JP2954408B2 - Image reading device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus used for reading optical image data and extracting it as a signal on a signal line in a facsimile or a scanner.

[0002]

2. Description of the Related Art FIG. 13 is a circuit diagram showing a conventional image reading apparatus. In FIG. 13, reference numeral 1 denotes a plurality of photodiodes each having a cathode connected to a power supply of a DC voltage Vd, that is, photodiodes as light receiving elements for n dots. Reference numeral 2 denotes each transistor corresponding to the number of dots whose collector is connected to the DC power supply and the base is connected to the anode of the photodiode 1, and 3 is an analog connected between the emitter of each transistor 2 and the signal line S. On / off control is performed by a switch in response to an external signal.

As shown in FIG. 14, 4a, 4b, and 4c sequentially output Q outputs from "H" to "L" in accordance with a start pulse SI synchronized with an external clock pulse (CLOCK).
In a plurality of flip-flops that operate so as to change from “H” to “H”, these Q output terminals are sequentially connected in cascade to the D input terminal. Each of these Q outputs is used to turn on and off each of the analog switches 3. Reference numeral 5 denotes a discharge switch connected between the signal line S and the ground, and here functions to discharge the charge of the photocurrent to the ground at the rise of the clock.

FIG. 15 is a signal output circuit, 8 is an amplifier for amplifying the signal SIG on the signal line S, 6 is a capacitor placed on the input side of the amplifier 8 to obtain the signal as a voltage, 7 Is an analog switch for dropping the electric charge of the signal line S to the ground, and is turned on and off by the clock.

Next, the operation will be described. First, when optical image data is detected by the photodiode 1,
The corresponding photocurrent is input to the base of the transistor 2, where it is amplified by hfe times and its charge is accumulated. The charge thus accumulated is sent out as an output to the signal line S when the analog switch 3 is turned on.

On the other hand, when a start pulse SI as shown in FIG. 14 is synchronized with a clock (CLOCK) and input to each flip-flop 4a, 4b, 4c, the Q output of each flip-flop 4a, 4b, 4c is changed. Sequentially ｀ L
As a result, the analog switch 3 is sequentially switched from off to on to off. For this reason, a photocurrent flows through the signal line S at the time of turning on. At this time, the analog switch 5 is closed by using the rising edge of the clock pulse without waiting for the opening of the analog switch 3 as shown in FIG. Then, the charge of the photocurrent, which is the signal SIG on the signal line S, is discharged to the ground.

Thus, assuming that the light accumulation time is constant,
Since the photocurrent is extracted, the photocurrent of the next read line can be read by setting the start pulse SI again after operating a plurality of light receiving units. As shown in FIG. 15, by inputting the signal on the signal line S to the load capacitor 6 provided on the input side of the amplifier 8, the signal can be extracted as a voltage. 7 on,
By turning off the charge, the charge on the signal line S can be completely removed, and as the final output, image data obtained by multiplying the above voltage by Av can be output to the SO terminal.

[0008]

Since the conventional image reading apparatus is configured as described above, the output of the photodiode 1 is amplified by hfe times by each transistor 2, so that the current when no light is irradiated is obtained. A certain dark current is also amplified, and for documents with low reflectance, that is, black documents,
The output is hidden by dark current, making it difficult to read.In addition, the photocurrent varies greatly due to changes in the ambient temperature due to the temperature characteristics of the light-receiving unit, and it is necessary to perform some kind of temperature correction by external operation. There was a problem.

In a minute current region such as a dark current,
Since the amplification factor hfe is not constant and small as compared with the photocurrent, there has been a problem that the resolution for reading a black document or the like is deteriorated.

In addition, the amplification by the transistor 1 causes the response characteristic to be worse than the case without the transistor, and the response characteristic further deteriorates in combination with the provision of the load capacitor 6 on the signal line S. There were problems such as doing.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is intended to improve the reading accuracy of a black document, suppress a change in photocurrent due to a change in ambient temperature, and use a transistor. It is an object of the present invention to obtain an image reading device capable of keeping the amplification factor of the image reading device constant.

[0012]

According to the first aspect of the present invention, there is provided an image reading apparatus comprising: a plurality of transistors for amplifying a photocurrent of a plurality of light receiving elements for photoelectrically converting optical image data; A switch for turning on / off the output to the line, and a switch connected in cascade to receive the clock pulse and output a signal sequentially shifted in timing to control the on / off operation of each switch. A flip-flop and a discharge switch connected to the signal line and receiving the clock pulse to discharge the charge of the photocurrent to the ground, between the base of the transistor and the emitter of the transistor or the ground. A high resistance for suppressing dark current output is connected.

