Abstract:
Cut-off control circuits implementing DC-coupling and AC-coupling to CRT cathodes can employ the same preamplifier integrated circuits with few additional components. The preamplifier includes a switching unit for receiving control data, generating a control signal according to control data, and outputting the control signal internally or externally. The switching unit provides a control signal internally to an amplification circuit, when the preamplifier operates in a cut-off control circuit having a DC-coupling to a CRT. With a DC coupling the amplification circuit controls a DC bias applied to a CRT cathode. The switching unit provides a bus control signal externally to a bias circuit, when the preamplifier operates in a cut-off control circuit having an AC-coupling to a CRT.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to video display devices, and more particularly, to amplifiers and cut-off control circuits for adjusting the white balance of display devices. 
     2. Description of the Related Art 
     In a display device such as a monitor, a white balance adjustment makes a white object appear white regardless of the color temperature. In particular, in a cathode ray tube (CRT), a white balance adjustment adjusts gains and biases of signals applied to red, green, and blue (RGB) guns or cathodes in the CRT. The bias adjustment is often referred to as a cut-off adjustment. Cut-off control circuits can employ a DC-coupling or an AC-coupling when driving cathodes of a CRT. The coupling mode of the cut-off control circuit does not affect the gain adjustment since the gain controls an AC signal component that both AC and DC couplings transfer. However, the coupling mode does affect the bias adjustment, and cut-off control circuits typically require different integrated circuit chips according to the type coupling employed. A further concern is that brightness control depends on a combined RGB signal, but the cut-off control, which controls cathode biases, is carried out for each of the R, G, and B signals separately. In order to adjust the white balance, the brightness level is adjusted first, and then the cut-off control is performed. 
     FIG. 1A is a circuit diagram of a conventional cut-off control circuit having a DC-coupling to the video portion of a monitor (e.g., a cathode in a CRT). The cut-off control circuit of FIG. 1A includes a video pre-amplifier  101  and a drive amplifier  102  for driving a CRT. For simplicity, FIG. 1A shows only one channel even though a color video system normally has three channels (R, G, and B). The cut-off control adjusts the respective CRT cathode bias voltages VA for the R, G, and B cathodes. In FIG. 1A, a video feedback voltage Vf provides a negative feedback to pre-amplifier  101 . Pre-amplifier  101  controls an output voltage Voutput so that video feedback voltage Vf is the same as or depends on a brightness control voltage Vbright. A current controller  103  controls video feedback voltage Vf and thus controls voltage Voutput and CRT cathode bias voltage VA. Equation 1 expresses CRT cathode bias voltage VA as a function of feedback voltage Vf, resistances R 1  and R 2  of respective resistors  104  and  105 , and a current Ic.                V                 A     =       V                 f   *       R1   +   R2     R2       +     Ic   *   R1                 Equation                 1        :                                               
     As can be seen in Equation 1, the CRT cathode bias voltage VA depends on the control current Ic, which a cut-off control signal Vcutoff can adjust. 
     FIG. 1B is a circuit diagram of the conventional video pre-amplifier  101  shown in FIG. 1A, in which an external feedback brightness control method is used. Referring to FIG. 1B, the video pre-amplifier  101  includes an adder  110   a , a drive amplifier  101   b , a comparator  101   d , and a switch  101   c.    
     FIG. 2A is a circuit diagram of a conventional cut-off control circuit using an AC-coupling mode. The cut-off control circuit of FIG. 2A includes a video pre-amplifier  201 , a drive amplifier  202 , a coupling capacitor  203 , and a comparator  204 . This circuit uses a separate DC bias circuit that controls cathode bias voltage VA. Only an AC component of the output signal from drive amplifier  202  passes through a coupling capacitor  203  to voltage VA. Equation 2 expresses cathode bias voltage VA as a function of a reference voltage Vref, resistances R 3 , R 4 , and R 5  of respective resistors  213 ,  214 , and  215 , and cutoff voltage Vcutoff.                V                 A     =       Vref   *       R3   +   R4   +   R5       R3   *   R5         -     Vcutoff   *     R4   R5                   Equation                                2        :                                               
     As shown in the equation 2, cathode bias voltage VA depends on cut-off control voltage Vcutoff which is applied to a comparator  204  to adjusts a control current Ic. 
     FIG. 2B is a circuit diagram of video preamplifier  201  shown in FIG. 2A, in which a built-in feedback brightness controlling method is used. The video pre-amplifier  201  of FIG. 2B includes an adder  201   a , a drive amplifier  201   b , a comparator  201   d , and a switch  201   c.    
     As mentioned above, the video pre-amplifiers  101  and  201  of the conventional cut-off control circuits respectively using DC-coupling and AC-coupling differ from each other. Thus, conventional cut-off control circuits using different coupling modes require different integrated circuits and external parts. Further, the cut-off control circuits require a number of external parts, such as an operational amplifier and resistors, in addition to the video pre-amplifier. This makes the circuits more complex. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a video pre-amplifier that can be implemented on a single IC chip and can be used in cut-off control circuits using both AC-coupling mode and DC-coupling mode. Another object of the invention is to provide a cut-off control circuit that requires fewer external components other than a video pre-amplifier IC chip. 
