Scanning circuit

Abstract: There is disclosed a scanning circuit composed of plural stages of unit circuit for releasing scanning pulses of two or more phases in succession from the unit circuits according to multiphase driving pulses, provided with set circuits for setting the unit circuits at a predetermined state and a switch circuit for activating the set circuit of a preceding unit circuit in response to the scanning pulse, wherein the switch circuits are activated by driving pulses different from the first-mentioned driving pulses.

Description: The present invention relates to a scanning circuit composed of plural stages of unit circuits for releasing scanning pulses of two or more phases in succession according to multi-phase driving pulses. Related Background Art Fig. 1 is a schematic circuit diagram of a conventional scanning circuit. Said conventional example is composed of n stages of units circuits, which release scanning pulses 11, 21, 12, ... in succession. In the unit circuit of the first stage, and in the presence of a start pulse hs, a transistor M1 is rendered conductive by a pulse h2 to elevate a potential V(1). A transistor M2 shows a conductance corresponding to said potential V(1) which is the gate potential of said transistor M2. Then, the pulse h2 is terminated and a pulse h1 is started, the potential of a terminal Op1 starts to rise through the transistor M2. Said potential rise is fed back to the gate of the transistor M2 through a capacitor C1, thereby elevating the potential V(1). As the rise of the potential V(1) increases the conductance of the transistor M2, the terminal Op1 provides a scanning pulse 11 without voltage drop. Also said first pulse 11 elevates the potential V(2) of a second stage through a transistor M3. Thus, at the upshift of the pulse h2, a terminal Op2 outputs a scanning pulse 21 through a transistor M6. At the same time, the pulse h2 turns on the transistor M4, thereby reducing the potential V(1) to a reference potential. Also a scanning pulse 12 of a third stage turns on a transistor M5, thereby reducing the potential V(2) to the reference potential. In this manner the scanning pulses are generated in succession according to the timing of the pulses h1 and h2. Fig. 2A is a circuit diagram showing an example of a signal reading device utilizing a conventional scanning circuit, and Fig. 2B is a timing chart showing an example of the function thereof. In the initial state it is assumed that capacitors Ct1, Ct2 respectively contain a sensor noise N and a sensor signal S containing noise. At the application of a high-level scanning pulse 11 with a pulse duration Tb, transistors Qt1 and Qs1 are turned on, whereby the sensor noise N stored in the capacitor Ct1 is released to an output line OUT1 through a bipolar transistor amplifier Q. After the noise N is read for a duration Tc (< Tb), a pulse bc is shifted to the high level while the scanning pulse 11 remains at the high level to turn on a transistor Qbc, thereby resetting the capacitor Ct1 and the base of a transistor Q. Then, by a scanning pulse 21 and a pulse bc, the sensor signal S stored in the capacitor Ct2 is released to an output line OUT2 for a duration Tc in a similar manner. The noise N and the sensor signal S supplied respectively to the output lines OUT1, OUT2 are subjected to a subtraction process, thereby eliminating the noise from the sensor signal S. At the same time the output of the bipolar transistor Q is supplied to the output line OUT1 or OUT2 through the turn-on resistance of a transistor Qs1 or QS2, thereby suppressing the feed-back through a diffusion capacitor and thus reducing the noise component. However, in such conventional structure, the effective period of reading the signal stored in the capacitors Ct1, Ct2 is Tc, which, as shown in Fig. 2B, is very short after the subtraction of the resetting period for the retentive component, in the duration Tb of the scanning pulse. For this reason the signal reading device utilizing the conventional scanning circuit is unable to provide a sufficiently high S/N ratio, thus being unable to achieve a high sensitivity when applied to an imaging sensor. Also in the conventional scanning circuit, since with the feedback resetting structure in which each stage is reset by the output scanning pulse of a succeeding stage, there is required an additional exclusive circuit for resetting the last stage, and the simultaneous resetting of all the stages prior to the start of the scanning circuit is not possible. In order to eliminate such drawbacks there has been proposed a structure of connecting resetting transistors respectively to the gate electrodes of the transistors M2 and giving a resetting pulse commonly to the gate electrodes of said resetting transistors, but