JPH0222590B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an image sensor, and more particularly to a one-dimensional image sensor using bipolar IC technology that makes it possible to read document information at high speed and high resolution.

2. Description of the Related Art In recent years, close-contact type image sensors that read a same-magnification image of a document have been actively developed in order to make document reading scanners smaller, have higher resolution, and make it easier to adjust the optical system. The close-contact type uses silicon crystal.
CCD multi-chip image sensor, bipolar
There are IC multi-chip image sensors, and thin film sensors such as CdS image sensors and a-S image sensors.
An example of a conventional bipolar multi-chip image sensor will be described below with reference to the drawings.

In FIG. 6, a selection input signal from the outside is received at the selection input signal terminal 50, and this is received at the complementary signal output terminal 5.
2 shows a circuit that generates complementary signals with different DC levels. Current from a constant current circuit 53 is used to convert a selected input signal to a logic reference voltage by a pair of transistors 54, 55, and 56 constituting a current switch. Therefore, the transistor 54a or 54b, 5
5a or 55b, 56a or 56b, and complementary voltages with different DC levels are generated by each current and resistance pair 57, 58, 59. For example, input terminal 5 of selection input signal 51
When 1a is input to the "H" level, A of the complementary output terminal 52 becomes "H" and "L". This logic amplitude is determined by the value of the resistor pair 57 and the current value of the constant current circuit 3. Although FIG. 6 shows the case of 3-bit input, this can be expanded to a case with a larger number of bits by repeating the same procedure.

FIG. 7 shows an input terminal 70 receiving complementary signals of different DC levels from the signal conversion circuit of FIG.
When the block selection signal is received at the block selection signal terminal 71, the current of the constant current source 72 activated by the block selection signal is transferred to the cascade-connected current changeover switches 74, 75, 76, 77,
78, 79 and current switches 80, 81, 8
2, 83, 84, 85, 86, 87 and only one phototransistor in the phototransistor array 89 selected by the select input signal can be placed in the read state. 88 is a power supply line, and 90 is a video output terminal. Next, the specific operation will be explained. Block input terminal is on, complementary input signal A,
When B and C are "L", . At this time, the PNP transistor 80a is turned on, and a charging current flows from the power supply to the phototransistor 89a, and is output to the video output terminal 90. At this time, only the phototransistor 89a is in the reading state. A complementary signal corresponding to the selection input signal of FIG. 6 allows any phototransistor in the phototransistor array to be placed in a read state one by one. (For example, Television Society Technical Report 1984
VOL.8.No.26.ED812) Problems to be solved by the invention However, the current switch as described above
In a configuration using a PNP transistor, the PNP transistor saturates and malfunctions. An example of malfunction is shown in FIG. FIG. 8 shows one phototransistor, a current switch, and a current switch. 9
1 is a load resistance, 92 is a parasitic capacitance between the base and collector of the PNP transistor, 93 is the base of the PNP transistor, and 94 is the collector of the PNP transistor. Figure 9 is a timing diagram;
5 is the waveform of the base of the PNP transistor, and 96 is the video output signal. First, during the period T ₁ , the current from the decoder does not flow through the resistor 80b, and the base of the PNP transistor is equal to the power supply voltage V _CC .Then, at the moment when t ₁ , the current from the decoder flows,
The state is read. At this time, the base potential of the PNP transistor becomes (V _CC -V _BE ), the PNP transistor is turned on, a charging current flows to the phototransistor 89a, and a video signal is output.
As the phototransistor is charged during period _T2 , the collector potential of the phototransistor increases. In other words, the collector potential of the PNP transistor also rises accordingly, and finally, when charging of the phototransistor is completed, the PNP transistor becomes saturated, and the potential of the collector 94 becomes (V _CC −V _CE(sat) ). . In this state, the parasitic capacitance 92 between the base and collector of the PNP transistor
is charged with a voltage of (V _BE −V _CE(sat) ), with the collector side 94 being positive and the base side 93 being negative.
Next, at the moment t ₂ is reached, the reading state ends and the transition to the integration state T ₃ occurs. At this time, no current flows from the decoder, and the base 93 becomes the power supply voltage V _CC . That is, the base 93 converts V _BE from (V _CC −V _BE ) to V _CC
The potential increases by The collector 94 is (V _CC −
The potential becomes higher than the power supply voltage of V _CE(sat) +V _BE ), and the charge stored in the parasitic capacitance 92 flows into the phototransistor 89a and is recharged. The signal at this time becomes the video signal occurring at _t2 in FIG.
From the above explanation, there has been a problem in that the video output signal appears twice in each phototransistor at the moment _t1 when it is placed in the readout state and at the moment _t2 when it is placed in the integration state, resulting in crosstalk.

In view of the above problems, the present invention solves the above problems.
This is because the potential of the collector 94 of the PNP transistor becomes higher than the potential of the base 93.This phenomenon is solved to provide an image sensor that eliminates the malfunction of recharging.

Means for Solving the Problems In order to solve the above problems, the image sensor of the present invention is configured to have a clamping function to prevent the collector potential of each PNP transistor constituting the current switch group from becoming higher than the base potential. It is something.

Effects In the present invention, the collector potential of the PNP transistor is clamped and does not become higher than the base potential due to the above-described configuration, so that the charge accumulated in the parasitic capacitance between the base and the collector is reduced at the time t ₂ in FIG. There is no flow into the phototransistor, which solves malfunctions caused by recharging.

