Patent Publication Number: US-8125506-B2

Title: Electro-optical device and electronic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2006-146430 filed in the Japanese Patent Office on May 26, 2006, the entire disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     Exemplary embodiments of the present invention relate to techniques for controlling electro-optical elements, such as organic light-emitting diodes. 
     2. Related Art 
     Electro-optical devices including an arrangement of a plurality of electro-optical elements are used for various applications, such as exposure devices of electrophotographic image forming apparatuses or display devices of various electronic apparatuses. For this type of electro-optical device, techniques for correcting characteristic errors (differences from design values or variations among the elements) of the electro-optical elements or active elements for driving the electro-optical elements have been proposed. For example, JP-A-8-39862 (FIG. 6) discloses, as shown in  FIG. 14 , a technique for supplying a drive current I, which is the sum of a predetermined drive current Ia and a correction current Ib according to a correction value of each light-emitting element, to the light-emitting element over a time length T according to a tone level specified to the light-emitting element. 
     Since the correction current Ib in the above structure is a current for correcting a small characteristic error of each light-emitting element or active element, the current value of the correction current Ib is significantly smaller than that of the drive current Ia. In general, a circuit that can generate such a small current with high precision and implement correction over a wide range is difficult to realize. If such a circuit is realized, the size of the circuit becomes large. 
     SUMMARY 
     Exemplary embodiments of the invention correct the tone level of each light-emitting element with a simple structure. 
     According to an exemplary embodiment, there is provided an electro-optical device including the following elements: a potential generating circuit that generates a first power supply potential (e.g., a potential VAN 1  shown in  FIG. 4 ) and a second power supply potential (e.g., a potential VAN 2  shown in  FIG. 4 ) that is different from the first power supply potential; a first signal processing circuit that selectively outputs one of a first high potential (e.g., a potential VDD 1  shown in  FIG. 4 ) and a first low potential (e.g., a potential VSS 1  shown in  FIG. 4 ) according to a data signal at one of at least three levels; a second signal processing circuit that selectively outputs one of a second high potential (e.g., a potential VDD 2  shown in  FIG. 4 ) that is different from the first high potential and a second low potential (e.g., a potential VSS 2  shown in  FIG. 4 ) that is different from the first low potential according to the data signal; a first current source that generates a first current in accordance with an output of the first signal processing circuit and the first power supply potential; a second current source that generates a second current in accordance with an output of the second signal processing circuit and the second power supply potential; and an electro-optical element that provides a tone level according to the first current and the second current. 
     With the above structure, the generation of the first current by the first current source and the generation of the second current by the second current source are individually controlled according to the multi-value data signal. Therefore, a period during which the second current is generated can be set to be shorter than a period during which the first current is generated. With this structure, the current value of the second current can be greater than that in the case where the tone level of each electro-optical element is always controlled according to the sum of the first current and the second current. Therefore, the tone level of each electro-optical element can be corrected with high precision, while the circuit size of the second current source is reduced. Compared with the case in which the first current and the second current are individually controlled using different data signals (e.g., binary signals), the rate at which the data signal is transferred or the operation speed of the first and second signal processing circuits is reduced, and the number of data signal transfer lines or the number of signal input terminals is reduced. 
     According to an exemplary embodiment, the first high potential differs from the second high potential, and the first low potential differs from the second high potential. Therefore, if a common power supply potential is supplied to the first current source and the second current source, the relationship between the output of the first signal processing circuit and the first current becomes different from the relationship between the output of the second signal processing circuit and the second current. According to an exemplary embodiment, since the first power supply potential supplied to the first current source and the second power supply potential supplied to the second current source are set to different values, the relative relationship between the first current and the second current can be easily understood at the time of design. For example, in the case that the first and second current sources include transistors, the first current and the second current can be easily and reliably set to an expected ratio according to the characteristics (e.g., the channel widths) of the transistors, provided that the voltage between the gate and the source of the first current source (the difference between the first high potential or the first low potential and the first power supply potential) is made substantially equal to the voltage between the gate and the source of the second current source (the difference between the second high potential or the second low potential and the second power supply potential) by appropriately selecting the first power supply potential and the second power supply potential. Even in the case that the first and second signal processing circuits include logic circuits (e.g., memories M 1  and M 2  shown in  FIG. 5 ) driven by different voltages, no conversion circuit for equalizing the logic levels of the logic circuits is necessary. This advantageously reduces the circuit size of the electro-optical device. 
     The electro-optical element is an element that changes its optical characteristics, such as brightness or transmission factor, by applying electrical energy, such as current or voltage, to the electro-optical element. For example, some embodiments of the invention are applicable to an electro-optical device (light-emitting device) using light-emitting elements, such as organic light-emitting diodes, as electro-optical elements. Needless to say, the structure having, besides the first and second signal processing circuits and the first and second current sources, an additional signal processing circuit or an additional current source (e.g., the structure having at least three pairs of a signal processing circuit (C 1  to C 3 ) and a current source (Q 1  to Q 3 ) shown in  FIG. 9 ) may belong to the scope of the invention. 