the connection of another transistor to the gate electrode of the transistor M2 increases the parasite capacity thereof, thereby reducing the boot strap effect. Also it is possible to activate the resetting transistor M4 of each stage by another pulse, instead of the scanning pulse from the succeeding stage, but the scanning circuit becomes inevitably complex for generating such another pulse. French Patent Specification No. FR-A-2,532,777 and GB Patent Specification GB-A-2,022,953 both disclose circuits which generate scanning pulses in manners similar to that discussed and both have the problem that the duty cycle of the scanning pulses produced thereby is limited. An embodiment of the present invention eliminates or reduces the drawbacks of the conventional technology explained above. An embodiment of the present invention provides a signal reading device with an improved S/N ratio. An embodiment of the present invention provides an imaging device of a high sensitivity. An embodiment of the present invention provides a scanning circuit of a simple resetting structure. In accordance with one aspect of the present invention, there is provided a circuit for providing successive scanning pulses as set out in claim 1. The above-explained structure allows to obtain scanning pulses overlapped in time, thus having a duty ratio in excess of 50%. For example in case of driving a signal reading system, such scanning pulses allows to achieve an effective signal reading period longer than in the prior technology, thus achieving an improved S/N ratio and a higher sensitivity even in the high-speed operation. Such use of the driving pulse supplied to the succeeding stage as the resetting pulse for the preceding stage enables the resetting of the preceding stage and the simultaneous resetting of all the stages with a simple structure. Embodiments of the present invention, given by way of non-limiting example, will now be described with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic circuit diagram of a conventional scanning circuit; Fig. 2A is a circuit diagram showing an example of a signal reading device utilizing such conventional scanning circuit; Fig. 2B is a timing chart showing an example of the function thereof; Fig. 3A is a schematic circuit diagram constituting an embodiment of the scanning circuit of the present invention; Fig. 3B is a timing chart showing the function thereof; Fig. 3C is a timing chart showing a collective resetting in said embodiment; Fig. 3D is a timing chart showing a collective high-level setting in said embodiment; Fig. 4 is a schematic circuit diagram of a signal reading system of an image sensor utilizing said embodiment; Fig. 5A is a chart showing the function of a signal reading system utilizing said embodiment; Fig. 5B is a chart showing the function of a conventional system as a reference example; Figs. 6A through 6C are schematic circuit diagrams showing other examples of the signal reading system; Figs. 7A through 7C are block diagrams showing current limiting means; Fig. 8 is a block diagram showing an imaging device utilizing the above-explained image sensor; Fig. 9 is a chart showing the function of said imaging device; Fig. 10 is a schematic circuit diagram showing a signal processing system in another imaging device; Fig. 11 is a chart showing the function of said signal processing system; Fig. 12A is a block diagram of an example of color imaging device; Fig. 12B is a schematic view showing an example of arrangement of color filters thereof; Fig. 13A is a schematic view showing the principle of enlarged reading; and Fig. 13B is a schematic timing chart showing the function of an image sensor at the enlarged reading. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. Fig. 3A is a schematic circuit diagram showing an embodiment of the scanning circuit of the present invention, wherein transistors of same or similar functions as those in the conventional example shown in Fig. 1 are represented by same symbols. In the present embodiment, driving pulses c1, c2, different from the driving pulses h1, h2 are utilized for on/off control of the transistors M3, M4, M7, M8, M11, M12 etc. Also in circuits for the feedback of an arbitrary scanning pulse to the unit circuits of stages back for turning on the transistors M5, M9, M13 etc., the feedback lines are equipped with transistors Q1, Q2, ... which are on/off controlled by the driving pulse c1 or c2. In the following the function of the present embodiment will be explained with reference to a timing chart shown in Fig. 3B. At first, in a unit circuit of a preceding stage, during the presence of a start pulse hs, a transistor M1 is rendered conductive by the pulse h1 to elevate the potential V1. A transistor M2 shows a conductance