Embodiment An image sensor according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a phototransistor and a current switch of an image sensor in a first embodiment of the present invention. 1 is a diode-connected transistor array for clamping, 80a is a PNP transistor, 80b is a resistor, 89a is a phototransistor, 88 is a power supply, 90 is a video signal output terminal, and 91 is a load resistor.

The operation of the image sensor configured as described above will be explained below using FIG. 1.

During period T ₁ in Figure 2, the power does not flow through the resistor 80b, and the base 93 of the PNP transistor is at the power supply voltage.
equal to V _CC , then in the read state T ₂ the current flows through resistor 80b and the base 93 is V _CC −V _BE2
(V _BE2 is the voltage between the base and emitter of the PNP transistor) and becomes the potential of the PNP transistor 80a.
turns on. When the PNP transistor 80a is turned on, a charging current flows through the phototransistor 89a, and this is output to the video signal output terminal 90.
As the phototransistor 89a is charged, the potential of the collector 94 of the PNP transistor rises, and when the potential exceeds nV _BE1 (V _BE1 is the rise voltage of the NPN transistor), the current flows to the n transistor string connected as a diode. The potential of the collector 94 does not rise any further and is clamped. The value (number) of this n is (V _CC −V _BE2 ) < nV _BE1
It is sufficient that the range satisfies the following relationship. In this way, the collector 94 of the PNP transistor 80a will not rise above the base 93, and even if the reading state changes to the integrating state at the time t ₂ , there will be no recharging and no malfunction will occur, and at the time t ₂ in FIG. , no video output signal appears.

FIG. 3 shows an example in which the clamping function is performed by an array of n diodes. In this case as well, if the forward voltage drop of each diode is V _D , the number n may be within the range that satisfies the relationship (V _CC - V _BE2 ) > nV _D , and the collector of the PNP transistor 89a is clamped at the potential of nV _D. The potential will not rise above this point.

FIG. 4 shows an example in which the clamping function is performed by a resistor. In this case, the collector 94 of the PNP transistor 89a is clamped by a voltage equal to the product of the current I _R flowing through the resistor 3 and the resistor 3 after the completion of charging the phototransistor.

As described above, by providing a clamping function using a diode-connected transistor array, a diode array, a resistor, etc., malfunctions caused by recharging the phototransistor are eliminated. Furthermore, in the case of use under low illuminance, conventionally, the collector potential of the PNP transistor rises to near the emitter potential, so the voltage between the collector and emitter becomes small, and at the time of t ₁ when the reading state is started, V as shown in Fig. 5 is applied. It operated at _CE1 , and the current capacity was I _C1 . However, if the clamp function of the present invention is applied, the collector potential of the PNP transistor will be nV _BE when a diode-connected transistor string is used, nV _D when a diode is used, and I _R × ( value of resistor 3)
The voltage between the collector and emitter does not rise above
It is larger than before, and in FIG. 5, V _CE2 > V _CE1 , the current capacity is I _C2 > I _C1 , the rush current value during charging becomes large, and the output value also becomes large.

Effects of the Invention As described above, the present invention provides the collector of the PNP transistor which is the current switch of the image sensor.
By providing a clamp circuit composed of a diode-connected transistor string, a diode string, or a resistor, it is possible to prevent malfunctions due to recharge caused by the inflow of charges accumulated in the parasitic capacitance between the collector and base of the PNP transistor. It has an excellent prevention effect. Another advantageous effect is that the current supply capacity of the PNP transistor at the moment it enters the reading state (at time t ₁ ) is greater than that of the conventional technology, increasing the sensitivity of the image sensor.

[Brief explanation of drawings]

Fig. 1 is a diagram in which the collector of the PNP transistor of the current switch of an image sensor according to the first embodiment of the present invention is clamped by a diode-connected transistor string, and Fig. 2 is a waveform of the video signal output from the circuit of Fig. 1. Figure 3 is a diagram where the clamp function is performed by a diode string, Figure 4 is a diagram where the clamp function is performed by a resistor, Figure 5 is a diagram of the operating point of the PNP transistor at the moment it shifts to the reading state, Figure 6 is a circuit diagram for generating complementary selection signals with different DC levels from the selection signal input of a conventional image sensor. A circuit diagram of the decoder, current switch, photodetector array, etc. that puts it into the reading state, Figure 8 is a circuit diagram showing a malfunction of a conventional current switch, and Figure 9 is a waveform diagram of the video signal output due to malfunction. It is. 1... Diode-connected transistor string for clamping, 2... Diode string for clamping, 3
... Clamping resistor, 92 ... Parasitic capacitance between the base and collector of the PNP transistor.

Claims (1)

[Claims] 1. A PNP transistor constituting a current switch, a phototransistor whose collector is connected to the collector of the PNP transistor, and
Connected to the collector of the PNP transistor and said
An image sensor comprising a clamping means for preventing a PNP transistor from becoming saturated. 2. The image sensor according to claim 1, wherein the clamping means is composed of one or more diode-connected transistors. 3. The image sensor according to claim 1, wherein the clamping means is composed of one or more diodes. 4. The image sensor according to claim 1, wherein the clamping means is constituted by a resistor.