     To achieve the above objects, a preamplifier according to an embodiment of the invention includes a switching unit for receiving control data, generating a bus control signal according to the control data, and outputting the bus control signal internally or externally. The switching unit provides a bus control signal internally, when the preamplifier operates in a cut-off control circuit that uses DC-coupling. The switching unit provides a bus control signal to an external bias circuit when the preamplifier operates in a cut-off control circuit that uses AC-coupling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1A illustrates a conventional cut-off control circuit using a DC-coupling; 
     FIG. 1B is a circuit diagram of a video pre-amplifier shown in FIG. 1A; 
     FIG. 2A illustrates a conventional cut-off control circuit using a AC-coupling; 
     FIG. 2B is a circuit diagram of a video pre-amplifier shown in FIG. 2A; 
     FIG. 3 illustrates a cut-off control circuit with a DC-coupling and a video amplifier of a preferred embodiment according to the present invention; 
     FIG. 4 illustrates a cut-off control circuit with an AC-coupling and the video amplifier from FIG. 3; 
     FIG. 5 is a circuit diagram of another embodiment of the video pre-amplifier according to the present invention; 
     FIG. 6 is a circuit diagram of yet another embodiment of the video pre-amplifier according to the present invention; and 
     FIGS. 7A through 7C are graphs showing simulated performance of a cut-off control circuit according to the present invention. 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a cut-off control circuit in accordance with an embodiment of the invention. The cut-off control circuit includes a video pre-amplifier  300  and a drive amplifier  350  that uses a DC-coupling to a cathode of a cathode ray tube (CRT), not shown. In accordance with an aspect of the invention, video pre-amplifier  300  can be formed on an integrated circuit (IC) chip and employed in a cut-off circuit using either a DC coupling (as shown in FIG. 3) or an AC coupling (as shown in FIG.  4  and described below). In FIG. 3, video pre-amplifier  300  amplifies a sum of a video input signal Vinput and a bias control voltage Vbc and applies an amplified output signal Vo to drive amplifier  350 . Drive amplifier  350  amplifies signal Vo and drives the CRT in the video system. Video pre-amplifier  300  includes a first adder  301 , an amplifier  302 , a second adder  303 , a switching unit  304 , an analog comparator  305 , and a clamping switch (or transistor)  306 . Switching unit  304  includes a bus control block  304   a , a digital-to-analog (D/A) converter  304   b , a switch  304   c , and an I/V converter  304   d.    
     Adders  301  and  303  add analog voltages. In operation, adder  301  adds video input signal Vinput to bias control signal Vbc and applies the resultant sum to amplifier  302 . Amplifier  302  amplifies signal output from adder  301 , and outputs the amplified signal as video output signal Vo from video-preamplifier  300 . Adder  303  adds an output signal Vda from switching unit  304  to a brightness control signal Vbright and applies the resultant signal Vc to an input terminal of comparator  305 . A microcontroller (not shown) or an on-screen display (OSD) controller (not shown) in the video system generates brightness control signal Vbright to indicate a desired brightness of the CRT image. Comparator  305  compares signal Vc from second adder  303  with video output signal Vo from amplifier  302  and generates an output signal having a level that depends on the difference between signal Vc and video output signal Vo. Clamping switch  306  closes or opens in response to pulses in a signal CGPulse from the microcontroller and connects or disconnects the output signal from comparator  305  to adder  301  and a capacitor C 1 . The signal CGPulse is activated during a back porch period of the video input signal. Capacitor C 1  is typically external to a pre-amplifier integrate circuit  300 . When switch  306  is closed, comparator  305  charges (or discharges) capacitor C 1  by drawing or supplying a current to capacitor C 1  depending on the compared result. The voltage difference across capacitor C 1  is provided as the bias control signal Vbc to the adder  301 . Due to the operation of the feedback loop, signal Vbc reaches such a voltage that signals Vo and Vc have equal magnitude. When switch  306  is open, capacitor C 1  clamps or holds signal Vbc at a nearly constant voltage. In particular, capacitor C 1  limits an AC component of bias control voltage Vbc which may result from the AC component of signal Vo. 
     In switching unit  304 , bus control block  304   a  receives serial control data from the microcontroller via an inter-IC bus (IIC bus) and stores such data in an internal register for output as a parallel signal. (For example, 8-bit parallel data signals indicating a cut-off level for an R, G, or B cathode in the CRT.) A digital-to-analog (D/A) converter  304   b  receives the bus control data from bus control block  304   a  and converts the bus control data into an analog control current signal. Switch  304   c  transfers the control current signal from D/A converter  304   b  either to current-to-voltage (I/V) converter  304   d  or to the terminal for cut-off signal Icutoff depending on a selection control signal from bus control block  304   a . Here, the control current signal is negative, i.e., D/A converter  304   b  draws current from either I/V converter  304   d  or the Icutoff terminal. When I/V converter  304   d  receives the control current signal, converter  304   d  converts the current signal into control voltage signal Vda. The microcontroller can provide the selection control signal with a value that indicates the type of coupling used in the cut-off control circuit. In FIG. 3, the cut-off control circuit uses a DC coupling to the CRT, and switching unit  304  operates in a mode referred to herein as DC-coupling mode or internal mode where switch  304   c  provides a path from D/A converter  304   b  to IN converter  304   d.    
     For control of the bias voltage using a DC coupling, comparator  305  (or charged capacitor C 1 ), adder  301 , and amplifier  302  form a negative feedback loop which sets the steady state or DC level of voltage Vo equal to control voltage Vc. Equation 3 shows the relationship of the DC component of video output voltage Vo to signals Vc, Vbright, and Vda. 
     
