Abstract:
A bias circuit to prevent distortion of color in a cathode ray tube (CRT) comprises a power supply part, a first amp to amplify and output a brightness control voltage applied to control brightness, a second amp to respectively amplify and output RGB-bias control voltages having a ratio of R:G:B supplied from a video power amp, an addition amp to add the brightness control voltage output from the first amp respectively to the RGB-bias control voltages amplified and output from the second amp and to amplify the respective added values to output the respective added RGB-bias control voltages, and an inverting amp to restore the respective added RGB-bias control voltages input from the addition amp and to calculate respective output RGB bias control voltages. Accordingly, color distortion of a video image caused after changing brightness of a screen can be prevented so that a correct color of the image can be implemented.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2004-8393, filed Feb. 9, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present general inventive concept relates to a bias circuit in a cathode ray tube (CRT). More particularly, the present general inventive concept relates to a bias circuit to prevent distortion of color in a CRT by equalizing a color combination rate of red-green-blue (RGB) colors even after a user changes brightness of a screen.  
         [0004]     2. Description of the Related Art  
         [0005]     General video display apparatuses are input with a video signal and a synchronization signal received from a video card of a computer, thereby displaying the video on a cathode ray tube (CRT) screen. The CRT used in a video display apparatus is based on a principle where electron beams of different amounts according to intensity of the video signals hit red, green, and blue (RGB) phosphors coated on a surface of the CRT, thereby emitting lights of different brightness and colors. The CRT is widely used due to its low price and its excellent display performance.  
         [0006]      FIG. 1  illustrates a block diagram of a general video display apparatus comprising a deflection part  1 , a high-pressure part  2 , a micro computer  3 , a video preamplifier (preamp)  4 , a video power amplifier (amp)  5 , and a bias circuit part  40 . The video preamp  4  primarily amplifies RGB video signals input from a computer, and the video power amp  5  re-amplifies the pre-amplified RGB video signals to drive a cathode of the CRT. The bias circuit part  40  supplies electric power to the respective RGB video signals to control a voltage level of the RGB video signals, which are supplied from the video power amp  5  to the cathode of the CRT.  
         [0007]      FIG. 2  illustrates a circuit view of a conventional bias circuit part. Referring to  FIG. 2 , the operation of the conventional bias circuit part  40  will be described.  
         [0008]     The conventional bias circuit part  40  comprises bias voltage amps  44 ,  46 , and  48  for respective RGB colors and a brightness controller  42 . The RGB-bias voltage amps  42 ,  44 , and  46  separately amplify RGB-bias voltages input by the video power amp  5  ( FIG. 1 ) to respective RGB-bias input terminals into desired voltages. The brightness controller  42  controls levels of the RGB-bias voltages as a whole.  
         [0009]     Since the RGB-bias voltage amps  44 ,  46 , and  48  operate in the same manner, only the operation of the R-bias voltage amp  42  will be described. The R-bias voltage supplied to the R-bias input terminal is amplified when it passes through a first transistor T 1  and a fourth transistor T 4  and then output through an R-bias output terminal to supply bias voltage to the R video signal.  
         [0010]     During this process, an amplification rate is controlled by the brightness controller  42 . If a brightness control voltage input to a base terminal B of a transistor T 7  is changed, voltage of an emitter terminal E of the transistor T 7  is also changed. As a result, a voltage of an emitter terminal E of the first transistor T 1  that amplifies the R-bias voltage is also changed. Accordingly, the amplification rate of the R-bias voltage is controlled.  
         [0011]     When the brightness controller  42  of the bias circuit part  40 , operated in the manner described above, controls the brightness control voltage, the RGB-bias voltage amps  44 ,  46 , and  48  are controlled thereby increasing and decreasing the RGB-bias voltages by the same rate.  
         [0012]     Color of video images in the CRT is determined according to an R:G:B cathode voltage ratio. A cathode voltage is determined by the RGB-bias voltages.  
         [0013]     In the conventional bias circuit part  40 , as the RGB-bias voltages are increased or decreased according to a change in brightness, the R:G:B cathode voltage ratio is also increased or decreased by the same rate. For instance, if a ratio of the RGB-bias voltages is R:G:B before the change in brightness, and the RGB-bias voltages are increased by 6 according to the change in brightness, the ratio of the RGB-bias voltages becomes R+δ:G+δ:B+δ after the brightness change.  
         [0014]     However, R:G:B is typically not equal to R+δ:G+δ:B+δ. In other words, since the RGB-bias voltage ratios before and after the brightness change are not equal, the R:G:B cathode bias voltage ratios are also changed after the change in brightness. As a result, distortion is generated in the color of the video images.  
         [0015]     Due to the color distortion caused by the change in brightness, a user&#39;s demand for exact color implementation cannot be satisfied even though display products that can implement exact colors become necessary for graphic works, home shopping, and internet shopping.  
       SUMMARY OF THE INVENTION  
       [0016]     The present general inventive concept provides a bias circuit to prevent distortion of color in a cathode ray tube (CRT), in which a ratio of red-green-blue (RGB) bias voltages is stabilized even after brightness of a screen is changed by compensating each of the RGB-bias voltages by a different amount according to input RGB-bias voltages.  
         [0017]     Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
         [0018]     The foregoing and/or other aspects and advantages of the present general inventive concept are achieved by providing a bias circuit to prevent distortion of color in a cathode ray tube (CRT) comprising a power supply part, a first amplifier (amp) to amplify and output a brightness control voltage applied to control brightness, a second amp to amplify and output RGB-bias control voltages having a ratio of R:G:B supplied from a video power amp, an addition amp to add the brightness control voltage output from the first amp respectively to the RGB-bias control voltages amplified and output from the second amp and to amplify the respective added values and output the respective added RGB bias control voltages, and an inverting amp to restore the respective added RGB bias control voltages input from the addition amp to the ratio of R:G:B and to calculate respective restored output RGB bias voltages.  