corresponding to said potential V1, which is the gate potential of said transistor M2. Then, when the pulse h1 is downshifted and the pulse h2 is upshifted, a potential V2 is elevated through the transistor M2 and is fed back to the gate thereof through a capacitor C1, thereby further elevating the potential V1. Thus the conductance of the transistor M2 is further increased to transmit the pulse h2 as the potential V2 without voltage drop. A driving pulse c2 of a shorter duration is entered in this state, whereby a transistor M3 is turned on to elevate a voltage V3 of the unit circuit of the first stage. Then the driving h1 of longer duration is upshifted, thereby elevating a voltage V4 through a transistor M6 and further elevating the voltage V3 through a capacitor C2. Consequently the pulse h1 is released as a voltage V4, which is externally released as a scanning pulse 11. Simultaneously the pulse h1 turns on the transistor M1, thereby reducing the potential V1 to the reference potential. While the voltage V4 is at the high level state, the driving pulse c1 of shorter duration is upshifted, thereby turning on a transistor M7 of the unit circuit of the first stage and elevating a voltage V5 of the unit circuit of the second stage. Then, the upshift of a driving pulse h2 elevates a voltage V6 through a transistor M10 and a capacitor C3, thereby releasing a scanning pulse 21. At this moment the transistor Q1 of the feedback line remains turned off, whereby a transistor M5 also remains in the off state. Consequently the voltage V3 of the first stage remains at the high level, so that the scanning pulse 11 remains at the high level state. Subsequently, at the downshift of the driving pulse h1, the voltage V4 (scanning pulse 11) is shifted down, and the voltage V3 is also lowered. A subsequent upshift of the driving pulse c2 turns on a transistor M11 to elevate a voltage V7, and also turns on transistors M8, Q1 and M3. The turning-on of the transistor M8 resets the voltage V4 to the reference potential Vns. Also the turning-on of the transistor Q1 turns on a transistor M5 by means of the scanning pulse 21, and reduces the voltage V3 to the ground potential. The turning-on of the transistor M3 resets the voltage V2 to the ground potential. Therefore, as shown in Fig. 3B, scanning pulses 11, 21, 12, ... are released in succession and in mutually overlapping manner at the timing of the driving pulses h1 and h2. The obtained scanning pulses have a long duration with a duty ratio in excess of 50%. Fig. 3C is a timing chart showing a collective resetting in the present embodiment, and Fig. 3D is a timing showing a collective high-level setting. As shown in Fig. 3C, the collective resetting is achieved by shifting the driving pulses c1 and c2 to the high level state at the same time while the reference voltage Vns is at a low level state. A period T1 shows a collective resetting in the course of releasing of scanning pulses, while a period T2 shows a collective resetting at the start of scanning operation. Such collective resetting function is useful in an enlarged reading operation in an imaging device, as will be explained later. The collective high-level setting is achieved, as shown in Fig. 3D, by shifting the driving pulses c1 or c2 during a high level state of the reference voltage Vns. In a period T4, the shift of the pulse c2 to the high level state sets the scanning pulses 11, 12, ..., 1n at the high level, and, in a period T5, the shift of the pulse c1 to the high level state sets the scanning pulses 21, 22, ..., 2n at the high level. A period T3 indicates the collective resetting operation explained above. Fig. 4 is a schematic circuit diagram of a signal reading system of an image sensor, utilizing the foregoing embodiment. Said image sensor is provided with an m x n matrix arrangement of photosensor cells S11, S12 etc. Said cells are selected in succession by an unrepresented vertical scanning circuit, and sensor signals S are respectively released from the cells of a selected row. As will be explained later, said sensor signals S contain noise components N of the cells. For example, when the first row is selected, a pulse t2 turns on transistors Qb2, and the sensor signals S are stored in capacitors Ct2 by the reading operation of the cells S11 - S1n. Then the cells S11 - S1n are reset, and transistors Qb1 are turned on by a pulse t1. Subsequently said cells are read again to accumulate the noise components N of the cells into capacitors Ct1. In this manner the noise components N of the cells are stored in the capacitors Ct1, while the sensor signals S containing the noises are stored in the capacitors Ct2, and are supplied to output lines OUT1 - OUT4 according to the function of the present embodiment, as will be