     To control each of the first and second current sources on a binary basis according to the multi-value data signal, for example, the first signal processing circuit controls the first current source on a binary basis according to a first threshold and the level of the data signal, and the second signal processing circuit controls the second current source on a binary basis according to a second threshold differing from the first threshold and the level of the data signal (e.g., the structure shown in  FIG. 7 ). In this structure, however, it is necessary to make the characteristics (e.g., the channel widths) of active elements of the first and second signal processing circuits different in order to achieve an expected relative ratio of the first and second thresholds. This imposes a difficulty in reducing the size of the first and second signal processing circuits. Accordingly, it is preferable that the first signal processing circuit include a first logic circuit that selectively outputs one of the first high potential and the first low potential according to the data signal, the first high potential and the first low potential serving as logic levels, and that the second signal processing circuit include a second logic circuit that selectively outputs one of the second high potential and the second low potential according to the data signal, the second high potential and the second low potential serving as logic levels. Accordingly, the size of the first and second signal processing circuits can be reduced to be smaller than that in the case where the multi-value data signal is converted into binary signals according to the relative ratio of the first and second thresholds. 
     It is preferable that the first low potential and the second low potential be the same potential, and that the data signal be at one of the levels of the first high potential, the first low potential, and the second low potential. Accordingly, the total number of potentials supplied from the outside to the electro-optical device is reduced. This further reduces the power necessary for driving the electro-optical device. 
     It is also preferable that the electro-optical device further include a first selection circuit that selects a first on-potential (e.g., a potential VG 1  shown in  FIG. 8 ) or a first off-potential (e.g., the potential VDD 1  shown in  FIG. 8 ) according to an output of the first logic circuit; and a second selection circuit that selects a second on-potential (e.g., a potential VG 2  shown in  FIG. 8 ) that is different from the first on-potential or a second off-potential (e.g., the potential VDD 2  shown in  FIG. 8 ) that is different from the first off-potential according to an output of the second logic circuit. In this case, the first current source generates the first current on the basis of the potential selected by the first selection circuit and the first power supply potential, and the second current source generates the second current on the basis of the potential selected by the second selection circuit and the second power supply potential. Accordingly, the first current and the second current can be adjusted by appropriately adjusting the first on-potential, the second-on potential, the first off-potential, and the second off-potential, without needing to change the first high potential and the first low potential used in the first signal processing circuit and the second high potential and the second low potential used in the second signal processing circuit (that is, without affecting the operation of the first and second signal processing circuits). This will be described later as a second embodiment. 
     It is preferable that the first current source be a transistor having a source to which the first power supply potential is supplied and a gate having a potential according to the output of the first signal processing circuit, and that the second current source be a transistor having a source to which the second power supply potential is supplied and a gate having a potential according to the output of the second signal processing circuit. Accordingly, the current values of the first current and the second current can be easily set by appropriately selecting the characteristics (e.g., the channel lengths and channel widths) of the transistors used as the first and second current sources. 
     The electro-optical device according to an exemplary embodiment of the invention is used in various electronic apparatuses. A typical example of an electronic apparatus is an electrophotographic image forming apparatus using the electro-optical device according to an exemplary embodiment of the invention to expose an image supporting member, such as a photosensitive drum or the like. The image forming apparatus includes the image supporting member on which a latent image is formed by exposure, the electro-optical device according to an exemplary embodiment of the invention, which is used to expose the image supporting member, and a developing unit that develops an image by applying a developer such as toner to the latent image on the image supporting member. However, the use of the electro-optical device according to an exemplary embodiment of the invention is not limited to exposing the image supporting member. For example, the electro-optical device according to the aspect of the invention can be used to illuminate a document in an image scanning apparatus such as a scanner. The image scanning apparatus includes the electro-optical device according to an exemplary embodiment and a light receiving device (such as a charged coupled device (CCD)) that converts light emitted from the electro-optical device and reflected from an object to be scanned (e.g., the document) into an electrical signal. The electro-optical device having electro-optical elements arranged in a matrix may also be used as a display section of various electronic apparatuses, such as a personal computer and a cellular phone. 
     Some embodiments of the invention can be specified as a method of driving the electro-optical device. The drive method includes generating a first power supply potential and a second power supply potential that is different from the first power supply potential; generating a first current in accordance with one of a first high potential and a first low potential selected according to a data signal at one of at least three levels and the first power supply potential; generating a second current in accordance with one of a second high potential that is different from the first high potential and a second low potential that is different from the first low potential selected according to the data signal and the second power supply potential; and controlling an electro-optical element to provide a tone level according to the first current and the second current. With this method, advantages similar to those of the electro-optical device according to an embodiment of the invention can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective view of the structure of part of an image forming apparatus according to a first embodiment. 
         FIG. 2  is a block diagram of the structure of an electro-optical device. 
         FIG. 3  is a timing chart showing the relationship between selection signals and a data signal. 
         FIG. 4  is a conceptual diagram of the level of each potential generated by a potential generating circuit. 
         FIG. 5  is a block diagram of the structure of a unit circuit. 
         FIG. 6  is a timing chart for describing the waveform of a drive current. 
         FIG. 7  is a block diagram of the structure of a unit circuit according to a comparative example. 
         FIG. 8  is a block diagram of the structure of a unit circuit according to a second embodiment. 
         FIG. 9  is a block diagram of the structure of a unit circuit according to a third embodiment. 
         FIG. 10  is a conceptual diagram of the level of each potential generated by the potential generating circuit. 
         FIG. 11  is a timing chart for describing the waveform of the drive current. 
