Abstract:
The disclosed embodiments relate to a current reduction circuit that reduces offset current from at least one driver circuit. The current reduction circuit comprises circuitry that measures the offset current and produces a measurement current and circuitry that receives the measurement current and responds by maintaining an auto kine bias measurement voltage within a predetermined range.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the field of auto kine bias (AKB), and in particular, to an offset current reducing circuit for AKB  
       BACKGROUND OF THE INVENTION  
       [0002]     This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.  
         [0003]     A cathode ray tube (CRT) driver integrated circuit, such as the Philips TDA6120 CRT driver integrated circuit, can be used as a CRT driver. The TDA6120 has a cathode current sample output pin that can be used for AKB. The design of the measurement circuit is such that there can be as much as +/−30 microamperes of offset current added to the desired CRT cathode measurement current for each of the red, green and blue drives. However, a back end signal and sync processor such as the Toshiba TA1316AN has an AKB sampling system with a limited dynamic range, and cannot handle the potential total of +/−90 microamperes of offset (3 X+/−30 μA). The AKB will not function under this condition. This invention is intended to remove enough of the offset current that the remainder is within the dynamic range of the measurement circuit of the TA1316AN. Data sheets of detailed operation for the TDA6120 and the TA1316AN are available from Philips and Toshiba respectively.  
         [0004]     The invention represents an improvement over a concept used previously utilized in the manufacture of televisions. That concept, when modified for projection TV, required three current clamps, one on each of the red, green and blue CRT drive boards. While this arrangement works very well, it is complicated and has a large number of parts. Moreover, if any one of the three clamps malfunctions the AKB system will not operate correctly. This can cause the instrument to shut down. Accordingly, there is a need to simplify this design to provide the same performance, yet at the same time, to be more robust and be subject to fewer problems. Moreover, reducing the number of parts can significantly reduce the cost of production.  
       SUMMARY OF THE INVENTION  
       [0005]     The disclosed embodiments relate to a current reduction circuit that reduces offset current from at least one driver circuit. The current reduction circuit comprises circuitry that measures the offset current and produces a measurement current and circuitry that receives the measurement current and responds by maintaining an auto kine bias measurement voltage within a predetermined range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     In the drawings:  
         [0007]      FIG. 1  is a circuit schematic for an offset current reducing circuit for AKB in accordance with the inventive arrangements. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0008]     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.  
         [0009]     In accordance with the inventive arrangements, a single comparator can be utilized instead of the three current clamps used by the previous system. More particularly, and in the presently preferred embodiment, the comparator is responsive to a voltage derived from the sampled beam current and a reference voltage. The comparator output is a current substantially free of the influences of offset current, and therefore, assured of remaining within a predetermined dynamic range. The inventive arrangements reduce the offset current from the CRT driver IC sufficiently to allow a back end signal and sync processor&#39;s AKB measurement circuit, even one subject to a limited dynamic range, to operate correctly. The inventive arrangements substantially remove both positive and negative offset currents. As compared to the previously described current clamp system, the total part count is advantageously reduced by approximately 85 components. Other advantages of the inventive arrangements may be applicable to other kinds of circuits.  
