 103. Further, a charging current flows from the charging circuit 103 to each phototransistor to charge it, and the charging current is sequentially output as an output current from the output signal line 102 according to the amount of light incident on each phototransistor.

Next, the configuration will be described with reference to FIG. In FIG. 4, 111 is a power supply line, 112 is a PNP transistor of the charging circuit 103, 113 is a phototransistor of the photosensor array 101, 114 is a base bias resistor of the PNP transistor 112, and 115 is a PNP transistor connected to the scanning circuit 104. A switch for activating 112, and 116 a current source.

Next, the operation of this circuit will be described.
First, when a scanning signal is transmitted from the scanning circuit 104 to the switch 115 and the switch 115 is closed, a current flows from the power supply line 111 through the resistor 114, the base potential of the PNP transistor 112 is lowered, and the PNP transistor 112 is turned on. PNP transistor 11
When 2 is turned on, a charging current flows through the phototransistor 113. The charging current is phototransistor 1
It appears as an output current on the output signal line 102 in accordance with the amount of light incident on the line 13. This output current is the phototransistor 1
It is proportional to the current amplification factor (hFE) of 13.

Next, the operation of the circuit of the conventional image sensor will be described with reference to the timing chart of FIG. In FIG. 5, 121 is a start signal and 122 is P
The waveforms of the base potential of the NP transistor 112 are shown respectively. The start signal 121 is continuously applied at intervals of time T. The base of the PNP transistor 112 drops for a time t ₁ (≦ T) after a delay of time t ₀ from the start signal, during which the PNP transistor 112 charges the phototransistor 113 and outputs an output according to the amount of light incident on the phototransistor 113. Current appears on the output signal line. This operation is repeated again after a fixed time t (≈T) to perform charging.

As described above, since the phototransistor 113 operates in the charge storage mode, the output current is {(output current) ∝ (incident light amount) × (integration time T) ×
(Current amplification factor hFE)}. (JP-A-60-1
14182).

[0009]

However, in the conventional structure as described above, the ratio of the output current to the amount of incident light depends on the characteristics of each phototransistor in the photosensor array, that is, the current amplification factor (hFE). Has been.
However, since the current amplification factor (hFE) of each phototransistor varies, the ratio of the amount of incident light to the output current of each phototransistor is unlikely to be the same. Therefore, there is a problem that a uniform amount of light is incident on each phototransistor and each output current varies even if the integration time is constant.

The present invention has been made in view of the above problems, and provides an image sensor in which the output current is less likely to vary even if the current amplification factor (hFE) of each phototransistor in the photosensor array varies. The purpose is to provide.

[0011]

In order to achieve the above object, an image sensor according to the present invention has a phototransistor array that operates in a charge storage mode as a photosensor array, and a charging device that charges each phototransistor of the phototransistor array. An image sensor comprising a circuit and a scanning circuit for sequentially switching the charging circuit, wherein the charging circuit has at least a current amplification factor (h
FE) including a transistor having the same current amplification factor (hFE) characteristic, and controlling the integration time of the phototransistor according to the discharge time of the transistor corresponding to the current amplification factor (hFE) of the phototransistor. It is a feature.

Further, in order to achieve the above-mentioned object, the image sensor according to the present invention includes a capacitor connected to the base of the transistor, and the capacitor is discharged through the base of the transistor to control the discharge time. It is characterized by doing so.

In summary, in order to achieve the above object, the image sensor according to the present invention includes, in the charging circuit, a transistor having the same characteristic as the current amplification factor (hFE) of the phototransistor, and a capacitor connected to the base of the transistor. By including the discharge capacity proportional to the current amplification factor (hFE) that discharges through the transistor and providing it with a timer function, the current amplification factor (hFE) of each phototransistor can be adjusted. It is characterized in that the integration time is controlled.

[0014]

The image sensor according to the present invention is constructed as described above and operates as follows. First, a start signal is continuously applied at an interval of time T, a scanning signal is delayed by time t ₀ to close the switch, the base of the NPN transistor is raised and turned on to lower the base potential of the PNP transistor, The PNP transistor is turned on and a charging current is passed through the phototransistor. The capacity is charged when the switch is closed, and after the switch is opened N
Discharged through the base of the PN transistor, the base potential of the NPN transistor is attenuated to draw a sawtooth wave.

When the base potential of the NPN transistor falls below its threshold voltage after the discharge time t1 from the turn-on of the NPN transistor, the NPN transistor is turned off, the base potential of the PNP transistor rises and is turned off, and the phototransistor is not charged. At the end, the integration time t begins. In this way, when the current amplification factor (hFE) of the phototransistor is large, the discharge time t1 is long and the integration time t is short. When the current amplification factor (hFE) is small, the discharge time t1 is short and the integration time t is small. Is long, the output current can be maintained substantially the same despite variations in the current amplification factor (hFE) of the phototransistor.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An image sensor according to an embodiment of the present invention will be described in detail below with reference to the accompanying drawings and FIGS. FIG. 1 is a circuit diagram showing a basic configuration of an image sensor in one embodiment of the present invention, and FIG. 2 is a waveform diagram showing timings of respective parts for explaining the operation of the circuit shown in FIG.

In FIG. 1, 1 is a power source, 2 is a PNP transistor for a charging circuit, 3 is a phototransistor for a photosensor array, 4 is a base bias resistor of the PNP transistor 2, and 5 is an NPN for integration time control. Transistor, 6 is a current source, 7 is a switch that is closed by a scanning signal, 8 is a capacitor that charges a reference voltage when switch 7 is closed, 9 is a reference voltage, and 10 is an output signal line that outputs an output current according to the amount of incident light. Is. Here, the NPN transistor 5 and the phototransistor 3 are arranged close to each other by, for example, an IC process and have the same characteristics, and in particular, the characteristics of the current amplification factor hFE are the same.