         FIG. 12  is a timing chart showing the waveform of the drive current according to a modification. 
         FIG. 13  is a sectional view of the structure of the image forming apparatus. 
         FIG. 14  is a conceptual diagram showing the relationship between a drive current and a correction current according to a known technique. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A. First Embodiment 
       FIG. 1  is a perspective view of the structure of part of an electrophotographic image forming apparatus using an electro-optical device according to a first embodiment of the invention as an exposure device (line head). As shown in  FIG. 1 , the image forming apparatus includes an electro-optical device H, a converging lens array  60 , and a photosensitive drum  70 . The photosensitive drum  70  is supported by a rotating shaft extending in a main scanning direction and rotates in such a manner that the peripheral surface of the photosensitive drum  70  faces the electro-optical device H. The converging lens array  60  is a condenser having an array of many gradient index lenses and is positioned between the electro-optical device H and the photosensitive drum  70 . 
       FIG. 2  is a block diagram of the structure of the electro-optical device H. As shown in  FIG. 2 , the electro-optical device H includes an array of n electro-optical elements E arranged in the main scanning direction and n unit circuits U 1  to Un corresponding to the electro-optical elements E, respectively (where n is a natural number). The electro-optical elements E and the unit circuits U 1  to Un are formed on an insulating substrate. The unit circuit Ui at an i-th stage (where i is an integer satisfying 1≦i≦n) supplies a drive current IDR to the electro-optical element E at the i-th stage. The electro-optical elements E are organic light-emitting diodes (light-emitting elements) each having a light-emitting layer made of an organic electroluminescence (EL) material between an anode and a cathode facing each other. Each of the electro-optical elements E emits light with a brightness according to the current value of the drive current IDR. As shown in  FIG. 1 , a light beam emitted from each of the electro-optical elements E passes through the converging lens array  60  and reaches the surface of the photosensitive drum  70 . With this exposure, a latent image in accordance with a desired image is formed on the surface of the photosensitive drum  70 . 
     As shown in  FIG. 2 , the electro-optical device H includes a control circuit  12 , a selection circuit  14 , and a potential generating circuit  16 . The control circuit  12  controls each element by supplying a control signal (e.g., a clock signal) and outputs a data signal D to a signal line  18 . The data signal D is a voltage signal for specifying a tone level to each electro-optical element E. The selection circuit  14  is a section that generates and outputs selection signals S 1  to Sn for sequentially selecting the n unit circuits U 1  to Un (e.g., an n-bit shift register). As shown in  FIG. 3 , the selection signals S 1  to Sn sequentially reach an active level (high level) every predetermined period (hereinafter referred to as a “unit period”) T. 
     The potential generating circuit  16  shown in  FIG. 2  generates a plurality of potentials (VDD 1 , VSS 1 , VDD 2 , VSS 2 , VAN 1 , VAN 2 , and VCT) used in the electro-optical device H.  FIG. 4  schematically shows the level of each potential generated by the potential generating circuit  16 . As shown in  FIG. 4 , the potential VDD 1  and the potential VSS 2  are the same potential. In the following description, however, the potential VDD 1  and the potential VSS 2  are distinguished from each other for convenience using different reference numerals. The potential VSS 1  is lower than the potential VDD 1 , and the potential VDD 2  is higher than the potential VSS 2 . The potential VAN 1  is a potential between the potential VSS 1  and the potential VDD 1 , and the potential VAN 2  is a potential between the potential VSS 2  and the potential VDD 2 . The potential VCT is, as shown in  FIG. 2 , commonly supplied to the cathode of each electro-optical element E and serves as a reference potential (e.g., a ground potential) for the voltage of each element. The potential VCT is lower than the potential VAN 1  or the potential VAN 2 . 
     Referring now to  FIG. 5 , the structure of each of the unit circuits U 1  to Un will be described. Although only one unit circuit U 1  at the i-th stage is shown in  FIG. 5 , the unit circuits U 1  to Un have the same structure. As shown in  FIG. 5 , the unit circuit Ui includes two signal processing circuits C (C 1  and C 2 ) and two current sources Q (Q 1  and Q 2 ). 
     The current source Q 1  is a section that generates a drive current I 1 . The current source Q 2  is a section that generates a correction current I 2 . According to the first embodiment, p-channel thin-film transistors are shown as examples of the current sources Q 1  and Q 2 . The gate of the current source Q 1  is connected to an output end of the signal processing circuit C 1 , and the gate of the current source Q 2  is connected to an output end of the signal processing circuit C 2 . The potential VAN 1  generated by the potential generating circuit  16  is supplied to the source of the current source Q 1 , and the potential VAN 2  is supplied to the source of the current source Q 2 . The drains of the current sources Q 1  and Q 2  are connected to a node Z. The node Z is connected to the anode of the electro-optical element E. As shown in  FIG. 6 , the sum of the drive current I 1  and the correction current I 2 , that is, the drive current IDR, is supplied to the electro-optical element E. The electro-optical element E is controlled to emit light whose amount is controlled according to a time integral of the drive current IDR, which will be described later. 