       
           Vo=Vc=V bright+ Vda.   Equation 3: 
       
     
     Signals Vbright, Vda, and Vc respectively denote a brightness control voltage, the output voltage of switching unit  304 , and the output voltage of second adder  303 . Sequential conversion of data from bus control block  304   a  can change voltage Vda to perform the cut-off control for each of the RGB cathodes and therefore change output voltage Vo to appropriate values for each of the RGB cathodes. 
     FIG. 4 shows a cut-off control circuit employing pre-amplifier  300  and a drive amplifier  450  with an AC-coupling to the video system. The configuration of video pre-amplifier  300  in FIG. 4 differs from the corresponding configuration in FIG. 3 in that switching unit  304  operates in an AC-coupling mode. In particular, switch  304   c  routes the control current signal from D/A converter  304   b  to an output terminal for cut-off signal Icutoff. To implement the AC coupling, the cut-off circuit of FIG. 4 includes a capacitor  462  coupled between drive amplifier  450  and an output node  465 . Amplifier  450  provides an AC video signal to node  465 . A DC bias circuit for node  465  includes resistors R 1  and R 2  and a transistor Q 1 . Resistor R 1  is between node  465  and a supply voltage Vb. Transistor Q 1  has a collector connected to node  465 , a base connected to a bias voltage Vbb, and an emitter connected via resistor R 2  to switching unit  304 . 
     When switch  304   c  routes the control current signal to the output terminal for signal Icutoff, the DC voltage to the second terminal of adder  303  is zero. The feedback loop including comparator  305  or capacitor C 1 , adder  301 , and amplifier  302  still drives output voltage Vo to the level of control voltage Vc. Accordingly, Equations 4 express the steady state or DC component of video output voltage Vo and CRT cathode bias VA. 
     
       
           Vo=V bright 
       
     
     
       
           VA=Vb−R 1 *Ic   Equations 4: 
       
     
     In Equations 4, Vbright, Ic, and Vb respectively denote a brightness control voltage, the magnitude of the output current from D/A converter  304   b , and a supply voltage. As can be seen from Equations 4, D/A converter  304   b  by controlling voltage Vda controls the DC component of CRT bias voltage VA. 
     FIG. 5 illustrates a video pre-amplifier  500  in accordance with another embodiment of the invention. Video pre-amplifier  500  includes a first adder  501 , an amplifier  502 , a second adder  503 , a switching unit  504 , a comparator  505 , and a clamping switch  506  which are respectively similar or identical to first adder  301 , amplifier  302 , second adder  303 , a switching unit  304 , comparator  305 , and clamping switch  306  of FIG. 3. A primary difference between pre-amplifier  300  of FIG.  3  and pre-amplifier  500  of FIG. 5 is that second adder  503  has a positive input terminal coupled to the output terminal of amplifier  502  and a negative input terminal coupled to the output terminal of switching unit  504 . An output voltage Vf′ from adder  503  is thus equal to output voltage Vo minus the voltage from switching unit  504 . 
     Cut-off control circuits using DC-coupling and AC-coupling, similar to those of FIGS. 3 and 4 can use pre-amplifier  500  in place of pre-amplifier  300 . In a DC coupling mode of pre-amplifier  500 , a negative feedback loop including comparator  505  (or charged capacitor C 1 ), adder  501 , amplifier  502 , and adder  503  drives voltage Vf′ to an equilibrium level equal to brightness control voltage Vbright. Accordingly, in DC coupling mode, Equation 5 gives output voltage Vo in terms of voltages Vbright and Vda. 
     
