Abstract:
A gamma correction circuit includes: a first set of operational amplifiers each configured as a voltage follower, each of the first set of operational amplifiers including a phase compensation circuit that has a variable current source; and a second operational amplifier configured as a voltage follower, the gamma correction circuit dividing an input voltage so as to generate a plurality of divided voltages, and outputting a plurality of grayscale voltages based on the plurality of divided voltages generated, the first set of operational amplifiers generating a most significant voltage and a least significant voltage of the plurality of grayscale voltages, based on a predetermined divided voltage of the plurality of divided voltages, the second operational amplifier generating an intermediate voltage of the plurality of grayscale voltages, but not the most significant voltage nor the least significant voltage of the grayscale voltages, based on a predetermined divided voltage of the plurality of divided voltages, an output capability of each of the first set of operational amplifiers being larger than an output capability of the second set of operational amplifier, and the variable current source being allowed to change and set a current value to an arbitral value.

Description:
[0001]    The entire disclosure of Japanese Patent Application No. 2007-110122, filed Apr. 19, 2007 is expressly incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a gamma (γ) correction circuit, for instance, to the one that produces a grayscale voltage corresponding to a gamma characteristic of a display device. 
         [0004]    2. Related Art 
         [0005]    A common gamma correction circuit includes, for instance, as shown in  FIG. 5 , a voltage divider circuit  2  composed of resistors RA 1  to RA 7 , a voltage divider circuit  3  composed of resistors RB 1  and RB 7 , changeover switches SW 0  to SW 7 , operational-amplifiers (op-amps) OP 0  to OP 7  that constitute a voltage follower, and resistors RC 1  to RC 5  provided on an output side of the circuit. Moreover, smoothing capacitors C 1  and C 2  having a relatively large capacity are provided in order to reduce the oscillation of the op-amps OP 0  and OP 7 . 
         [0006]    Here, as shown in  FIG. 6 , the resistors RA 1  to RA 7  and the resistors RB 1  to RB 7  each include a selector and a resistor having intermediate taps, and one of the intermediate taps can be selected by the selector. The overall resistance of the resistors RA 1  to RA 7  is equal to that of the resistors RB 1  to RB 7 . 
         [0007]    This gamma correction circuit allows the selective use of the voltage divider circuits  2  and  3 , by switching between the changeover switches SW 0  to SW 7 . 
         [0008]    A grayscale voltage V 0  obtained from the op-amp OP 0  has a voltage VDD supplied from a grayscale power source  1 , when using any of the voltage divider circuits  2  and  3 . Similarly, a grayscale voltage V 63  obtained from the op-amp OP 7  has a voltage VSS, when using any of the voltage divider circuits  2  and  3 . On the other hand, the grayscale voltages V 1  to V 62 , obtained from output terminals of the op-amps OP 1  to OP 6  as well as from the intermediate taps of the resistors RC 1  to RC 5 , have different set of values, depending on which one of the voltage divider circuits is selected, i.e. the voltage divider circuit  2  composed of the RA 1  to RA 7 , or the voltage divider circuit  3  composed of the RB 1  to RB 7 . 
         [0009]    In the gamma correction circuit shown in  FIG. 5 , the grayscale voltage V 0  stays at the voltage VDD, and the grayscale voltage V 63  stays at the voltage VSS, when using any of the voltage divider circuits  2  and  3 . The grayscale voltages V 1  to V 62  are changeable, while the grayscale voltages V 0  and V 63  remain fixed, when selectively using the voltage divider circuits  2  and  3 . Therefore, it is desirable that the grayscale voltages V 0  and V 63  be also changeable, not only the grayscale voltages V 1  to V 62 , even when any one of the voltage divider circuits  2  and  3  are being used. 
         [0010]    The common gamma correction disclosed in JP-A-2005-10276 is known as another example of a common gamma correction circuit. 
         [0011]    The gamma correction circuit according to JP-A-2005-10276 includes a plurality of gamma correction resistors provided at the input side of the circuit, another plurality of gamma correction resisters provided at the output side of the circuit, and a plurality of op-amps provided therebetween. Among the plurality of gamma correction resistors at the input side, the ones in the vicinity of the input voltage are variable resistors, and others are fixed resistors. Similarly, among the plurality of gamma correction resistors at the output side, the ones in the vicinity of the input voltage are variable resistors, and others are fixed resistors. 
         [0012]    In the gamma correction circuit having a structure described above, in order to obtain desired grayscale voltages according to gamma characteristics, the potentials of the op-amps at the input and the output side are equalized by simultaneously modifying the gamma correction resistors at both the input side and the output side. Consequently, an excessive current is prevented from flowing between the gamma correction resisters and the op-amps, and the consumption current is reduced, thereby stable grayscale voltages are produced. 
         [0013]    As described, the gamma correction circuit described in JP-A-2005-10276 reduces the consumption current when the desired grayscale voltages are obtained according to gamma characteristics, while modification of the gamma correction resistors is required at both the input side and the output side every time. 
         [0014]    Based on this background, in the case of obtaining a desired grayscale voltages such as two types of grayscale voltages while taking advantage of the gamma correction circuit shown in  FIG. 5 , a novel gamma correction circuit is desired for reducing the consumption current and achieving power saving, without using capacitors, while reducing the oscillation of the op-amps. 
       SUMMARY 
       [0015]    An advantage of the invention is to provide, without using capacitors, a gamma correction circuit that allows reducing the consumption current and achieving power saving, while reducing the oscillation of the op-amps, in the case of obtaining a desired grayscale voltage. 
         [0016]    In order to achieve this advantage, an aspect of the present invention is provided in the following configuration. 
         [0017]    According to the invention, a gamma correction circuit includes: a first set of operational amplifiers each configured as a voltage follower, each of the first set of operational amplifiers including a phase compensation circuit that has a variable current source; and a second operational amplifier configured as a voltage follower. Here, the gamma correction circuit divides an input voltage so as to generate a plurality of divided voltages, and outputs a plurality of grayscale voltages based on the plurality of divided voltages generated. Further, the first set of operational amplifiers generates a most significant voltage and a least significant voltage of the plurality of grayscale voltages, based on a predetermined divided voltage of the plurality of divided voltages. Still further, the second operational amplifier generates an intermediate voltage of the plurality of grayscale voltages, but not the most significant voltage nor the least significant voltage of the grayscale voltages, based on a predetermined divided voltage of the plurality of divided voltages. Moreover, an output capability of each of the first set of operational amplifiers is larger than an output capability of the second operational amplifier; and the variable current source is allowed to change and set a current value to an arbitral value. 
         [0018]    In this case, the second operational amplifier includes a set of the second operational amplifiers, and the second set of operational amplifiers includes: a third set of operational amplifiers generating a maximum voltage and a minimum voltage of the intermediate voltages; and a fourth operational amplifier generating a voltage among the intermediate voltages, but not the maximum voltage nor the minimum voltage of the intermediate voltages. An output capability of each of the third set of operational amplifiers is larger than an output capability of the fourth operational amplifier. 
         [0019]    In this case, the variable current source includes: a plurality of first transistors each generating a predetermined current, so as to function as a current source; and a plurality of second transistors respectively coupled with each of the plurality of first transistors in series, so as to function as switches. A predetermined bias voltage is applied to gates of the plurality of first transistors, and on-off control signals are applied on gates of the plurality of second transistors. 
         [0020]    Therefore, the invention configured as above described allows reducing the consumption current and achieving power saving, while reducing the oscillation of the op-amps, in the case of obtaining a desired grayscale voltage without using capacitors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0022]      FIG. 1  is a circuit diagram illustrating a configuration of an embodiment according to an aspect of the invention. 
           [0023]      FIG. 2  is a circuit diagram illustrating an example of a specific configuration of the op-amps OP 0  and OP 7  illustrated in  FIG. 1 . 
           [0024]      FIG. 3  is a circuit diagram illustrating an example of a specific configuration of the op-amps OP 1  and OP 6  illustrated in  FIG. 1 . 
           [0025]      FIG. 4  is a circuit diagram illustrating an example of a specific configuration of the op-amps OP 2  through OP 5  illustrated in  FIG. 1 . 
           [0026]      FIG. 5  is a circuit diagram of a common gamma correction circuit. 
           [0027]      FIG. 6  is a drawing illustrating a structure of a resistor used in a voltage divider circuit. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    An embodiment of the invention will now be described with references to the accompanying drawings. 
         [0029]    As shown in  FIG. 1 , this embodiment of a gamma correction circuit according to the aspects of the invention includes voltage divider circuits  12  and  13  provided on an input side of the circuit, a plurality of changeover switches SW 0  to SW 7 , a plurality of op-amps OP 0  to OP 7  that constitute a voltage follower, and a plurality of resistors RC 1  to RC 5  provided on an output side of the circuit. 
         [0030]    Here, as shown in  FIG. 6 , resistors RA 0  to RA 8  and resistors RB 0  to RB 8  each include a selector and a resistor element that has intermediate taps, and one of the intermediate taps can be selected by the selector. The overall resistance of the resistors RA 0  to RA 8  is equal to that of the resistors RB 0  to RB 8 . 
         [0031]    The voltage divider circuit  12  includes a series circuit in which the resistors RA 0  to RA 8  are coupled in series. The voltage divider circuit  13  includes a series circuit in which the resistors RB 0  to RB 8  are coupled in series. One end of each of the voltage divider circuits  12  and  13  receives the voltage VDD as an input voltage supplied from the grayscale power source  1 , and other end of each of the voltage divider circuits  12  and  13  receives the voltage VSS as an input voltage. 
         [0032]    The changeover switches SW 0  to SW 7  select divided voltages of the voltage divider circuits  12  and  13 , and supply these selected divided voltages to the op-amps OP 0  to OP 7 . Normally, the changeover switches SW 0  to SW 7  select and output the divided voltage of the voltage divider circuit  13 , as shown in  FIG. 1 . 
         [0033]    If the changeover switches SW 0  to SW 7  select the divided voltages of the voltage divide circuit  12 , then the plurality of the op-amps OP 0  to OP 7  constituting the voltage follower output the divided voltages of the voltage divider circuit  12  as a first set of grayscale voltages V 0  to V 63 . If the changeover switches SW 0  to SW 7  select the divided voltages of the voltage divide circuit  13 , then those divided voltages are output as a second set of grayscale voltages V 0  to V 63  that are different from the first set. 
         [0034]    The plurality of resistors RC 1  to RC 5  couples the output terminal of one of the op-amps OP 1  to OP 6  to another. These resistors RC 1  to RC 5  each include the intermediate taps which subdivide the grayscale voltages V 1  to V 62 . 
         [0035]    No resistor is coupled between the output terminals of the op-amp OP 0  and the op-amp OP 1 , nor between the output terminals of the op-amp OP 62  and the op-amp OP 63 . This prevents the current from flowing into the op-amps OP 1  to OP 62  in a drive mode in which only the op-amps OP 0  and OP 63  are driven. 
         [0036]    In this embodiment, the resistors RA 0  and RA 8  are included in the voltage divider circuit  12 , and the resistors RB 0  and RB 8  are included in the voltage divider circuit  13 . Consequently, the voltage divider circuit  12  and the voltage divider circuit  13  produce different sets of grayscale voltages V 0  and V 63 , by selecting the intermediate taps of the resistors RA 0  and RA 8 . If, in this embodiment, the capacitors C 1  and C 2  are present and are coupled to the output terminals of the op-amps OP 0  and OP 7 , similar to the configuration shown in  FIG. 5 , the different sets of grayscale voltages V 0  and V 63  of those voltage divider circuits  12  and  13  cause the input voltage of the op-amps OP 0  and OP 7  to fluctuate when switching between the voltage divider circuits  12  and  13 . As a result, the output voltages of the op-amps OP 0  and OP 7  also fluctuate, causing the current to flow into the capacitors C 1  and C 2 , resulting in wastage in power consumption. 
         [0037]    Therefore, in this embodiment, unlike the configuration shown in  FIG. 5 , the capacitors C 1  and C 2  are not used at the output terminals of the op-amps OP 0  and OP 7 . Instead, those op-amps OP 0  to OP 7  are set to have different output capabilities, in order to reduce the wastage in power consumption. The description thereof follows. 
         [0038]    The output capabilities of the op-amps OP 0  to OP 7  are the indicators of voltage levels (voltage values) of their self-output voltages within the voltage range of power voltages (input voltages) VDD and VSS used in the voltage divider circuits  12  and  13 . 
         [0039]    The op-amps OP 0  and OP 7  both have the largest output capabilities, respectively outputting the most significant voltage (V 0 ) and the least significant voltage (V 63 ) of the grayscale voltages V 0  to V 63 , so that the values of the output voltages thereof become close to that of the power voltages VDD and VSS used in the voltage divider circuits  12  and  13 . The output capabilities of the op-amps OP 1  to OP 6  that output intermediate voltages V 1  to V 62  are set to have the output capabilities smaller than those of the op-amps OP 0  and OP 7 . 
         [0040]    Moreover, the op-amps OP 1  to OP 6  are configured so that the op-amps OP 1  and OP 6  both have the largest output capabilities, the op-amps OP 1  and OP 6  generating the maximum voltage V 1  and the minimum voltage V 62  of the intermediate voltages V 1  to V 62 . Other op-amps OP 2  to OP 5  are set to have the output capabilities smaller than those of the op-amps OP 1  and OP 6 . 
         [0041]    Hereafter, the specific configuration of the op-amps OP 0  to OP 7  sorted according to their output capabilities will be described while referring to  FIGS. 2 to 4 . 
         [0042]    Each of the op-amps OP 0  and OP 7  illustrated in  FIG. 1  has a configuration described in  FIG. 2 . 
         [0043]    Each of these op-amps includes, as shown in  FIG. 2 , a first input unit  31 , a second input unit  32 , a first intermediate unit  41 , a second intermediate unit  42 , an output unit  51 , a first phase compensation circuit  61 , and a second phase compensation circuit  62 , all of which together constitute a voltage follower. 
         [0044]    The first input unit  31  includes p-type MOS transistors Q 1  and Q 2  which together constitute a differential input pair, a p-type MOS transistor Q 3  that functions as a power source, and n-type MOS transistors Q 4  and Q 5  that function as loads. These together constitute a differential amplifier circuit, The first input unit  31  also includes n-type MOS transistors Q 6  and Q 7 . 
         [0045]    The second input unit  32  includes n-type MOS transistors Q 11  and Q 12  which together constitute a differential input pair, an n-type MOS transistor Q 13  that functions as a power source, and p-type MOS transistors Q 14  and Q 15  that function as loads. These together constitute a differential amplifier circuit. The second input unit  32  also includes p-type MOS transistors Q 16  and Q 17 . 
         [0046]    The first intermediate unit  41  includes n-type MOS transistors Q 21  and Q 22  which together constitute a differential input pair, an n-type MOS transistor Q 23  that functions as a power source, and p-type MOS transistors Q 24  and Q 25  that constitute a current mirror. These together constitute a differential amplifier circuit. 
         [0047]    The second intermediate unit  42  includes p-type MOS transistors Q 31  and Q 32  which together constitute a differential input pair, a p-type MOS transistor Q 33  that functions as a power source, and n-type MOS transistors Q 34  and Q 35  that constitute a current mirror. These together constitute a differential amplifier circuit. 
         [0048]    The output unit  51  includes a p-type MOS transistor Q 41  and an n-type MOS transistor Q 42 , and an output voltage Vout is retrieved from a common coupling portion present between those transistors. 
         [0049]    The first phase compensation circuit  61  is a circuit that carries out phase compensation in order to prevent an oscillation of the op-amp caused by a feedback circuit included in the op-amp which constitute the voltage follower. As shown in  FIG. 2 , the first phase compensation circuit  61  includes a capacitor C 3 , a MOS transistor Q 27 , a resistor R 1 , and a variable current source  611 . 
         [0050]    The variable current source  611  produces a variable current, and the values of the currents can be arbitrarily set, for instance, with an external unit. In other words, the variable current source  611  adjusts the capability of the first phase compensation circuit  61 , setting the capability to an arbitral value as needed. 
         [0051]    The variable current source  611  therefore generates a predetermined current, and includes a plurality of first transistors that function as a current source, and a plurality of second transistors that function as switches, each of the plurality of second transistors coupled with the plurality of first transistors in series. None of these components are illustrated. A predetermined bias voltage is impressed to the gates of the plurality of first transistors, and on-off control signals are impressed on the gates of the plurality of second transistors, so as to optionally set the current value, depending on the number of second transistors that are switched on. 
         [0052]    Similarly, the second phase compensation circuit  62  includes a capacitor C 4 , a MOS transistor Q 37 , a resistor R 2 , and a variable current source  621 . The configuration of the variable current source  621  is similar to that of the variable current source  611 . 
         [0053]    Each of the op-amps OP 1  and OP 6  illustrated in  FIG. 1  has a configuration described in  FIG. 3 . 
         [0054]    Each of these op-amps includes, as shown in  FIG. 3 , the first input unit  31 , the second input unit  32 , the first intermediate unit  41 , the second intermediate unit  42 , the output unit  51 , a first phase compensation circuit  61   a,  and a second phase compensation circuit  62   a,  all of which together constitute a voltage follower. 
         [0055]    These op-amps are substantially similar to that of the configuration illustrated in  FIG. 2 , except that the first phase-compensation circuit  61  and the second phase compensation circuit  62  are respectively altered with the first phase compensation circuit  61   a  and the second phase compensation circuit  62   a.    
         [0056]    The first phase compensation circuit  61   a  includes, as illustrated in  FIG. 3 , the capacitor C 3 , the MOS transistor Q 27 , the resistor R 1 , and a fixed current source  611   a  having a pre-fixed current value. Similarly, the second phase compensation circuit  62   a  includes the capacitor C 4 , the MOS transistor Q 37 , the resistor R 2 , and a fixed current source  621   a  having a pre-fixed current value. 
         [0057]    Each of the op-amps OP 2  to OP 5  illustrated in  FIG. 1  has a configuration described in  FIG. 4 . 
         [0058]    Each of these op-amps includes, as shown in  FIG. 4 , a first input unit  71 , a second input unit  72 , and an output unit  81 , all of which together constitute a voltage follower. 
         [0059]    The first input unit  71  includes n-type MOS transistors Q 51  and Q 52  which together constitute a differential input pair, an n-type MOS transistor Q 53  that functions as a power source, and p-type MOS transistors Q 54  and Q 55  that constitute a current mirror. These together constitute a differential amplifier circuit. 
         [0060]    The second input unit  72  includes p-type MOS transistors Q 61  and Q 62  which together constitute a differential input pair, a p-type MOS transistor Q 63  that functions as a power source, and n-type MOS transistors Q 64  and Q 65  that constitute a current mirror. These together constitute a differential amplifier circuit. 
         [0061]    The output unit  81  includes a p-type MOS transistor Q 71  and an n-type MOS transistor Q 72 , and the output voltage Vout is retrieved from a common coupling portion present between those transistors. 
         [0062]    As described, in this embodiment, the resistors RA 0  and RA 8  are included in the voltage divider circuit  12 , and the resistors RB 0  and RB 8  are included in the voltage divider circuit  13 . Consequently, not only the grayscale voltages V 1  to V 62  but also the grayscale voltages V 0  and V 63  become changeable. Therefore, in the case of switching between the voltage divider circuits  12  and  13  during their use, two different sets of grayscale voltages, i.e. the most significant voltage V 0  (maximum value) and the leas significant voltage V 63  (minimum value), are generated. 
         [0063]    Moreover, according to this embodiment, the output capabilities of the op-amps OP 0  and OP 7  are relatively larger than those of the op-amps OP 1  to OP 6 , and the op-amps OP 0  and OP 7  respectively include the first phase compensation circuit  61  that has the variable current source  611 , and the second phase compensation circuit  62  that has the variable current source  621 . This prevents the oscillation of the op-amps OP 0  and OP 7  without using the capacitors C 1  and C 2  shown in  FIG. 5 . 
         [0064]    Therefore, this embodiment allows reducing, without using capacitors, the consumption current as well as achieving power saving, while reducing the oscillation of the op-amps, in the case of obtaining two types of grayscale voltages.