     A period during which the drive current I 1  is generated (hereinafter referred to as a “drive period”) P 1 , and a period during which the correction current I 2  is generated (hereinafter referred to as a “correction period”) P 2  are individually controlled according to the data signal D. The drive period P 1  is set to a time length according to a tone level specified to the electro-optical element E. That is, as shown in  FIG. 6 , the drive current I 1  is supplied to the node Z for the drive period P 1  consisting of unit periods T (T 1  to T 9 ), the number of which corresponds to the tone level specified to the electro-optical element E, of a period F corresponding to 15 unit periods T (T 1  to T 15 ). For the remaining period (T 10  to T 15 ) of the period F, the supply of the drive current I 1  is stopped. 
     The time length of the correction period P 2  is determined in accordance with the results of preliminary observations and measurements of the characteristics of the unit circuits U 1  to Un (particularly the current sources Q 1  and Q 2 ) and the electro-optical elements E such that the actual tone levels of the electro-optical elements E to which the same tone level is specified are equalized. That is, as shown in  FIG. 6 , the correction current I 2  is supplied to the node Z for the correction period P 2  consisting of the unit periods T (T 1  to T 4 ), the number of which corresponds to the characteristics of the unit circuit U and the electro-optical element E, of the period F. For the remaining period (T 5  to T 15 ) of the period F, the supply of the correction current I 2  is stopped. As shown in  FIG. 6 , the drive period P 1  and the correction period P 2  are periods starting from the beginning of the period F. The correction period P 2  is set to be shorter than the drive period P 1 . Therefore, the current value of the drive current IDR is controlled to be one of the following: (1) the sum of the drive current I 1  and the correction current I 2  (T 1  to T 4  of  FIG. 6 ); (2) the current value of the drive current I 1  (T 5  to T 9 ); and (3) zero (T 10  to T 15 ). 
     Assume that the time integral of the drive current IDR of the first embodiment is the same as the time integral of the drive current I shown in  FIG. 14  (that is, when the electro-optical elements E are controlled to provide the same tone level). According to the first embodiment, the correction period P 2  during which the correction current I 2  is supplied is set to be shorter than a period P of  FIG. 14 . Therefore, if the drive current I 1  of  FIG. 6  and the current Ia of  FIG. 14  have the same current value and the same pulse width, the correction current I 2  of  FIG. 6  is set to be larger than the current Ib of  FIG. 14 . That is, according to the first embodiment, it is no longer necessary to set the correction current I 2  to a very small current value (the current Ib of  FIG. 14 ). Therefore, a large circuit that can generate a small current becomes no longer necessary, and the current value of the correction current I 2  can be controlled with high precision. As shown in  FIG. 5 , when the current sources Q 1  and Q 2  are provided for each electro-optical element E, the number of current sources Q 1  and Q 2  increases as the electro-optical elements E provide higher resolution. In that case, it becomes difficult to provide sufficient space for the current sources Q 1  and Q 2 . According to the first embodiment, the space needed for the current source Q 2  is reduced. Accordingly, the resolution of the electro-optical elements E can be easily improved. 
     The control circuit  12  shown in  FIG. 12  generates the data signal D such that the drive current IDR satisfying the following conditions is generated. The data signal D according to the first embodiment is a multi-value voltage signal specifying the generation and stopping of the drive current I 1  and the correction current I 2  for each unit time T. As shown in  FIG. 3 , the data signal D has one of three potentials d 1  to d 3  according to the tone level specified to the unit circuit Ui in a period during which a selection signal Si reaches an active level. The potential d 1  specifies the stopping of both the drive current I 1  and the correction current I 2 . The potential d 2  specifies the generation of the drive current I 1  and the stopping of the correction current I 2 . The potential d 3  specifies the generation of both the drive current I 1  and the correction current I 2 . As shown in  FIG. 4 , the potential VSS 1  generated by the potential generating circuit  16  is used as the potential d 1 . Similarly, the potential VSS 2  (=VDD 1 ) is also used as the potential d 2  of the data signal D, and the potential VDD 2  is also used as the potential d 3 . 
     As has been described above, the multi-value data signal D is used in the first embodiment. In comparison with the structure in which, for example, the generation and stopping of the drive current I 1  and the generation and stopping of the correction current I 2  are specified using different binary signals, the rate at which the data signal D is transferred or the operation speed of the signal processing circuits C 1  and C 2  is reduced, and the number of signal transfer lines or the number of signal input terminals is reduced. 
     As shown in  FIG. 5 , the data signal D output from the control circuit  12  is commonly supplied to the signal processing circuits C 1  and C 2  of each of the unit circuits U 1  to Un via the signal line  18 . The signal processing circuits C 1  and C 2  are sections that obtain and maintain the data signal D supplied from the control circuit  12  (latch circuits). The signal processing circuit C 1  includes a memory M 1  having an output end connected to the gate of the current source Q 1 , and a switch SW provided between the signal line  18  and the memory M 1  for controlling an electrical connection established between the signal line  18  and the memory M 1 . Similarly, the signal processing circuit C 2  includes a memory M 2  having an output end connected to the gate of the current source Q 2 , and a switch SW for controlling an electrical connection established between the signal line  18  and the memory M 2 . Each of the switches SW of the unit circuit Ui is selectively turned on during the period in which the selection signal Si is at the active level (high level). The memories M 1  and M 2  each obtain the data signal D from the signal line  18  during the period in which the corresponding switch SW is turned on and maintain an output according to the potential of the data signal D until the next time the selection signal Si reaches the active level. 