[0013]

The high resistance according to the first aspect of the present invention suppresses the dark current flowing through the light receiving element from flowing between the base and the emitter of the transistor, thereby improving the SN ratio of the photocurrent and reading near the black original. The temperature characteristics of the photocurrent can be improved by adding a temperature-sensitive element as needed, or the amplification factor can be made constant by inputting a constant current to the base of the transistor.

[0014]

[Embodiment 1] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In FIG. 1, reference numeral 1 denotes a plurality of photodiodes whose cathodes are connected to a power supply of a DC voltage Vd, that is, photodiodes as light receiving elements for n dots, 2 denotes a collector to the DC power supply, and a base to the anode of the photodiode 1. Each transistor for the number of dots connected above, 3
Is an analog switch connected between the emitter of each transistor 2 and the signal line S, and is turned on and off in response to an external signal.

4a, 4b, and 4c sequentially output Q outputs from "H" to "L" in accordance with a start pulse SI synchronized with an external clock pulse (CLOCK), as shown in FIG.
In a plurality of flip-flops that operate so as to change from “H” to “H”, these Q output terminals are sequentially connected in cascade to the D input terminal. Each of these Q outputs is used to turn on and off each of the analog switches 3. Reference numeral 5 denotes a discharge switch connected between the signal line S and the ground, and here functions to discharge the charge of the photocurrent to the ground at the rise of the clock. Also,
Reference numeral 91 denotes a high resistance for suppressing a dark current output as a base resistance connected between the base and the emitter of each transistor 2. A load-side circuit as shown in FIG. 9 is connected to the signal line S.

Next, the operation will be described. First, when image data is received by the photodiode 1 serving as a light receiving element, a corresponding photocurrent is output from the photodiode 1, and the photocurrent is amplified by hfe times by the current amplifying action of the transistor 2 and output. Is done. In this case, even when the photocurrent is small or not present at all, the dark current is amplified.
When no photocurrent is present, the dark current can be bypassed from the base of the transistor to the emitter via the high resistance 91 and flown out, so that the amplified output of the dark current is reduced. Note that the emitter side of the transistor 2 needs a storage function and has a relatively high impedance.
The base resistance also needs to be high.

FIG. 2 shows the resistance value RB of the base resistor 91.
The output voltage characteristics when (A) is set to 100 MΩ and (B) is set to 10 MΩ. Here, tone number 1 indicates the density of the black document 1.7D, and as the tone number increases, it approaches the density of the white document, and tone number 6 has a density of 0.76D. As described above, the addition of the base resistor 91 reduces the output voltage, but improves the linearity from the gradation 2 onward. (C) is the output voltage when RB = ∞ and the base resistor 91 is not added, that is, in the case of the conventional example.
This output voltage is output to the SO terminal in FIG.

Embodiment 2 FIG. FIG. 3 shows another embodiment of the present invention. In this embodiment, a base resistor 92 and a diode 102 as a temperature sensing element are connected in series between the connection point between the photodiode 1 and the base of the transistor 2 and the ground. Here, the cathode of the diode 102 is grounded.

According to this embodiment, the photodiode 1
Is amplified by hfe times by the current amplification of the transistor 2. By the way, if the photocurrent Ip ignores the series resistance and the internal resistance of the photodiode 1, Ip = Ie−Id = Ie−Is ｛exp (eVp /
kT) -1}, the photocurrent generating Ie, Id
Is the reactive current flowing through the diode, Is is the reverse saturation current, e
Is the charge, Vp is the voltage generated across the diode 102, k is the Boltzmann constant, and T is the absolute temperature. It can be seen that the photocurrent Ip changes at the ambient temperature.

On the other hand, since the forward voltage of the diode 102 decreases as the ambient temperature rises, a change in the photocurrent with respect to the ambient temperature can be reduced by appropriately selecting the size of the base resistor 92. In this case, the base resistor 92 and the diode 1 are connected to the emitter of the transistor 2.
02 may be connected, but if it is connected to the ground as described above, the temperature change of the forward voltage of the diode 102 can be made large.