       
           Vo−Vda=V bright or  Vo=V bright+ Vda   Equation 5: 
       
     
     In AC coupling mode, switching circuit  504  grounds the negative input to adder  503 , and output voltage Vo is the same as for pre-amplifier  300  in FIG.  4 . 
     FIG. 6 illustrates a video preamplifier  600  in accordance with yet another embodiment of the invention. Pre-amplifier  600  of FIG. 6 can replace pre-amplifier  300  in cut-off control circuits using DC-coupling and AC-coupling, similar to those of FIGS. 3 and 4. 
     In FIG. 6, video pre-amplifier  600  includes a video input clamping unit  601 , an amplifier  602 , a first adder  603 , a switching unit  604 , and a second adder  605 . Switching unit  604  includes a bus control block  604   a , a digital-to-analog (D/A) converter  604   b , a switch  604   c , and an I/V converter  604   d . Video input clamping unit  601 , which is optional, receives a video input signal Vinput and an OSD signal from an OSD controller (not shown) and matches the black level of video input signal Vinput to that of the OSD signal. Video input clamping unit  601  applies a level-adjust video input signal to amplifier  602  for amplification. 
     In switching unit  604 , D/A converter  604   b  receives digital bus control data from bus control block  604   a  and converts that data into an analog control current signal. Switch  604   c  directs the analog control current signal from D/A converter  604   b  either to I/V converter  604   d  or to the output terminal for cut-off signal Icutoff according to a selection control signal from bus control block  604   a . When I/V converter  604   d  receives the control current signal, converter  604   d  converts the current signal into control voltage signal Vda. Second adder  605  adds the output signal VDA from switching unit  604  to brightness control signal Vbright. First adder  603  adds the output signal from amplifier  602  to the output signal of second adder  605  and outputs the sum as video output signal Vo. Accordingly, in DC coupling mode, adder  603  shifts the DC component of output from amplifier  602  by the sum of signals Vbright and Vda. 
     FIGS. 7A through 7C show plots of the results of a simulation of the circuit of FIG.  4 . In the simulation, the bus control data from bus control block  404   a  sweeps from 00h to FFh and then to 00h again. FIGS. 7A and 7B respectively show the waveforms of a collector current It and an emitter current Ic of transistor Q 1 . FIG. 7C shows the waveform of the DC component of output voltage VA, which is output to a CRT cathode. As can be seen in FIG. 7C, the DC bias of the output voltage VA varies across a range according to the bus control data. Therefore, the bus control data can set the DC bias of output voltage VA as required within the range. Further, a specific value of the bus control data can be easily selected and set during manufacture. 
     As described above, the pre-amplifier includes a built-in bus control block and D/A converter that is usable in the cut-off control circuits using both DC-coupling and AC-coupling. The chip size of the pre-amplifier is not greatly increased for this added flexibility since separate sets of control blocks and D/A converters, one for DC-coupling mode and one for AC-coupling, are not required. Instead, a switch selects the operating mode and use of the control block and D/A converter. Furthermore, in either a control circuit employing DC or AC coupling, the number of required components in addition to the video pre-amplifier IC chip is low, which lowers manufacturing cost of the cut-off control circuits. 
     In alternative embodiments of the present invention where only DC-coupling or only AC coupling is required, switch  304   c ,  504   c , or  604   c  may be omitted from in the respective pre-amplifier  300 ,  500 , or  600 . In such embodiments, the output signal of the D/A converter  304   b ,  504   b , or  604   b  is provided directly to either the I/V converter or the Icutoff terminal depending on the coupling mode for the pre-amplifier  300 ,  500 , or  600 . When the pre-amplifier is employed in a DC coupled cut-off control circuit, adder  303 ,  503 , or  605 , which is used adding the output signal of the switching unit to the video output signal Vo or the brightness control signal Vbright, may be omitted. 
     Having described and illustrated principles of the invention in specific embodiments, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.