     The memory M 1  is a logic circuit activated by the potential VDD 1  and the potential VSS 1  serving as power supply potentials. The memory M 1  outputs a control signal G 1  generated by inverting the logic level of the data signal D to the gate of the current source Q 1 . That is, as shown in  FIG. 6 , the memory M 1  outputs the control signal G 1  at the potential VDD 1  (high level) in the case that the data signal D obtained from the signal line  18  has the potential d 1  (low level), and the memory M 1  outputs the control signal G 1  at the potential VSS 1  (low level) in the case that the data signal D has the potential d 2  or d 3  (high level). 
     The memory M 2  is a logic circuit activated by the potential VDD 2  and the potential VSS 2  serving as power supply potentials. As in the memory M 1 , the memory M 2  outputs a control signal G 2  generated by inverting the logic level of the data signal D to the gate of the current source Q 2 . That is, as shown in  FIG. 6 , the memory M 2  outputs the control signal G 2  at the potential VDD 2  (high level) in the case that the data signal D obtained from the signal line  18  has the potential d 1  or d 2  (low level), and the memory M 2  outputs the control signal G 2  at the potential VSS 2  (low level) in the case that the data signal D has the potential d 3  (high level). As has been described, since the memories M 1  and M 2  have different potentials corresponding to the logic levels, the control signals G 1  and G 2  in the case that the data signal D has the potential d 2  have different logic levels. The higher potential VDD 1  of the control signal G 1  is different from the higher potential VDD 2  of the control signal G 2 , and the lower potential VSS 1  of the control signal G 2  is different from the lower potential VSS 2  of the control signal G 2 . 
     The current sources Q 1  and Q 2  shown in  FIG. 5  operate in a saturation region. Therefore, the drive current I 1  has a current value according to a voltage Vgs 1  between the gate and the source of the current source Q 1  (current value independent of the voltage between the source and the drain). That is, the current source Q 1  generates the drive current I 1  in the case that the control signal G 1  has the potential VSS 1  and stops generating the drive current I 1  in the case that the control signal G 1  has the potential VDD 1 . Similarly, the correction current I 2  has a current value according to a voltage Vgs 2  between the gate and the source of the current source Q 2 . That is, the current source Q 2  generates the correction current I 2  in the case that the control signal G 2  has the potential VSS 2  and stops generating the correction current I 2  in the case that the control signal G 2  has the potential VDD 2 . Therefore, as shown in  FIG. 6 , in the case that the data signal D obtained in the unit circuit Ui has the potential d 1 , the current value of the drive current IDR output to the electro-optical element E is zero. In the case that the data signal D has the potential d 2 , the drive current I 1  is output as the drive current IDR. In the case that the data signal D has the potential d 3 , the sum of the drive current I 1  and the correction current I 2  is output as the drive current IDR. 
     The potential VAN 1  at the source of the current source Q 1  and the potential VAN 2  at the source of the current source Q 2  are set to individual potentials such that the voltage Vgs 1  of the current source Q 1  and the voltage Vgs 2  of the current source Q 2  are substantially equal. Specifically, assume a case that the control signals G 1  and G 2  are at a low level (potentials VSS 1  and VSS 2 ) (that is, the drive current I 1  and the correction current I 2  are generated). The voltage Vgs 1 , which is the difference between the potential VSS 1  at the gate of the current source Q 1  and the potential VAN 1  at the source of the current source Q 1  (Vgs 1 =VSS 1 −VAN 1 ) is equal to the voltage Vgs 2 , which is the difference between the potential VSS 2  at the gate of the current source Q 2  and the potential VAN 2  at the source of the current source Q 2  (Vgs 2 =VSS 2 −VAN 2 ). Therefore, the potential VAN 1  and the potential VAN 2  satisfy the following equation:
 
 VAN 2 −VAN 1 =VSS 2 −VSS 1  (1)
 
     As has been described above, as the structure in which the current sources Q 1  and Q 2  are binary-controlled on the basis of the multi-value data signal D, for example, as shown in  FIG. 7 , the structure in which a multi-value-to-binary conversion circuit L is provided between the signal line  18  and the current sources Q 1  and Q 2 , as disclosed in JP-A-61-43827, may also be available. In the structure shown in  FIG. 7 , a logic circuit L 1  including a transistor having a threshold voltage TH 1  is positioned between the signal line  18  and the current source Q 1 , and a logic circuit L 2  including a transistor with a threshold voltage TH 2  that is different from the threshold voltage TH 1  is positioned between the signal line  18  and the current source Q 2 . With the structure shown in  FIG. 7 , however, it is necessary to make characteristics (e.g., the channel widths) of the transistors of the logic circuits L 1  and L 2  different so that the threshold voltages TH 1  and TH 2  are set to an expected relative ratio. It is thus difficult to reduce the size of the logic circuits L 1  and L 2 . In contrast, according to the first embodiment, the current sources Q 1  and Q 2  are controlled on a binary basis in accordance with the multi-value data signal D using the structure in which the signal processing circuits C 1  and C 2  have different power supply potentials. It thus becomes unnecessary to have such large circuits, such as the logic circuits L 1  and L 2  shown in  FIG. 7 . However, it is not intended that the structure shown in  FIG. 7  be excluded from the scope of the invention. 