         [0010]     With reference to  FIG. 1 , an offset current reducing circuit  10  is illustrated. The kine current measurement outputs from pin  7  of each of three TDA6120 CRT driver ICs  12 ,  14  and  16  (one each for red, green and blue) are coupled to the node labeled “I K  FROM KINE SOCKETS”. Each of the kine current measurement outputs from the CRT drivers  12 ,  14  and  16  has a series resistor ( 20 ,  22  and  24 , respectively) coupled thereto to provide arc protection. An 11 K ohm resistor  26  is also coupled to the node and is returned to a +12 volt power supply. The AKB circuit is completed by the red, green and blue, low-level drive signals that are output from integrated circuit  18  pins  43 , 42  and  41 , respectively, being applied to the input pin  2  of respective CRT driver integrated circuits  12 ,  14  and  16 . The basic operation of the AKB circuit is for the DC bias of low-level red, green and blue drive signals to be controlled by the voltage applied to pin  45  of integrated circuit  18  during each channel&#39;s measurement interval, thus controlling the bias applied to the red, green and blue cathodes  82 ,  78  and  74 , respectively. The cathode current measurement signal at pin  7  of each CRT driver is reflective of the cut-off bias of each cathode connected to pin  12  of that CRT driver. The error correction signal for each channel of the AKB control loop is held on capacitors  80 ,  76  and  72 , which are connected to the red, green and blue filter pins  48 ,  47  and  46  of integrated circuit  18 . The time constant associated with each channel&#39;s dominant pole is determined by the value of capacitors  80 ,  76  and  72  and is typically on the order of several fields, the specific value being chosen for loop stability and noise performance. The dominant pole of the offset current reduction circuit is formed by the values of capacitor  56  and resistor  54 . The charge rate of capacitor  56  is determined by resistor  54  and the value of transistor  50  collector current, while the discharge rate is determined by resistor  54 . The time constant of the dominant pole of the offset current reduction circuit must be much longer than the dominant pole time constant of the AKB loop. In the preferred embodiment the offset current reduction circuit time constant is several seconds and the power-on charge is set to stabilize the loop in about four seconds in order to avoid a visible brightness “bounce” when power is applied. Those of ordinary skill in the art will appreciate that the values of components, component identification numbers, voltage levels and the like referred to herein and in  FIG. 1  are exemplary only. Other values may be used in some cases as a matter of design choice.  
         [0011]     A 5V peak-to-peak pulse signal (0V to 5V) whose width is approximately 64 microseconds and whose repetition rate is approximately 16.7 milliseconds is coupled to a resistor  70 . This pulse is an attenuated version of the vertical drive pulse. The pulse occurs shortly after the start of the vertical blanking interval. At this time, the total CRT beam current is theoretically zero. As a practical matter, there is a small amount of flyback signal pickup, but it is small enough to be neglected. The pulse saturates a transistor  68 , which develops a voltage of nominally 5.88 volts at the base of a transistor  46 . This turns on a transistor  46 , whose collector current is approximately 900 microamperes. The transistor  68  is connected to a voltage source as illustrated in  FIG. 1  through a resistor  64  and a resistor  66 . The collector current of the transistor  46  activates a differential amplifier  47  made up of a transistor  50  and a transistor  48 . The base of transistor  50  is biased by the divider made up of resistors  58  and  60 . This voltage divider establishes a base voltage of approximately 2.1 volts. The base of the transistor  50  is additionally coupled to ground via a capacitor  62 . The collector of the transistor  50  is connected to ground via a resistor  52  and a resistor  54 . The base of transistor  48  is connected to resistors  34  and  32 , capacitor  36 , and pin  45  of a Toshiba TA1316AN integrated circuit  18 . Pin  45  of integrated circuit  18  is the AKB measurement point. The differential pair measures the difference between the divider voltage at the base of transistor  50  and the voltage resulting from the sum of the CRT cathode currents, the offset currents from the CRT driver ICs, the current provided by the 11 K ohm resistor  26  and the collector current of transistor  40 . The offset current component must be substantially removed for AKB to function properly.  
         [0012]     If the “zero current” voltage at pin  45  of the TA1316AN  18  at the time of the vertical rate pulse can be held between −0.5 volts and 3 volts, an internal clamp in the TA1316AN  18  can substantially reduce the remainder of the offset. However, the total voltage offset could theoretically be between −2.43 volts (−90 microamperes×27 K ohms) and +2.43 V (+90 microamperes×27 K ohms). One could, for example, simply pump approximately 70 microamperes of current into the node, and that would raise the minimum to −0.5 volts. However, the maximum would then become 4.36 volts, and this value is outside the dynamic range of the measurement system. If, however, the voltage at pin  45  can be maintained at approximately 2 volts, the system will function properly.  
         [0013]     If the voltage at the base of transistor  48  is lower than the voltage at the base of transistor  50 , indicating a negative offset current, the collector current of transistor  50  will be reduced, and the voltage on capacitor  56  and the base of transistor  40  will fall. This will reduce the collector current of transistor  40 . The current of transistor  40  is subtracted from the sum of the “zero beam” current during the vertical pulse measurement period, the offset current, and the current that flows through the 11 K ohm resistor  26 . Because the current in transistor  40  is reduced, the current through resistors  32  and  34  increases, and the voltage at pin  45  of the TA1316AN  18 , which was low initially, increases.  
         [0014]     Conversely, if the voltage at the base of transistor  48  is higher than the voltage at the base of transistor  50 , indicating a positive offset current, the current in transistor  50  will increase, which will raise the voltage on the base of transistor  40 . This increases the current in transistor  40 , which reduces the current through resistors  32  and  34 , dropping the voltage at pin  45  of the TA1316AN  18 . The 11 K ohm resistor  26  advantageously adds an offset in the total current so that transistor  40  is always active over the total range of offset currents from the CRT driver ICs.  
         [0015]     In an AKB system it is desirable to limit the voltage at the measurement point that results from high cathode currents that can occur during active video, since the cathode current always flows in the system, including negative first derivative currents resulting from discharging the parasitic cathode capacitances. This limiting of the measurement point voltage is advantageously accomplished in this system by passing the current through resistor  32  as well as resistor  34 . As the sum of the cathode currents increases, the drop across these two resistors will eventually saturate transistor  30 , the collector of which is connected to the node labeled “I K  FROM KINE SOCKETS” by a resistor  28 . The voltage drop across the resistors  32  and  34  limits the voltage on pin  45  of the TA1316AN  18  to approximately 4 volts, and eliminates the need for a Zener diode or PNP emitter follower to clip the measurement voltage during active scan. A capacitor  36  is connected from integrated circuit  18  pin  45  to ground to filter any residual video rate signals at pin  45 .  
         [0016]     Development of the circuit resulted in several advantageous circuit component configurations and value selections that were not apparent in the initial design stage. The original value of resistor  44  was 5.6 K ohms. This value was reduced to 2.7 K ohms to improve the transient response of the clamp. Resistor  32  was originally 1 K ohm, but this value made the saturated collector voltage of transistor  30  too high and somewhat unpredictable. A value of 39 K ohm for resistor  32  provides a lower, more stable saturated collector voltage.  
         [0017]     The Zener diode  38  was not originally in the system, but was added when it became apparent that the negative currents associated with the crosshatch pattern (discharging the parasitic cathode capacitances) caused the voltage at pin  45  of the TA1316AN  18  to try to go below ground. However, the base to collector junction of transistor  40  became forward biased before the voltage could get to ground via a resistor  42 . This would start discharging capacitor  56  and cause the displayed picture to go green because of dynamic range limitation of the AKB measurement system. During active scan with a 100 IRE crosshatch pattern, the lower voltage at the base of transistor  40  would decrease its collector current, which resulted in higher current in resistor  34 , thus raising the voltage at pin  45  of the TA1316AN  18 . The clamp was not fast enough to restore the charge on the capacitor, and the voltage remained below the normal equilibrium point. With this condition, the green AKB pulse would start to go beyond the linear range of the AKB detector, and the system would increase the green video&#39;s bias level to compensate. This caused the green shift observed. With a Zener diode  38  installed, the collector of transistor  40  is advantageously always at or above 2.2V. This is sufficiently high to keep the base collector junction from becoming forward biased. It also advantageously allows the integrated circuit  18  pin  45  voltage to operate at the desired 2 volts DC since the level at pin  45  of the TA1316AN resulting from a voltage at the collector of transistor  40  of 2.2 volts is 0.9V. This means that the Zener is advantageously off when the AKB current measurement is made.  
         [0018]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.