Abstract:
A gamma conversion circuit of the present invention comprises: an input terminal; an output terminal; and a plurality of a voltage controlled amplifiers each coupled between the input terminal and the output terminal, each inputting an input voltage, a gain setting voltage, and a region setting voltage, and outputting an output voltage. The gain setting voltage sets an increasing rate of a gain of the output voltage during a unit period and the region setting voltage sets an amplifying operation region of the output voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gamma conversion circuit which converts input-output characteristics especially in accordance with needs of users. 
     2. Description of the Prior Art 
     A gamma conversion circuit converts an input-output voltage in order to realize characteristics, especially those which users need. Such a gamma conversion circuit generally undergoes linear approximation with a differential amplifier circuit comprising an emitter resistance, but in recent years circuit design which can freely change gain setting of gamma conversion has been required. 
     Conventionally, in supposing the case that a gamma conversion curb is approximated with three straight lines, each block is configured by a differential amplifier circuit with emitter resistances RElb and RE 2 b as shown in FIG.  12 . 
     Incidentally, a block diagram covering from an input (Vin) to an output (Vout) will be one as shown in FIG. 13 in which the above described differential amplifier circuit (FIG. 12) with an emitter resistance undergoes three-stage cascade connection. 
     However, the conventional has problems described below. 
     Each block, which is a differential amplifier circuit with an emitter resistance as shown in FIG. 12, can only undergo offsets adjustment in up-to-down and left-to-right directions as shown in FIG. 14 even if an external setting differential voltage V 2  is changed. FIG. 14 shows three kinds of offset settings, and when an external setting voltage V 2  for each block is adjusted, an input-output characteristics will be indicated in black dots and a thick line in FIG.  14 . 
     A first problem of this circuit configuration (FIG.  12  and FIG. 13) is that the linear portion which approximates a gamma conversion curb is configured by a differential amplifier circuit with an emitter resistance. This will make it difficult to attain a gamma conversion curb having a free gain since any combination of various external setting voltages V 2  will only stay within an adjustment range as in FIG.  15 . 
     A reason therefor is that a differential amplifier circuit with an emitter resistance can not alter inclination of a line with the external setting voltage V 2 . 
     A second problem is that due to a circuit configuration in which a gain is set with a resistance the gain changes in accordance with dispersion of resistance in a product, thus a yield factor will decrease. 
     BRIEF SUMMARY OF THE INVENTION 
     By contemplating such problems, the present invention is achieved and objects thereof are to provide a gamma conversion circuit characterized by a gamma conversion curb having a free gain and with a high yield factor of products. 
     A gamma conversion circuit of the present invention comprises: an input terminal; an output terminal; and a plurality of variable gain circuits each coupled between the input terminal and the output terminal, each having an operation amplifier circuit inputting two input voltages and producing an output voltage; 
     wherein gain of said output voltage changes in accordance with a difference between two input voltages. 
     A gamma conversion circuit of the present invention comprises: an input terminal; an output terminal; and a plurality of a voltage controlled amplifiers each coupled between the input terminal and the output terminal, each inputting an input voltage, a gain setting voltage, and a region setting voltage, and outputting an output voltage; 
     wherein the gain setting voltage sets an increasing rate of a gain of the output voltage during a unit period, the region setting voltage sets an amplifying operation region of the output voltage. 
     A gamma conversion circuit having a voltage controlled amplifier, the amplifier comprises: a first differential amplifier responding a gain setting voltage to produce a first control signal; a second differential amplifier responding an input voltage and a region setting voltage to produce a second control signal; and a third differential amplifier responding the first control signal and the second control signal to produce an output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This above-mentioned and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of an electric circuit showing a practical embodiment of a gamma conversion circuit of the present invention; 
     FIG. 2 is a block diagram showing an example in which an image processing circuit  2  comprising a gamma conversion circuit  5  of the present invention is applied to an image display system; 
     FIG. 3 is a block diagram on an electric circuit showing a configuration example of an image processing circuit  2  comprising a gamma conversion circuit  5  of the present invention; 
     FIG. 4 is a block configuration diagram showing input output characteristics of a gamma conversion circuit  5  of the present invention; 
     FIG. 5 is an electronic circuit diagram showing an internal circuit configuration of a VCA circuit block VCA 1  shown in FIG. 1; 
     FIG. 6 is a graph showing a range of outputs Vout for inputs Vin of a VCA circuit block VCA 1  shown in FIG. 1; 
     FIG. 7 is a graph showing operation regions as well as outputs Vout for inputs Vin of VCA circuit blocks VCA 1 , VCA 2 , and VCA 3  shown in FIG. 1; 
     FIG. 8 is a graph showing input-output characteristics under a mid gain for all VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 ; 
     FIG. 9 is a graph showing input-output characteristics under various combinations of gains for VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 ; 
     FIG. 10, which shows another practical embodiment of the present invention, is an electronic circuit diagram showing an internal circuit of VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 , to which a Gilbert-type multiplier circuit is applied; 
     FIG. 11 is a graph showing input-output characteristics of a VCA circuit and of a Gilbert-type multiplier circuit; 
     FIG. 12 is an electronic circuit diagram showing a conventional gamma conversion circuit; 
     FIG. 13 is a block configuration diagram showing input-output characteristics of the conventional gamma conversion circuit; 
     FIG. 14 is a graph showing ranges of gain changes in each block of the conventional gamma conversion circuit; and 
     FIG. 15 is a graph showing input-output characteristics of the conventional gamma conversion circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, a multistage connection of voltage-controlled variable gain circuits (Voltage Controlled Amplifiers: VCA circuits), in which a gain of an output voltage can be optionally altered with voltages being set outside, is made in a portion of a gamma conversion circuit in a display unit, and thus a free gamma conversion curb can be set. 
     In the configuration of an image processing circuit  2  as shown in FIG. 3, a gamma conversion circuit  5  is configured so that VCA circuit blocks VCA 1 , VCA 2 , and VCA 3  are connected in parallel as shown in FIG. 1 in accordance with the present invention. The VCA circuit block VCA 1  is configured by a basic circuit shown in FIG. 5, and the VCA circuit blocks VCA 2  and VCA 3  have the same configuration as that of the VCA circuit block VCA 1 . 
     These VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 , respectively, cover a certain range of an input voltage Vin as shown in FIG. 6, and a gain thereof can be optionally altered by outside gain setting voltages VG 1 , VG 2 , and VG 3 . 
     Accordingly, such an effect is attained that a gamma conversion curb will be able to be set freely by linear approximation. 
     Practical embodiments of the present invention will be detailed with reference to drawings as follows. 
     FIG. 2 is a block diagram showing an example of an image processing step to which the present invention is applied. An image output  1  in FIG. 2 is an image signal output device such as a TV tuner and a personal computer, etc., and this image signal is displayed in a display  3  via an image processing circuit  3 . Here, in a display in the display  3 , adjustment in characteristics in a display device or coloring will be necessary. 
     FIG. 3 is a block diagram showing a configuration of an image processing circuit  2  in the present practical embodiment, and an image signal being inputted undergoes clamping in a clamp circuit  4  as well as an input-output conversion of the image signal in a gamma conversion circuit  5 , and thereafter is supplied to a display by an output buffer  6 . 
     An internal connection in this gamma conversion circuit  5  is configured by VCA circuit blocks VCA 1 , VCA 2 , and VCA 3  being disposed in parallel as shown in FIG.  1 . 
     As shown in FIG. 1, the gamma conversion  5  related to the present practical embodiment comprises VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 , and a load Ro being commonly connected thereto. An inputted voltage, that is, an input Vin, is inputted to VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 , and in accordance with a range of input voltages of the input Vin, each block of the VCA circuit blocks VCA 1  through VCA 3  is operated. A gain for each block is set by gain setting voltages VGl through VG 3 . An output from each VCA circuit block connected together, and is connected with a load Ro provided between itself and a power supply Vcc. A voltage (output Vout) outputted form this load Ro is supplied to the display  3  via the output buffer  6 . 
     As described above, the gamma conversion circuit  5  is realized with VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 . In the present practical embodiment, the gamma conversion circuit  5  is configured by VCA circuit blocks VCA 1 , VCA 2 , and VCA 3  undergoing three-stage connection as shown in FIG.  4 . 
     An internal basic circuit configuring the VCA circuit blocks VCA 1 , VCA 2 , and VCA 3  is shown in FIG.  5 . Here, for the purpose of simplification, a case where a gamma conversion curb is approximated with three straight lines will be considered. 
     The following is the explanation of the circuit structure of each VCA as shown in FIG.  5 . 
     A NPN bipolar transistor Q 1  has a collector coupled to a power source line, which is supplied with a power source Vcc. A NPN bipolar transistor Q 2  has a collector coupled to the power source line via a resistor RO, and an emitter coupled to an emitter of the transistor Q 1 . An output terminal OUTPUT is coupled to the collector of the transistor Q 2 . A NPN bipolar transistor Q 6  has a collector coupled to the emitters of the transistors Q 1  and Q 2 , an emitter coupled to a constant current source, and a base supplied with a region setting reference voltage V R1b . A NPN bipolar transistor Q 5  has a collector coupled to the power source line, an emitter coupled to a constant current source, and a base supplied with an input voltage (signal) Vin. A resistor is coupled between the emitters of the transistors Q 5  and Q 6 . A NPN bipolar transistor Q 3  has a collector coupled to the power source line via a diode, an emitter coupled to a constant current source, and a base supplied with a gain setting voltage VG 1 . The collector of the transistor Q 3  is coupled to a base of the transistor Q 1 . A NPN bipolar transistor Q 4  has a collector coupled to the power source line via a diode, an emitter coupled to a constant current source, and a base supplied with an inclination setting reference voltages V R1a . The collector of the transistor Q 4  is coupled to a base of the transistor Q 2 . A resistor is coupled between the emitters of the transistors Q 3  and Q 4 . 
     As shown in FIG. 6, when an input voltage Vin is being varied as a horizontal axis, it is necessary for a display to undergo input-output conversion (gamma conversion) with a curb of γ=2.2. Here, as shown in FIG. 7, a region setting reference voltage V R1b  shown in FIG. 5 is adjusted so that each one third of an input voltage range is allocated to each block. 
     Since each VCA circuit block VCA 1 , VCA 2 , and VCA 3  is a VCA circuit, any gain can be set up by a gain setting voltage VG 1  being an external setting difference in FIG.  5 . If the transistor Q 1  and the transistor Q 2  are balanced, a current Io flowing through the load Ro will be Io=11. When the gain setting voltage VG 1  is altered, the transistor Q 1  and the transistor Q 2  undergo changes by a base voltage of ±ΔI, and therefore the balance between the transistor Q 1  and the transistor Q 2  will fall apart, giving rise to, for example, Io=(1/2)×I 1 . That is, for an input change, the following transition takes place: 
     
       
         (1/2)× I   1 →(2/3)× I   1   
       
     
     And therefore, a voltage drop ratio of the load Ro changes. 
     Here, inclination setting reference voltages V R1a , V R2a , and V R3a  are for determining an inclination level of a gain inclination, and region setting reference voltages V R1b , V R2b , and V R3b  are for determining an inclination center of a gain inclination, and all of them can be set up from outside. That is, the region is an amplifying operation region which means an amplifying operation is controllable, capable, or operatable within its region. Beyond the region, the output voltage is a constant. The output voltage keeps a constant, that is, the amplifying operation is not performed when an input voltage is changed outside of the region. 
     FIG. 4 shows the case where three types of gains are set up for VCA circuit blocks VCA 1 , VCA 2 , and VCA 3  to which each region is allocated and, in the case where all blocks select a mid gain, input-output characteristics will be indicated by black dots and a thick line as shown in FIG. 8, and thus linear approximation of a desired gamma curb will become possible. 
     Thus, examples of gamma conversion curbs for cases where various gains are combined are shown in FIG.  9 . 
     When the maximum output voltage of each of VCA 1 , VCA 2 , and VCA 3  (an output voltage (a black dot) at its right end of each VCA in FIG. 4) is controllable by changing values of the constant current sources coupled to the transistors Q 5  and Q 6  in its up and down direction of FIG.  4  and changing the region setting reference voltage VR 1 b in its right and left direction of FIG.  4 . 
     When the region setting voltage VR 1 b changes in a case that the difference voltage between the gain setting voltage VG 1  and the inclination setting reference voltage VR 1 a is a constant, the region of each VCA changes in its right and left direction in FIGS. 4,  7 - 9  with keeping the width of the region. 
     When the difference voltage between the gain setting voltage VG 1  and the inclination setting reference voltage VR 1 a changes in a case that the region setting voltage VR 1 b is a constant, the inclination of the output voltage changes from the maximum output of each VCA as its starting point in FIGS. 4,  7 - 9 . 
     The gamma conversion circuit  5  related to the practical embodiment, which is configured as described above, thus will give rise to effects described as follows. 
     A first effect is that a voltage set up from outside can freely perform gamma conversion at any time, compared with a conventional gamma conversion circuit with a fixed inclination. The reason therefor is that a gamma conversion curb undergoes linear approximation with a VCA circuit so that a gain can be set up with an external differential voltage. 
     A second effect is that dispersion in products can be adjusted with a voltage externally set up. The reason therefor is that gains for VCA circuits VCA 1 , VCA 2 , and VCA 3  can be adjusted with differential voltages VG 1 , VG 2 , and VG 3  externally set up. 
     In an embodiment of the present invention, a gamma conversion curb undergoes linear approximation with three VCA circuit blocks VCA 1 , VCA 2 , and VCA 3 , but there are no limitations on a number of blocks. In addition, an increase in a number of blocks enables it to be applied to a device having a special gamma conversion curb such as liquid crystal display, etc. 
     As another embodiment of the present invention, the one having its basic configuration as described above, in which, however, a VCA circuit is replaced with a Gilbert-type multiplier circuit, is shown in FIG.  10 . 
     The following is the explanation of the circuit structure of the Gilbert-type multiplier circuit as shown in FIG.  10 . 
     A NPN bipolar transistor Q 7  has a collector coupled to a power source line, which is supplied with a power source Vcc. A NPN bipolar transistor Q 8  has a collector coupled to an output terminal OUTPUT and an emitter coupled to an emitter of the transistor  7 . A NPN bipolar transistor Q 9  has a collector coupled to the power source line and a base coupled to a base of the transistor  8 . A NPN bipolar transistor Q 10  has a collector coupled to the power source line via a resistor RO and an emitter coupled to an emitter of the transistor Q 9 . A NPN bipolar transistor Q 11  has a collector coupled to the emitters of the transistors Q 7  and Q 8 , an emitter coupled to a constant current source, and a base supplied with an input voltage Vin. A NPN bipolar transistor Q 12  has a collector coupled to the emitters of the transistors Q 9  and Q 19 , an emitter coupled to a constant current source, and a base supplied with a region setting reference voltage V R1b . A resistor RE 1  is coupled between the emitters of the transistors Q 11  and Q 12 . A NPN bipolar transistor Q 13  has a collector coupled to the power source line via a diode, an emitter coupled to a constant current source, and a base supplied with a gain setting voltage VG 1 . The collector of the transistor Q 13  is coupled to a base of the transistor Q 7 . A NPN bipolar transistor Q 14  has a collector coupled to the power source line via a diode, an emitter coupled to a constant current source, and a base supplied with an inclination setting reference voltages V R1a . The collector of the transistor Q 14  is coupled to a base of the transistor Q 10 . A resistor RE 2  is coupled between the emitters of the transistors Q 13  and Q 14 . 
     In FIG. 10, its basic operation remains same with the circuit in FIG. 5, but a gain inclination thereof can be altered with a mid point in an input voltage range as shown in FIG.  11 . 
     Incidentally, the present practical embodiment, to which the present invention is not limited, can be applied to a suitable mode to which the present invention is applied. 
     In addition, quantities, positions, and shapes, etc. are not limited to the above described practical embodiment, but suitable quantities, positions, and shapes, etc. can be selected for the present invention to be embodied. 
     Incidentally, in each drawing, a same reference numeral denotes a same configuring element. 
     The present invention, which is configured as described above, is featured by a gamma conversion curb having a free gain, and in addition, gives rise to an effect that a gamma conversion circuit with a high yield factor on products can be provided.