         [0019]     The bias circuit may further comprise a plurality of resistances to generate a predetermined voltage by dividing a power voltage supplied from the power supply part.  
         [0020]     The predetermined voltage is added to the brightness control voltage and the respective RGB-bias control voltages in order to control voltages input to the addition amp according to the predetermined voltage.  
         [0021]     The first amp may comprise at least one resistance connected in series with a brightness control voltage input terminal, an operational amp connected in series with an end of the at least one resistance by a negative (−) input terminal and grounded by a positive (+) terminal, and at least one transistor connected to the operational amp in parallel.  
         [0022]     The at least one transistor is connected in a manner that a collector terminal is connected to a connection point between the (−) input terminal of the operational amp and the at least one resistance, an emitter terminal is connected to an output terminal of the operational amp, and a base terminal is grounded.  
         [0023]     The second amp comprises an R-bias control voltage amp having at least one resistance, at least one transistor, and at least one operational amp to amplify and output the R-bias control voltage; a G-bias control voltage amp having at least one resistance, at least one transistor, and at least one operational amp to amplify and output the G-bias control voltage; and a B-bias control voltage amp having at least one resistance, at least one transistor, and at least one operational amp to amplify and output the B-bias control voltage.  
         [0024]     The addition amp comprises a first addition amp to add the amplified brightness control voltage, the amplified R-bias control voltage, and the predetermined voltage, and to amplify and output the added R-bias control voltage; a second addition amp to add the amplified brightness control voltage, the amplified G-bias control voltage, and the predetermined voltage, and to amplify and output the added G-bias control voltage; and a third addition amp to add the amplified brightness control voltage, the amplified B-bias control voltage, and the predetermined voltage, and to amplify and output the added B-bias control voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0026]      FIG. 1  illustrates a block diagram of a general video display apparatus;  
         [0027]      FIG. 2  illustrates a schematic circuit view of a conventional bias circuit of a cathode ray tube (CRT);  
         [0028]      FIG. 3  illustrates a schematic circuit view of a bias circuit to prevent color distortion in a CRT according to an embodiment of the present general inventive concept. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures.  
         [0030]      FIG. 3  illustrates a schematic circuit view of a bias circuit to prevent color distortion in a CRT according to an embodiment of the present general inventive concept. Referring to  FIG. 3 , the bias circuit to prevent color distortion comprises a first amplifier (amp)  110 , a second amp  120 , an addition amp  130 , and an inverting amp  140 .  
         [0031]     The first amp  110  comprises at least one resistance, at least one operational amp, and at least one transistor. The first amp  110  amplifies and outputs a brightness control voltage VBRin, which is input from a video power amp  5  ( FIG. 1 ).  
         [0032]     The second amp  120  comprises an R-bias voltage amp  122 , a G-bias voltage amp  124 , and a B-bias voltage amp  126 . The respective bias voltage amps  122 ,  124 , and  126  independently amplify RGB-bias voltages input to respective input terminals. The respective bias voltage amps  122 ,  124 , and  126  have at least one resistance, at least one operational amp, and at least one transistor. The second amp  120  amplifies and outputs the RGB-bias voltages input from the video power amp  5  ( FIG. 1 ).  
         [0033]     The addition amp  130  comprises a first addition amp  132 , a second addition amp  134 , and a third addition amp  136 . The first addition amp  132  is connected to an output terminal of the R-bias voltage amp  122  through a resistance R 4  and is connected to an output terminal of the first amp  110  through a resistance R 10 . The first addition amp  132  is also connected to a connection spot @ between a resistance R 19  and a resistance R 20  through a resistance R 7 .  
         [0034]     The second addition amp  134  is connected to an output terminal of the G-bias voltage amp  124  through a resistance R 5  and is connected to the output terminal of the first amp  110  through a resistance R 11 . The second addition amp  134  is also connected to the connection spot @ between the resistance R 19  and the resistance R 20  through a resistance R 8 .  
         [0035]     Similarly, the third addition amp  136  is connected to an output terminal of the B-bias voltage amp  126  through a resistance R 6  and is connected to the output terminal of the first amp  110  through a resistance R 12 . The third addition amp  136  is also connected to the connection spot@ between the resistance R 19  and the resistance R 20  through a resistance R 9 .  
         [0036]     The brightness control voltage VBRin, amplified and output by the first amp  110 , is added to each of the respective input RGB-bias voltages amplified and output by the bias voltage amps  122 ,  124 , and  126 . The added voltages VR 1 , VG 1 , and VB 1  are then amplified and output.  
         [0037]     The inverting amp  140  comprises an R-bias voltage calculator  142 , a G-bias voltage calculator  144 , and a B-bias voltage calculator  146 . The inverting amp  140  calculates respective output RGB-bias voltages by restoring the added voltages VR 1 , VG 1 , and VB 1  amplified and output by the addition amp  130 .  
         [0038]     The RGB-bias voltage calculators  142 ,  144 , and  146  each comprise at least one transistor, at least one resistance, and at least one operational amp.  
         [0039]     The brightness control voltage VBRin is applied to an input terminal of the first amp  110  from the video power amp  5  ( FIG. 1 ). The brightness control voltage VBRin is amplified when it passes through a first operational amp  112  in the first amp  110 , which comprises a transistor Q 7  and a resistance R 21 . Assuming that an emitter current of the transistor Q 7  is I E , the relationship between the I E  and V BE  (i.e., a voltage between a base B and the emitter E) is expressed by [Equation 1] as follows. 
 
 I   E   =I   S ·exp( V   BE   /V   T )   [Equation 1]
 
         [0040]     In [Equation 1], V T =kT/q, I s  denotes a reverse saturation current constant of the transistor Q7, V T  denotes a thermal voltage constant of the transistor Q7, k denotes Baltzman&#39;s constant (1.38×10 −23  J/°K), T denotes absolute temperature, and q denotes electric charge (1.6×10 −19  Coulomb).  
         [0041]     When the brightness control voltage VBRin input to the first amp  110  changes, a current  121  at the resistance R 21  is thoroughly absorbed by the transistor Q 7 , and a voltage input to a negative (−) terminal of the first operational amp  112  becomes 0. Therefore, I 21 =VBRin/R 21 =I c =αI E . I E  is expressed below. 
 
 I   E   =VBR in/α R   21 ; 
 
 wherein, α denotes an amplification rate of the transistor Q 7 . 
 
         [0042]     An output voltage VBRout of the first amp  110  (i.e., ah amplified brightness control voltage) is calculated as follows. 
 
 VBR out=− V   T   ·ln ( I   E   /I   S )=− V   T   ·ln ( VBR in/α R   21   I   s ) 
 
         [0043]     In the above equation, a variable may be substituted for a constant related to a property of matter as in [Equation 2] below. 
 
 VBR   out   =−a·ln ( VBR in/ R   21 )+ b    [Equation 2]
 
         [0044]     A relationship between the brightness control voltage VBRin input to the first amp  110  and the amplified brightness control voltage VBRout output from the first amp  110  can be understood from [Equation 2].  
         [0045]     When input RGB-bias voltages VRin, VGin, and VBin are applied to the respective RGB-bias input terminals of the second amp  120 , the second amp  120  operates in the same manner as the first amp  110 . Further, since the RGB-bias voltage amps  122 ,  124 , and  126  of the second amp  120  operate in the same manner, only operation of the R-bias voltage amp  122  will be described.  
         [0046]     An output R-bias voltage VRout 1  of the R-bias voltage amp  122  in the second amp  120  can be expressed using [Equation 3] as follows. 
 
 VR out 1 =− a 1 ·ln ( VR in/ R   1 )+ b 1   [Equation 3]
 
         [0047]     In [Equation 3], VRin denotes the input R-bias voltage. Likewise, an output G-bias voltage and B-bias voltage are expressed as VGout 1 =−a2·ln(VGin/R 2 )+b2 and VBout 1 =−a3·ln(VBin/R 3 )+b3, respectively.  
         [0048]     The addition amp  130  amplifies by adding the amplified brightness control voltage VBRout output by the first amp  110  to the respective output RGB-bias voltages VRout 1 , VGout 1 , and VBout 1 . The operation of the first addition amp  132  connected to the R-bias voltage amp  122  will be described hereinbelow.  
         [0049]     A current I 4  flowing in the resistance R 4  is expressed by VRout 1 /R 4 , a current I 10  flowing in the resistance R 10  is VBRout 1 /R 10 , and a current I 7  flowing in the resistance R 7  is V @ /R 7 . V @  denotes the voltage at the connection spot @ between the resistance R 19  and the resistance R 20 .  
         [0050]     An output voltage VR 1  of the first addition amp  132  is expressed as the following.  
             VR1   =       ⁢     -     R13   ⁡     (       I   4     +     I   10     +     I   7       )                     =       ⁢       -     R13   ⁡     (     VRout1   /   R4     )         +     (     VBRout   /   R10     )     +     (       V   @     /   R7     )                 
 
         [0051]     Here, since R 4 =R 7 =R 10 , the above equation can be expressed by [Equation 4] as follows.  
             VR1   =       R13   R4     ⁡     [       (       a   ⁢           ⁢     ln   ⁡     (     VBRin   R21     )         +   b     )     +     (       a   ⁢           ⁢     ln   ⁡     (     VBRin   R1     )         +   b     )     -     V   @       ]               [     Equation   ⁢           ⁢   4     ]             
 
         [0052]     In [Equation 4], by controlling a resistance ratio of the resistance R 19  and the resistance R 20  such that V @ , voltage of the spot@, becomes 2b, and the first resistance R 1  becomes the same as the resistance R 21 , the output voltage VR 1  of the first addition amp  132  can be expressed by [Equation 5] as below.  
             VR1   =       a1R13   R4     ⁢           ⁢     ln   ⁡     (       VBRin   ·   VRin     R1     )                 [     Equation   ⁢           ⁢   5     ]             
 
         [0053]     As in the first addition amp  132 , an output voltage VG 1  of the second addition amp  134  is expressed as below.  
       VG1   =       a2R14   R2     ⁢           ⁢     ln   (       VBRin   ·   VGin     R2     )           
 
         [0054]     An output voltage VB 1  of the third addition amp  136  is expressed as below.  
       VB1   =       a3R15   R3     ⁢           ⁢     ln   ⁡     (       VBRin   ·   VBin     R3     )             
 
         [0055]     The R-bias voltage calculator  142  in the inverting amp  140  restores the voltage amplified and output by the addition amp  130  to calculate a final output R-bias voltage VRout 2 . In [Equation 3], if VR 1  of [Equation 5] is applied for VRout 1 , and the final output R-bias voltage VRout 2  is applied for VRin, [Equation 6] is obtained as the following. 
 
 VR out 2 = C   1 *( VBR in* VR in)   [Equation 6]
 
         [0056]     Also, VGout 2 =C 2 *(VBRin*VGin), and VBout 2 =C 3 *(VBRin*VBin). Here, C 1 , C 2  and C 3  are constants.  
         [0057]     According to [Equation 5], a ratio of the final output RGB-bias voltages VRout 2 , VGout 2 , and VBout 2  becomes the same as the ratio of the input RGB-bias voltages VRin, VGin and VBin, even after the brightness change.  
         [0058]     In essence, the above description is summarized as follows. 
 
 R -bias output voltage  VR out 2 :  G -bias output voltage VGout 2 : B-bias output voltage  VB out 2 = C   1 *brightness control voltage  VBR in* R -bias input voltage  VR in:  C   2 *brightness control voltage  VBR in* G -bias input voltage  VG in:  C   3 *brightness control voltage  VBR in  B -bias input voltage  VB in= R -bias input voltage:  G -bias input voltage:  B -bias input voltage
 
         [0059]     Accordingly, distortion of color, which is usually caused after the brightness change, can be prevented.  
         [0060]     According to this embodiment of the present general inventive concept, color distortion is restrained even after brightness of an image in a display is changed by a user, thereby enabling correct colors.  
         [0061]     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.