FIG. 4 shows that the base resistor 92 is (d) 100 MΩ.
And (e) 10 MΩ, and a diode 1
02 shows the output voltage characteristics when No. 02 is connected in the forward direction and grounded. According to this, when the resistance value Rb of the base resistor 92 decreases as compared with the case where the base resistor 92 is not provided, that is, (f) RB = が な い, the rate of change of the output voltage with respect to the ambient temperature decreases. You can see that. However, the output voltage of gradation number 6 (original density 0.76D) also decreases as the resistance value Rb decreases. This is because the photocurrent is not amplified between the base and the emitter of the transistor 2 and partly flows out to the ground side.

Embodiment 3 FIG. FIG. 5 shows still another embodiment of the present invention. This means that a constant current of 5 to 50 μA is applied to the base resistor 93 from the VB terminal.
Is supplied to the base of the transistor 2 via According to this, current amplification can be performed at an operating point higher than the recombination current between the base and the emitter, so that the amplification factor hfe is constant. The output voltage characteristics according to this embodiment are as shown in FIG. 6 in the case where there is (g) the base current and in the case where there is no (h) base current, and the output voltage rises at gradation 1 (black). Thus, an output voltage with relatively good linearity can be obtained. Note that, with a base current of 50 μA or more, the charge injection efficiency deteriorates, and as the gradation increases, the linearity decreases and worsens.

Embodiment 4 FIG. In each of the above embodiments, the output of the signal line S is taken out by the capacitor 6 shown in FIG. 15 and amplified by the amplifier 8 as shown in FIGS. The output, that is, the output of the signal line S may be taken out by a load resistor instead of the capacitor 6.

Embodiment 5 FIG. In each of the above embodiments, the analog switch 3 is used to take out the output of each transistor 2 to the signal line S. However, another switch such as a relay or a buffer may be used. The switch 5 has a function of rapidly discharging the electric charge of the signal line S to the ground, and has an operation equivalent to that of the analog switch 7 in FIG. 15, so that one of them can be omitted in principle.

Embodiment 6 FIG. FIG. 7 shows an embodiment of the invention according to claim 2, wherein 1 is a plurality of photodiodes as light receiving elements, 21 is an operational amplifier provided by the same number as each photodiode 1, and 22 is one end of the photodiode 1 and the operational amplifier. 21 is a photocurrent reading resistor connected to the midpoint of connection with the positive input terminal 21 and the other end connected to ground.

Reference numerals 23 and 24 denote resistors connected between the output terminal and the negative input terminal of the operational amplifier 21 and between the negative input terminal and the ground, respectively.
25 is an output terminal of the operational amplifier 21 and the analog switch 3
Is a protection resistor connected between In addition, the same components as those shown in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted.

In the image reading apparatus having such a configuration,
The optical image signal input to the photodiode 1 is photoelectrically converted to generate a potential corresponding to the resistance value RI at both ends of the resistor 22. Then, a non-inverted amplified current is output from the operational amplifier 21 based on this potential, and this is output as a non-inverted amplified signal as shown in FIG. here,
The amplification factor of the operational amplifier 21 is RA / RB if the resistance values of the resistors 23 and 24 are RA and RB, and is approximately RA
The voltage is converted to a light voltage of / RB times.

The response speed τ of the circuit including the operational amplifier 21 is given by τ = K · C · R, where K is a constant, C is the junction capacitance of the photodiode 1 and R is the load resistance. The response speed is greatly improved as compared to τ = J · C · R · hfe where the constant and hfe are the amplification factors of the transistors, and the rise and fall are fast as shown in FIG. Waveform line signal S
IG is output. Note that, also in this case, a delay in response speed occurs due to the load capacitor 6 connected to the signal line S.

Here, each protection resistor 25 opens and closes each analog switch 3 based on the output of each flip-flop 4a, 4b, 4c to take out the output. When the discharge switch 5 resets the electric charge of the signal line S, In addition to discharging, the output of each operational amplifier 21 is prevented from being short-circuited.

Embodiment 7 FIG. FIG. 9 shows another embodiment of the present invention. This is such that a load resistor 26 is connected between the midpoint between the connection between the protection resistor 25 and the analog switch 3 shown in FIG. 7 and the ground, and the other configuration is the same as that of FIG. FIG. 10 shows an output circuit of a signal on the signal line S in FIG. 9. Numeral 8 multiplies the signal by Av and outputs it to the output terminal. Here, the load capacitor 6 shown in FIG. is there.

According to this embodiment, the photodiode 1
Since there is no capacitance component other than the junction capacitance of
And the response speed τ is given by τ = K · C ·, where Rl is the load resistance.
Assuming that R1 is, for example, K = 3, C = 2PF, and R1 = 10 KΩ, τ is about 60 nsec, which is 10 times or more faster than the case where the conventional transistor 2 is used. Although the output voltage can be increased as Rl increases, the optimum value is about 10 KΩ in consideration of a decrease in response speed.