     As has been described above, according to the first embodiment, the potential VSS 1  for activating the current source Q 1  is different from the potential VSS 2  for activating the current source Q 2 . Therefore, if the source of the current source Q 1  and the source of the current source Q 2  are at the same potential, it is necessary to select outputs of the memories M 1  and M 2  such that the expected drive current I 1  and correction current I 2  are generated, individually taking into consideration the relationship between the gate potential of the current source Q 1  and the drive current I 1  and the relationship between the gate potential of the current source Q 2  and the correction current I 2 . In contrast, according to the first embodiment, the source potentials (VAN 1  and VAN 2 ) of the current sources Q 1  and Q 2  are set to different potentials such that the voltage Vgs 1  of the current source Q 1  and the voltage Vgs 2  of the current source Q 2  become the same voltage. That is, the relationship between the gate potential of the current source Q 1  and the drive current I 1  is common to the relationship between the gate potential of the current source Q 2  and the correction current I 2 . Therefore, the drive current I 1  and the correction current I 2  can be easily and reliably set to the expected relative ratio according to the characteristics (e.g., the channel widths) of the current sources Q 1  and Q 2 , regardless of the difference in the potentials of the control signals G 1  and G 2 . This facilitates the design. 
     Even in the structure in which the sources of the current sources Q 1  and Q 2  are set to the same potential, the voltage between the gate and the source of the current source Q 1  can be made equal to that of the current source Q 2  by positioning a level shifter that changes each signal such that the potential of the control signal G 1  matches the potential of the control signal G 2  between each of the memories M 1  and M 2  and each of the current sources Q 1  and Q 2 . However, the size of the unit circuits U 1  to Un increases because of the arrangement of the level shifters. Since one level shifter is provided for each electro-optical element E, the size increase in the unit circuits U 1  to Un is especially a serious problem in the case that the resolution of the electro-optical elements E is increased. According to the first embodiment in which the potentials VAN 1  and VAN 2  are different, no level shifter is necessary. This suppresses the size increase in the unit circuits U 1  to Un. 
     B. Second Embodiment 
     Next, a second embodiment of the invention will be described. The elements of the second embodiment with operations and functions common to those of the first embodiment are denoted with the same reference numerals, and detailed descriptions thereof are omitted where appropriate. 
       FIG. 8  is a block diagram of the structure of the unit circuit Ui. As shown in  FIG. 8 , the unit circuit Ui according to the second embodiment additionally includes, besides the elements of the unit circuit Ui according to the first embodiment ( FIG. 5 ), a selection circuit  21  positioned between an output end of the memory M 1  and the gate of the current source Q 1 , and a selection circuit  22  positioned between an output end of the memory M 2  and the gate of the current source Q 2 . The potential generating circuit  16  additionally generates, besides the potentials shown in  FIG. 4 , potentials VG 1  and VG 2 . The potential VG 1  is a potential greater than or equal to the potential VSS 1  and less than or equal to the potential VDD 1  (VSS 1 ≦VG 1 ≦VDD 1 ), and the potential VG 2  is a potential greater than or equal to the potential VSS 2  and less than or equal to the potential VDD 2  (VSS 2 ≦VG 2 ≦VDD 2 ). 
     The selection circuit  21  shown in  FIG. 8  is a section that selectively outputs one of the potential VG 1  and the potential VDD 1  according to the control signal G 1  output from the memory M 1 . The selection circuit  21  includes a switch  211  positioned between a line to which the potential VG 1  is supplied and the current source Q 1 , and a switch  212  positioned between a line to which the potential VDD 1  is supplied and the current source Q 1 . The switches  211  and  212  operate in an exclusive manner. That is, in the case that the control signal G 1  is at a high level (potential VDD 1 ), the switch  212  is turned on. In the case that the control signal G 1  is at a low level (potential VSS 1 ), the switch  211  is turned on. Therefore, in the case that the control signal G 1  is at a low level, the potential VG 1  is supplied to the gate of the current source Q 1 , thereby generating the drive current I 1 . In the case that the control signal G 1  is at a high level, the potential VDD 1  is supplied to the gate of the current source Q 1 , thereby stopping the generation of the drive current I 1 . 
     The selection circuit  22  shown in  FIG. 8  is a section that selectively outputs one of the potential VG 2  and the potential VDD 2  according to the control signal G 2  output from the memory M 2 . As in the selection circuit  21 , the selection circuit  22  includes a switch  221  that is selectively turned on in the case that the control signal G 2  is at a low level (potential VSS 2 ) to supply the potential VG 2  to the current source Q 2 , and a switch  222  that is selectively turned on in the case that the control signal G 2  is at a high level (potential VDD 2 ) to supply the potential VDD 2  to the current source Q 2 . Therefore, as in the first embodiment, the correction current I 2  is generated in the case that the control signal G 2  is at a low level, and the generation of the correction current I 2  is stopped in the case that the control signal G 2  is at a high level. According to the second embodiment, advantages similar to those of the first embodiment can be achieved. 
     With the structure as in the first embodiment in which the power supply potentials VDD 1  and VSS 1  of the memory M 1  are directly supplied to the gate of the current source Q 1 , the power supply potentials VDD 1  and VSS 1  of the memory M 1  need to be changed to adjust the current value of the drive current I 1 . Similarly, the adjustment of the correction current I 2  requires the adjustment of the power supply potentials VDD 2  and VSS 2  of the memory M 2 . However, when the power supply potentials of the memories M 1  and M 2  are changed, a characteristic value that determines the time constant of each circuit, such as an on-resistor of the transistor of each of the memories M 1  and M 2 , is also changed. This may affect the operation speed or margin of the memories M 1  and M 2 . Accordingly, the range in which the drive current I 1  and the correction current I 2  can be adjusted is limited. In contrast, according to the second embodiment, the drive current I 1  is adjusted according to the potential VG 1 , and the correction current I 2  is adjusted according to the potential VG 2 . That is, the power supply potentials of the memories M 1  and M 2  need not be changed. Therefore, the drive current I 1  and the correction current I 2  can be adjusted within a wide range without affecting the operation of the memories M 1  and M 2 . 
     C. Third Embodiment 
     The number of signal processing circuits C or the number of current sources Q included in one unit circuit Ui is not limited to that in the above examples. For example, as shown in  FIG. 9 , the structure of one unit circuit Ui having three signal processing circuits C (C 1  to C 3 ) and three current sources Q (Q 1  to Q 3 ) may be adopted. The current source Q 3  is a p-channel thin-film transistor generating a correction current I 3  according to the voltage between the gate and the source of the current source Q 3  and supplying the correction current I 3  to the node z. 
       FIG. 10  is a conceptual diagram showing the level of each potential generated by the potential generating circuit  16 . As shown in  FIG. 10 , the potential generating circuit  16  according to a third embodiment additionally generates, besides the potentials shown in  FIG. 4 , three types of potentials (VDD 3 , VSS 3 , and VAN 3 ). The potential VDD 3  is the same potential as the potential VDD 2 . The potential VAN 3  is higher than the potential VSS 3  and is supplied to the source of the current source Q 3 . The potential VDD 3  is higher than the potential VAN 3 . As shown in  FIG. 10 , the data signal D has one of four potentials including the potentials d 1  to d 3 , as in the first embodiment, and a potential d 4  equivalent to the potential VDD 3 . 
     As shown in  FIG. 9 , a memory M 3  of the signal processing circuit c 3  is activated by the potential VDD 3  and the potential VSS 3  serving as power supply potentials. That is, the memory M 3  outputs a control signal G 3  at a high level (potential VDD 3 ) in the case that the data signal D obtained from the signal line  18  has one of the potentials d 1  to d 3 . Since the generation of the correction current I 3  by the current source Q 3  is stopped at this point, as shown in  FIG. 10 , the relationship between the data signal D and the current value of the drive current IDR is the same as that of the first embodiment. In contrast, in the case that the data signal D has the potential d 4 , the memory M 3  outputs the control signal G 3  at a low level (potential VSS 3 ), thereby allowing the current source Q 3  to generate the correction current I 3 . Accordingly, the drive current IDR, which is the sum of the drive current I 1 , the correction current I 2 , and the correction current I 3 , is supplied to the electro-optical element E. The potential VAN 3  is set such that a voltage Vgs 3  between the gate and the source of the current source Q 3  (Vgs 3 =VSS 3 −VAN 3 ) becomes equal to the voltage Vgs 1  of the current source Q 1  and the voltage Vgs 2  of the current source Q 2 . 
       FIG. 11  is a timing chart showing the waveform of the drive current IDR. As shown in  FIG. 11 , a correction period P 3  during which the current source Q 3  generates the correction current I 3  is a period starting from the beginning of the period F and is set to a period (T 1  and T 2  of  FIG. 11 ) shorter than the drive period P 1  or the correction period P 2 . Therefore, according to the third embodiment, the current value of the drive current IDR can be corrected with high precision, without inducing a significant increase in the size of the unit circuit Ui, thereby equalizing the tone levels of the electro-optical elements E. Although the structure with changes to the structure of the first embodiment is shown in  FIG. 9 , the structure of the third embodiment may be applied to the second embodiment. 
     D. Modifications 
     Various modifications can be made to the above embodiments. Specific modifications will be described below by way of example. The following modifications may be combined where appropriate. 
     (1) First Modification 
     In the above embodiments, the beginning of the drive period P 1 , the beginning of the correction period P 2 , and the beginning of the correction period P 3  match one another. However, the timing of each period may be changed appropriately. For example, as shown in  FIG. 12 , the correction period P 2  (the correction period P 3  in the third embodiment) may begin at a time delayed from the beginning of the drive period P 1 . Also, plural correction periods (P 2  and P 3 ) may be dispersed within the drive period P 1 . Because capacitors and resistors are added to a line from the unit circuit Ui to the electro-optical element E, the drive current IDR rises gradually at the beginning of the drive period P 1  and the correction periods (P 2  and P 3 ). If both the drive period P 1  and the correction periods (P 2  and P 3 ) start at the beginning of the period F, as shown in  FIGS. 6 and 11 , the rising of the waveform of the drive current IDR can be speeded up, compared with the case shown in  FIG. 12 . 
     (2) Second Modification 
     In the above embodiments, the single-line data signal D is distributed among the unit circuits U 1  to Un on a time division basis. Alternatively, different data signals D may be supplied on a parallel basis to the corresponding unit circuits U 1  to Un. The structure of the unit circuit Ui may be changed appropriately. For example, the current sources Q may include n-channel transistors, or a plurality of transistors connected parallel to one another may be used as one current source Q. In the case that n-channel transistors are used as the current sources Q, for example, each current source Q is preferably positioned at the cathode side of the electro-optical element E. That is, the anode of the electro-optical element E is set to a constant potential, one current source Q is positioned between the cathode of the electro-optical element E and the potential VAN 1 , and another current source Q is positioned between the cathode and the potential VAN 2 . 
     (3) Third Modification 
     Organic light-emitting diodes are only examples of the electro-optical elements E. Regarding the electro-optical elements to which various embodiments of the invention are applied, no distinction need to be made between a self-luminescent type that emits light and a non-luminescent type that changes the external light transparency (e.g., liquid crystal elements), or between a current-drive type that is driven by current supply and a voltage-drive type that is driven by voltage application. For example, various electro-optical elements, such as organic EL elements, field-emission (FE) elements, surface-conduction electron-emitters, ballistic electron surface emitters, light-emitting diodes (LEDs), liquid crystal elements, electrophoretic elements, and electrochromic elements can be used in embodiments of the invention. 
     E. Applications 
     Next, the structure of an image forming apparatus using the electro-optical device H according to the above embodiments will be described.  FIG. 13  is a sectional view of the structure of an image forming apparatus using the electro-optical devices H according to the above embodiments. The image forming apparatus is a tandem full-color image forming apparatus and includes four electro-optical devices H (HK, HC, HM, and HY) according to the above embodiments, and four photosensitive drums  70  ( 70 K,  70 C,  70 M, and  70 Y) corresponding to the electro-optical devices H, respectively. Each of the electro-optical devices H is positioned facing an image forming surface (peripheral surface) of a corresponding one of the photosensitive drums  70 . The subscripts “K”, “C”, “M”, and “Y” of the reference numerals mean that the elements are used to develop black (K), cyan (C), magenta (M), and yellow (Y) images. 
     As shown in  FIG. 13 , an endless intermediate transfer belt  72  is wound around a drive roller  711  and a driven roller  712 . The four photosensitive drums  70  are arranged near the intermediate transfer belt  72  at predetermined intervals. The photosensitive drums  70  rotate in synchronization with the driving of the intermediate transfer belt  72 . 
     Besides the electro-optical devices H, corona charging units  731  ( 731 K,  731 C,  731 M, and  731 Y) and developing units  732  ( 732 K,  732 C,  732 M, and  732 Y) are arranged near the corresponding photosensitive drums  70 . Each of the corona charging units  731  uniformly charges the image forming surface of a corresponding one of the photosensitive drums  70 . An electrostatic latent image is formed by exposing the charged image forming surface to light using each electro-optical device H. Each of the developing units  732  then develops an image (visible image) on the corresponding one of the photosensitive drums  70  by allowing a developer (toner) to be adhered to the electrostatic latent image. 
     The black, cyan, magenta, and yellow images developed on the photosensitive drums  70  are sequentially transferred onto the surface of the intermediate transfer belt  72  (first transfer), thereby developing a full-color image. Four first transfer corotrons (transfer units)  74  ( 74 K,  74 C,  74 M, and  74 Y) are arranged inside the intermediate transfer belt  72 . Each of the first transfer corotrons  74  electrostatically absorbs the developed image from a corresponding one of the photosensitive drums  70  and transfers the developed image to the intermediate transfer belt  72  passing between the photosensitive drum  70  and the first transfer corotron  74 . 
     Sheets (recording media)  75  are fed one at a time by a pickup roller  761  from a sheet feeding cassette  762  and transported to the nip between the intermediate transfer belt  72  and a second transfer roller  77 . The full-color image developed on the surface of the intermediate transfer belt  72  is transferred to one side of the sheet  75  (second transfer) by the second transfer roller  77 , and then fused onto the sheet  75  by allowing the sheet  75  to pass through a fusing roller pair  78 . A paper-expelling roller pair  79  expels the sheet  75  on which the developed image has been fused in the above steps. 
     Because the image forming apparatus described above uses the organic light-emitting diodes as light sources (exposure devices), the size of the image forming apparatus becomes smaller than the size of an image forming apparatus using a laser scanning optical system. The electro-optical device H is additionally applicable to image forming apparatuses with structures other than the above exemplary structure. For example, the electro-optical device H is applicable to a rotary developing image forming apparatus, an image forming apparatus that directly transfers an image developed on each photosensitive drum to a sheet without using an intermediate transfer belt, and an image forming apparatus that forms a monochrome image. 
     The use of the electro-optical device H is not limited to exposing an image supporting member. For example, the electro-optical device H is applied in an image scanning apparatus as an illuminating device for illuminating an object to be scanned, such as a document. This type of image scanning apparatus includes a scanner, a scanning section of a copier and a facsimile machine, a barcode reader, and a two-dimensional image code reader that reads a two-dimensional image code, such as a QR Code®. 
     The electro-optical device according to the embodiments of the invention can be used as display sections of various electronic apparatuses, such as portable personal computers, cellular phones, personal digital assistants (PDAs), digital still cameras, televisions, video cameras, car navigation systems, pagers, digital notebooks, electronic paper, calculators, word-processors, workstations, videophones, point-of-sale (POS) terminals, printers, scanners, copiers, video players, and apparatuses equipped with a touch panel.