Next, with reference to FIG. 2 in addition to FIG. 1, the operation of the image sensor configured as described above will be described. FIG. 2 is a waveform diagram showing the timing of each part of the circuit shown in FIG. In FIG. 2, 11 is a start signal,
Reference numeral 12 is a scanning signal output from the scanning circuit to operate the switch 7, 13 is a threshold voltage of the NPN transistor, and 15 is a base potential of the PNP transistor 2. Further, t ₀ is the delay time from the start signal 11 to the scanning signal for closing the switch 7, t ₁ is the discharge time of the capacitor 8 passing through the base of the NPN transistor 5, and t is the integration time of the output current.

The start signal 11 is continuously applied at intervals of time T, and the scanning signal 12 closes the switch 7 after a delay of time t ₀ from the start signal 11 and opens it after a predetermined time. At this time, the NPN transistor 5 turns on because its base rises to the reference voltage 9, and P
The base potential of the NP transistor 2 is lowered. for that reason,
The PNP transistor 2 is turned on to allow the charging current to flow through the phototransistor 3. The capacitor 8 is charged with the reference voltage 9 when the switch 7 is closed, and is discharged through the base current of the NPN transistor 5 after the switch 7 is opened, and the base potential 13 of the NPN transistor 5 is attenuated as shown in FIG. Draw a sawtooth wave.

When the base potential of the NPN transistor 5 drops below its threshold voltage 14 after the discharge time t1 from the turn-on of the NPN transistor 5, the NPN transistor 5 is turned on.
The transistor 5 turns off. Therefore, the base potential of the PNP transistor 2 rises to the power supply voltage and PN
The P-transistor 2 is turned off, and the charging of the phototransistor 3 is completed. Then, from that point on, the integration time t
Begins.

The discharge time t1 depends on the base current of the NPN transistor 5, and if the current source 6 is constant, the base current depends on the current amplification factor (hFE) of the NPN transistor 5, so the discharge time t1 depends on the current amplification factor. It depends on (hFE). Further, since the current amplification factors (hFE) of the phototransistor 3 and the NPN transistor 5 are the same,
When the current amplification factor (hFE) of the phototransistor 3 is large, the discharge time t1 becomes long and the integration time t becomes short. On the contrary, the current amplification factor (hFE) of the phototransistor 3 becomes shorter.
When is small, the discharge time t1 is short, and therefore the integration time t is long.

That is, the output current has a relation of {(output current) ∝ (incident light amount) × (integration time t) × (current amplification factor hFE)}. Therefore, if the current amplification factor (hFE) is large, If t is short and the current amplification factor (hFE) is small, the integration time t becomes long and the variation in output current becomes extremely small.

[0023]

The image sensor according to the present invention is configured as described above, and in particular, by connecting a transistor having the same characteristics as the current amplification factor (hFE) of the phototransistor to control the on / off of the power supply to the phototransistor. ,
After connecting the capacitor to the base of the transistor and switching off, the charge capacity is discharged through the base of the transistor to maintain the transistor on state, and when the base voltage drops to the threshold potential, the transistor is turned off. When the current amplification factor (hFE) of the phototransistor is large, the discharge time t1 is set by turning off the power supply to the phototransistor and starting the integration time.
, The integration time t becomes short, and the current amplification factor (hFE)
, The discharge time t1 is short and the integration time t is long, so that the output current can be maintained substantially the same despite the variation in the current amplification factor (hFE) of the phototransistor.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a basic configuration of an image sensor according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing the timing of each part for explaining the operation of the circuit shown in FIG.

FIG. 3 is a configuration diagram showing a basic configuration of a conventional image sensor.

4 is a circuit diagram showing a circuit of a sensor unit and a charging unit of the image sensor shown in FIG.

5 is a start signal and PN in the circuit shown in FIG.
Timing diagram showing the waveform of the base potential of the P-transistor

[Explanation of symbols]

 1 Power Supply 2 PNP Transistor 3 Phototransistor 4 Resistor 5 NPN Transistor 6 Current Source 7 Switch 8 Capacitance 9 Reference Voltage 10 Output Signal Line 11 Start Signal 12 Scan Signal from Scan Circuit Actuating Switch 13 NPN Transistor Base Potential 14 NPN Transistor Threshold voltage of 15 PNP transistor base potential 101 Photosensor array 102 Output signal line 103 Charging circuit 104 Scanning circuit 105 Start signal line 106 Clock signal line 111 Power supply line 112 PNP transistor 113 Phototransistor 114 Resistor 115 Switch 116 Current source 121 Start Signal 122 Base potential of PNP transistor

Claims (2)

[Claims] 1. An image including a phototransistor array that operates in a charge storage mode as a photosensor array, a charging circuit that charges each phototransistor of the phototransistor array, and a scanning circuit that sequentially switches the charging circuits. A sensor, wherein the charging circuit includes at least a transistor having the same current amplification factor (hFE) characteristic as the current amplification factor (hFE) of the phototransistor, the transistor corresponding to the current amplification factor (hFE) of the phototransistor. An image sensor characterized in that the integration time of the phototransistor is controlled according to the discharge time of. 2. The charging circuit includes a capacitor connected to the base of the transistor, the capacitor being discharged through the base of the transistor to control the discharge time. Image sensor.