explained in more detail in the following. [Structure of signal reading system] The capacitors Ct1 and Ct2 corresponding to each column of the area sensor are connected, respectively through transistors Qt1 and Qt2, in common to the base terminal of a buffer amplifier Q. Said base terminal is grounded through a transistor Qbc, and the gate electrodes of the transistors Qbc commonly receive a pulse bc. The collector electrode of the buffer amplifier Q receives a predetermined positive voltage. Also the emitter electrode of the buffer amplifier Q is connected, in an odd column of the area sensor, through a transistor Qs1 to the output line OUT1 and through a transistor QS2 to the output line OUT2, and, in an even column, to the output lines OUT3, OUT4 in a similar manner. The release of the output of the bipolar transistor Q through the transistor Qs1 or Qs2 to the output lines enables to limit the current by the on-state resistance of said transistors as will be explained later, thereby reducing the noises. This is effective when the scanning pulses are wide, as in the present invention. The scanning pulses 11 and 21 from the scanning circuit 1 are respectively supplied to the gate electrodes of the transistor Qt1 and Qs1 corresponding to the first and second columns of the area sensor. The scanning pulse 12 is supplied, in addition to the gate electrodes of the transistors Qt2 and Qs2 corresponding to the first column, to the gate electrodes of the transistors Qt1 and Qs1 corresponding to the third column. Also the scanning pulse 22 is supplied, in addition to the gate electrodes of the transistors Qt2 and Qs2 corresponding to the second column, to the gate electrodes of the transistors Qt1 and Qs1 corresponding to the fourth column. Subsequent scanning pulses 13, 23, 14, ... are also supplied in a similar manner. The output lines OUT1 - OUT4 are grounded respectively through transistors Qr1 - Qr4. The gate electrodes of the transistors Qr1 and Qr4 receive a driving pulse c2, while those of the transistors Qr2 and Qr3 receive a driving pulse c1. [Function of the signal reading system] Fig. 5A is a chart showing the function of a signal reading system of the present employing the present embodiment, and Fig. 5B is a chart showing the function of a conventional system as a reference example. When the scanning pulse 11 of a long duration is released from the scanning circuit 1 of the present embodiment while the noise component N and the sensor signal S are respectively stored in the capacitors Ct1 and Ct2, the transistors Qt1 and Qt2 corresponding to the first column are turned on as explained above. Thus the noise component N1 corresponding to the first column is released to the output line OUT1 through the buffer amplifier Q (see column of scanning pulse 11 in Fig. 5A). Then, when the scanning pulse 21 is released, the transistors Qt1 and Qs1 corresponding to the second columm are turned on, whereby the noise component N2 is released to the output line OUT3 through the buffer amplifier Q (see column of scanning pulse 21). Subsequently, in response to the release of the scanning pulse 12, the sensor signal S1 and the noise component N3 are respectively released to the output lines OUT2 and OUT1 in a similar manner. Thereafter the sensor signals and noise components are released in succession as shown in Fig. 5A. The output lines OUT1 and OUT2 are reset by the transistors Qr1, Qr2 at the timing of the driving pulses c2, while the output lines OUT3 and OUT4 are reset by the transistors Qr3, Qr4 at the timing of the driving pulses c1. After the lapse of a predetermined time from the upshift of each scanning pulse, the pulse bc is shifted up to turn on the transistor Qbc, whereby the capacitor Ct1 or Ct2 corresponding to a column from which the signals have been read is reset. Fig. 5B shows a conventional driving method for comparison. In the conventional system, since the signal reading is conducted by alternately using the driving pulses h1 and h2, the effective signal reading period is shortened as already explained in relation to Fig. 2B. The signal reading system is not limited to that of the above-explained image sensor, but can be modified in various manners. Figs. 6A through 6C are schematic circuit diagrams showing other embodiments of the signal reading system. Fig. 6A shows a circuit of output by capacity division, while Fig. 6B shows a circuit having a scanning switch at the base of a bipolar transistor, and Fig. 6C shows a circuit having a scanning switch at the emitter of a bipolar transistor. In the signal reading system of the present embodiment, as already explained above, the output of the bipolar transistor amplifier Q is released to the output line OUT through the turn-on resistance of the transistor Qs1 or Qs2. Said turn-on resistance limits the output current, thereby contributing to the noise reduction. The current limitation can be achieved not only by a resistor or the turn-on resistance of a transistor, but also by a slower upshift of the scanning pulse. Figs. 7A through 7C are schematic views showing different current limiting means. Fig. 7A shows a method of utilizing the turn-on resistance r, r′ of transistors, employed in the above-explained signal reading system. Fig. 7B shows a method of relaxing, with an RC time constant, the upshift of the driving pulses h1 and h2 supplied to the scanning circuit 1. Inversely, the time constant may become too large due to the wiring resistances and the parasite capacitances in the integrated circuit. In such case, as shown in Fig. 7C, there may be provided a buffer circuit, after the protective circuit, for reducing the impedance, followed by a resistor r″ and a capacitor C1 or C2 according to the design value. Fig. 8 is a schematic block diagram of an example of imaging device utilizing the image sensor explained above, and Fig. 9 is a chart showing the function thereof. In. Fig. 8, a sensor 101 corresponds to the image sensor shown in Fig. 3. The driving pulses h1, h2, c1, c2, hs, bc etc. are supplied from a driver 102, which functions according to a clock signal from a clock generator 103. As shown in Fig. 5A, the output lines OUT1 and OUT3 of the sensor 101 release the noise components N, while the output lines OUT2 and OUT4 release the sensor signals S with a delay of one cycle from the corresponding noise components, according to the timing of the scanning pulses h1 and h2. Consequently, in order to eliminate the noise component Ni from an arbitrary sensor signal Si, it is necessary to delay the noise component Ni by one cycle and to effect subtraction of two signals. For this purpose delay lines DL1 of one cycle period are connected to the output lines OUT1 and OUT3. The output lines OUT1 and OUT2 release the sensor signals and noise components of odd columns of the area sensor, while the output lines OUT3 and OUT4 release those of even columns. Consequently, in order to restore the original arrangement of the signals of the odd and even columns, there are provided a delay line DL2 and a sample-hold circuit 104, which selects a terminal a or b according to S/H pulses from the clock generator 103. As shown in Fig. 5, the output 10a of the delay line DL2 is the sensor signals S1′, S3′, ... of odd columms after noise component elimination, while the output 10b is the sensor signals S2′, S4′, ... of even columns after noise component elimination. Thus the sample-hold circuit 104 releases an output signal 10c composed of point-sequential sensor signals S1′, S2′, S3′, ... according to the S/H pulses. The output signal 10c is subjected to the elimination of high frequency components by a low-pass filter LPF1, and is converted for example into a television signal by a process circuit 105. The present embodiment, being capable of supplying scanning pulses of a large duration, allows to obtain the sensor signals S of reduced noises and the noise components N by the current limiting means. Consequently the imaging device can provide a sensor signal S′ of a high S/N ratio, obtained after the elimination of the noise component N from the sensor signal S, thereby achieving a high sensitivity. Fig. 10 is a schematic circuit diagram of a signal processing system in another imaging device, and Fig. 11 is a chart showing the function thereof. In this signal processing system, the sensor signals S and the noise components N are separately formed as point-sequential signals by switches SW3 and SW2. The output signals 10d of the switch SW2 are delayed by a delay line DL1 as output signals 10d′ for phase matching, and then subtracted from the point sequential sensor signals 10e to obtain sensor signals 10c without the noise components. This signal processing system can dispense with a delay line, in comparison with the circuit shown in Fig. 8. Fig. 12A is a schematic block diagram showing an example of color imaging device, and Fig. 12B is a schematic view showing an example of the arrangement of color filters thereof. A color sensor 201 has the structure of the embodiment shown in Fig. 3 on both sides of an area sensor, and has eight output lines OUT1 - OUT8. Fig. 12B shows an example of the arrangement, wherein W, R and B respectively stand for white, red and blue. The output line OUT2 releases the signals W1, W3, ... of odd rows, the output line OUT4 releases the signals W2, W4, ... of even rows, the output line OUT6 releases the signals R1, R3, ... of odd rows, and the output line OUT8 releases the signals B2, B4, ... of even rows. Other output lines OUT1, 3, 5 and 7 are used for the respectively corresponding noise components. The sensor signals of respective colors are subjected to the elimination of noise components by the subtractions explained above, whereby obtained are white signals w1, w2, a red signal r and a blue signal b. The white signals w1, w2 are constructed into the original arrangement in the above-explained manner by a sample-hold circuit 204, and are supplied, through a low-pass filter LPF1, as a luminance signal Y to a color process circuit 205. An adder 206 adds the red signal r and the blue signal b with suitable coefficients (k₁r + k₂b), and an adder 207 adds the white signals w1 and w2 (w1 + w2). The obtained results are subjected to subtraction in a subtractor 208 to obtain a green signal g. The color signals r, g, b thus obtained are supplied, through a low-pass filter LPF2, as color signals R, G, B to the color process circuit 205 together with the luminance signal Y, for conversion for example into a television signal. In the following there will be explained an application of the collective resetting function of the present invention. Fig. 13A is a schematic view showing the principle of enlarged reading, and Fig. 13B is a schematic timing chart showing the driving method of the image sensor in the enlarged reading. In case of enlarged reading of the signals of an area B of the image sensor 301, it is necessary to eliminate unnecessary portions a and c in a horizontal scanning 302, during the horizontal blanking period. During said horizontal blanking period, a start pulse hs is entered to release the scanning pulses in synchronization with high-speed driving pulses h1 and h2, whereby the signals of the unnecessary portion a are transferred at a high speed in a period ta. Then the signals of the area b to be enlarged are transferred at a low speed in an effective period tb, and pulses c1 and c2 are then shifted up for collective resetting to initialize the scanning circuit 1. In this manner the unnecessary portion c can be eliminated without the unnecessary scanning of the scanning circuit 1. Consequently the unnecessary scanning period of the scanning circuit 1 is limited to the period ta, thus significantly reduced in comparison with the prior technology. As detailedly explained in the foregoing, the scanning circuit of the present invention is capable of releasing the scanning pulses mutually overlapping in time, with a duty ratio in excess of 50%. Therefore, for example in an application for driving a signal reading system, the effective signal reading period can be made longer than in the conventional technology, so that a high S/N ratio and a high sensitivity can be attained even in a high-speed operation.

Claims: A circuit for providing successive scanning pulses in response to plural phase first drive signals (h1, h2), comprising a plurality of unit circuits connected in series, each unit circuit comprising: first switch means (M6, M10) for generating a scanning pulse (11, 21..) at an output in response to one of said first drive signals which scanning pulse also activates the succeeding unit circuit, a feedback line connected to the output of the succeeding unit circuit, and resetting means (M5, M9, M13, M17) connected to said feedback line for resetting said unit circuit, and characterised in that said feedback line comprises a second switch means (Q₁,Q₂...) for activating said resetting means, said second switch means being responsive to the scanning pulse from the succeeding unit circuit in combination with a second drive signal (C1, C2) which is different to the first drive signals (H1, H2). A circuit according to claim 1, wherein one unit circuit is adapted to generate a said scanning pulse in response to one of a pair of different phase first drive signals (H1), and the subsequent unit circuit is adapted to generate a said scanning pulse in response to the other of the different phase first drive signals (H2), and the resetting means are adapted to reset the unit circuits in response to a pair of different second drive signals (C1, C2) such that the scanning pulse generated by successive unit circuits overlap in time. A circuit according to either of claims 1 or 2, wherein each said resetting means is connected to the output of an adjacent unit circuit and resets its associated unit circuit in response to the combination of a said second drive signal (C1, C2) and a said scanning pulse (11, 21, ...) from said adjacent unit circuit. A circuit as claimed in any one of the preceding claims and further comprising at least a line of photoelectric converting elements connected to be scanned by said scanning pulses.

Inventors: HASHIMOTO SEIJI, OHZU HAYAO, HASHIMOTO, SEIJI, OHZU, HAYAO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G11C19
IPC codes: G11C19, H03K5