Embodiment 8 FIG. FIG. 11 shows still another embodiment of the present invention. This is obtained by connecting a diode 94 as a temperature sensing element between the load resistor 26 shown in FIG. 9 and the ground.
It can be further reduced.

FIG. 12 shows this change in the output voltage by using the capacitor 6 shown in FIG.
If there is, there is a diode 94 and the load resistance 2
6 has a resistance Rl of 10 KΩ, a diode 9
4 and the resistance value R1 of the load resistor 26 is 10 KΩ, and the output voltage change rate is about 0.6% / ° C. in the cases of (l) and (l). Against
In the case of (nu), it is 0.4% / ° C.

Although the diode 94 is provided on the output side of the operational amplifier 21 in the above embodiment, it may be connected in series with the resistor 22 and grounded. Further, a Zener diode or a thermistor as a temperature sensing element may be used in place of the diode 94, and the same effect as in the above embodiment can be obtained.

[0035]

As described above, according to the first aspect of the present invention, one transistor for amplifying the photocurrent of the plurality of light receiving elements for photoelectrically converting the optical image data, and one transistor from these transistors to the signal line. A switch for turning on and off the output of each of the flip-flops, and a flip-flop connected in cascade with each other for receiving a clock pulse and outputting a signal whose timing is sequentially shifted to control the on / off operation of each of the switches. And a discharge switch connected to the signal line for receiving the clock pulse and discharging the charge of the photocurrent to the ground, wherein a dark current flows between the base of the transistor and the emitter of the transistor or the ground. Since a high resistance for output suppression is connected, dark current flows between the base and emitter of the transistor.
The suppressed from being less black originals reflectance Ru prevented from becoming difficult to read. Further, if necessary, a temperature-sensitive element such as a diode can be added to suppress a change in photocurrent due to the ambient temperature. Further, if a constant current is input to the base of the transistor, the gain of the transistor can be made constant, and an effect that can read image data with good linearity can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an image reading apparatus according to an embodiment of the present invention.

FIG. 2 is an output voltage characteristic diagram showing a relationship between an output voltage and a document density according to the embodiment of FIG. 1;

FIG. 3 is a circuit diagram showing an image reading apparatus according to another embodiment of the present invention.

FIG. 4 is an output voltage characteristic diagram showing a relationship between an output voltage and an ambient temperature in the circuit of FIG. 3;

FIG. 5 is a circuit diagram showing an image reading apparatus according to still another embodiment of the present invention.

FIG. 6 is an output voltage characteristic diagram showing a relationship between an output voltage and a document density according to the embodiment of FIG. 5;

FIG. 7 is a circuit diagram showing an image reading apparatus according to an embodiment of the present invention.

8 is a timing chart showing signals of respective parts of the circuit in FIG. 7;

FIG. 9 is a circuit diagram showing an image reading apparatus according to another embodiment of the present invention.

FIG. 10 is a circuit diagram showing an output circuit according to the invention of claim 2;

FIG. 11 is a circuit diagram showing an image reading apparatus according to still another embodiment of the present invention.

FIG. 12 is an output voltage characteristic diagram showing a relationship between an output voltage and an ambient temperature in FIG. 11;

FIG. 13 is a circuit diagram showing a conventional image reading device.

FIG. 14 is a timing chart showing signals of respective parts of the circuit of FIG. 13;

FIG. 15 is a circuit diagram showing a conventional output circuit.

[Explanation of symbols]

 Reference Signs List 1 photodiode (light receiving element) 2 transistor 3 analog switch (switch) 4a, 4b, 4c flip-flop 5 analog switch (discharge switch) 21 operational amplifier 91, 92, 93 base resistance (high resistance) S signal line

Claims (1)

(57) [Claims] 1. A plurality of light-receiving elements for photoelectrically converting read optical image data, one transistor for amplifying a photocurrent of these light-receiving elements, and turning on / off an output from these transistors to a signal line. Each one switch, each one flip-flop cascaded to output ON / OFF operations of the switches by outputting a signal whose timing is sequentially shifted in response to the clock pulse, and A discharge switch connected to receive the clock pulse and discharge the photocurrent to ground, and a high resistance for suppressing dark current output connected between the base of the transistor and the emitter of the transistor or the ground. An image reading device comprising: