JPH08294016A - Dynamic focus circuit - Google Patents

Abstract

PURPOSE: To prevent a waveform distortion due to power fluctuation of a parabolic voltage of a horizontal period and a vertical period or the like superimposed onto a DC focus voltage of a cathode ray tube. CONSTITUTION: A 1st amplifier circuit is made up of an operational amplifier 1 and its DC operating point is set by a voltage obtained by feeding back its output terminal neutral voltage of a 2nd amplifier circuit (TR1, TR2 or the like) and a 1st power supply B1 via a resistor (at its noninverting input terminal). On the other hand, a 2nd amplifier circuit (cascade connection of the TRs 1, 2) as an output circuit is directly coupled with the 1st amplifier circuit in terms of DC and the power supply is supplied from the 2nd power supply B2. When a neutral voltage Vo of the 2nd amplifier circuit is decreased, the voltage at the noninverting input terminal is decreased and the neutral voltage of the output terminal is decreased by the noninverting input. The reduction acts like increasing the neutral voltage of the output circuit coupled directly. Thus, fluctuation in the neutral voltage of the output circuit is compensated and a deformed parabolic wave is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dynamic focus circuit, and more particularly, to a focus voltage of a cathode ray tube, which prevents waveform distortion due to power fluctuations of a horizontal period and a vertical period parabolic voltage superimposed on a DC focus voltage. Regarding

[0002]

2. Description of the Related Art Conventionally, in a display device using a cathode ray tube (CRT), a dime focus circuit is often used in order to obtain a good focus on the entire central portion and peripheral portion of the screen. FIG. 2 shows an example of a conventionally used dimic focus circuit.
The dimic focus is to change the focus voltage applied to the focus electrode of the cathode ray tube in a parabolic shape in the vertical and horizontal directions with respect to the center of the screen.
In this case, the peripheral portion is made higher than the central portion. Therefore, the horizontal and vertical parabolic voltages are superimposed on the DC voltage. Therefore, a circuit for amplifying the parabolic voltage is required, but FIG. 2 shows an operational amplifier circuit (opamp 1) for preamplifying the parabolic voltage and a transistor TR.
This is done by an output circuit in which 1 and TR2 are connected in cascade.

The positive phase input terminal (+) of the operational amplifier 1 has a first
A required DC voltage (bias voltage) is applied from a power source B1 of the above through a resistor to determine a DC operating point. Also, the parabola voltage Vp is input to the inverting input terminal (−) of the amplifier 1,
Amplification is performed to a required level centering on the DC operating point. As shown in FIG. 3, the parabola voltage Vp has a horizontal period (1H) and a vertical period (1V), and the input polarity to the inverting input terminal (−) is the polarity shown in FIG. The parabolic voltage amplified by the operational amplifier 1 is sent to an output circuit in which TR1 and TR2 are cascade-connected and further amplified. The output circuit is generally
Power is supplied from the second power supply B2 obtained by rectifying the pulse voltage (horizontal cycle) generated by the flyback transformer (FBT) 2, and the parabolic voltage of the required amplitude is output from the emitter of TR1 cascade-connected with TR2. To do. The flyback transformer is used because a high voltage is required as the second power source B2. The parabolic voltage amplified by the output circuit is superimposed on the DC focus voltage Vf via the capacitor C1 and applied to the focus electrode of the cathode ray tube (CRT) 3. The voltage applied to the focus electrode of the CRT 3 is as shown in FIG. 3, and the voltage is made higher than that at the central portion as it goes to the periphery of the screen (vertical, horizontal).

[0004]

As described above, since the second power source B2 used in the output circuit requires a high voltage, it is generally obtained by rectifying the pulse of the flyback transformer. The rectified voltage B2 fluctuates due to a change in the brightness of the image, a change in the horizontal frequency of the video signal in a multi-scan display device, or the like, which changes the rectified voltage B2. When this voltage B2 changes, the voltage (midpoint voltage Vo) that is the center of operation of the output circuit also changes. Also, this midpoint voltage
Vo also fluctuates due to variations in hfe (current amplification factor) of the transistors TR1 and TR2. When the midpoint voltage Vo fluctuates, the output parabolic voltage is crushed due to the dynamic range of the output circuit, resulting in waveform distortion and deterioration of focus quality. The conventional circuit of FIG. 2 has such drawbacks. The present invention is intended to improve the drawbacks of the conventional circuit as described above, and provides a dimic focus circuit in which the parabolic output voltage is not distorted by stabilizing the midpoint voltage of the output circuit. The purpose is to

[0005]

According to the present invention, a first amplifier circuit applies a required DC bias voltage from a first power source and amplifies an input parabolic voltage that changes in a horizontal cycle and a vertical cycle, and the first amplifier circuit. Is connected to the amplifier circuit in a DC manner, receives power from the second power source, amplifies the parabolic voltage from the first amplifier circuit, and superimposes the amplified parabolic voltage on the DC focus voltage via the capacitor. And a second amplifying circuit that feeds back the output operation center voltage via a resistor so as to be superimposed on the DC bias voltage of the first amplifying circuit, and the DC operating point of the first amplifying circuit The present invention provides a dimic focus circuit in which is changed according to the fluctuation of the output operation center voltage of the second amplifier circuit.

[0006]

When the first amplifier circuit is composed of an operational amplifier,
The DC operating point is set by the first power supply and the voltage obtained by feeding back the output terminal midpoint voltage of the second amplifier circuit through the resistor (positive phase input terminal). On the other hand, the second amplifier circuit (cascade connection output circuit) as the output circuit is directly connected to the first amplifier circuit in terms of direct current, and the power is supplied from the second power source. Now, when the midpoint voltage of the second amplifier circuit drops, the voltage at the positive phase input terminal also drops, and the center voltage at the output terminal also drops because of the positive phase input. This decrease acts to increase the midpoint voltage of the output circuit directly connected. That is, it acts so as to compensate for the fluctuation of the midpoint voltage of the output circuit, and the collapse of the parabola wave is prevented.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A dynamic focus circuit according to the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of essential parts showing an embodiment of a dynamic focus circuit according to the present invention. In FIG. 1, B1 is a first power supply, B2 is a second power supply, 1 is an operational amplifier (hereinafter, referred to as “op amp”) that constitutes a first amplifier circuit for amplifying an input parabola voltage Vp, TR1 and TR2 are A first transistor and a second transistor that form an output circuit (cascade connection) for parabolic amplification. Reference numeral 2 is a flyback transformer (FBT) that outputs high voltage, DC focus voltage, second power source pulse, etc., VR1 is a focus voltage adjusting volume, and 3 is a cathode ray tube (CRT).

Next, the operation of the present invention will be described. The first power source B1 and the voltage obtained by feeding back the midpoint voltage Vo of the output circuit via the resistor R7 are applied to the positive phase input (+) end of the operational amplifier 1, and the DC operating point of the required bias voltage is set. To do.
The parabola voltage is input to the inverting input terminal (−) of the operational amplifier 1. Therefore, the output phase is inverted with respect to the input phase.
As described above, the parabolic voltage is a combined parabolic wave having a horizontal period (1H) and a vertical period (1V), as shown in FIG. The output of the operational amplifier 1 is input to the base of the second transistor TR2 via the resistor R4. That is, the DC component is transmitted together with the parabola voltage which has been inverted and amplified. same
First transistor TR1 connected to TR2 as shown
The second power source B2 is applied to the collector of the. The parabolic wave input to the base of TR2 is phase-inverted and taken out from the emitter of TR1.

The second power source B2 is one in which a pulse having a horizontal period generated in the flyback transformer 2 is converted into a direct current by a rectifying diode D2 and a smoothing capacitor C4.
As described above, the power source B2 fluctuates due to changes in the brightness of the video signal. Now, for example, if the second power source B2 drops due to the above cause, the DC voltage of the TR1 emitter, that is, the midpoint voltage Vo also drops if the feedback resistor R7 is not provided. As a result,
The point A in FIG. 3 is lowered and is shifted to the ground potential side. When this drop is large, it finally reaches a saturation voltage and begins to collapse from the same point. However, by providing the feedback resistor R7 as shown in the figure, the positive-phase input terminal (+) of the operational amplifier 1 also decreases due to the decrease of the midpoint voltage Vo, and the output terminal also decreases due to the relationship of the positive-phase input. Due to this decrease, the base voltage of the direct-connection TR2 also decreases, and the decrease also acts to increase the emitter voltage of TR1 to compensate for the decrease in the midpoint voltage Vo. That is, the fluctuation of the output circuit midpoint voltage Vo is suppressed and stabilized. As a result, the parabolic wave is amplified without being crushed. The emitter output of the TR1 is superimposed on the DC focus voltage Vf via the capacitor C1 and the like and applied to the focus electrode of the cathode ray tube 3.

Contrary to the above example, when the midpoint voltage Vo rises, it operates in the opposite relationship to the above, and as a result, the midpoint voltage Vo
Vo rise is suppressed. In addition, even if the variation of hfe (current amplification factor) of TR1 or TR2 can cause variation in the midpoint voltage Vo, this variation is compressed by the negative feedback action of the resistor R7 and the collapse of the upper or lower part of the parabola wave is prevented. To do.
As described above, by feeding back the output circuit midpoint voltage to the pre-stage amplifier circuit (the operational amplifier 1), the fluctuation of the midpoint voltage is compensated.

[0011]

As described above, according to the present invention, in the dynamic focus circuit often used in the cathode ray tube display device, the operation of the output circuit due to the fluctuation of the power supply voltage or the variation of hfe (current amplification factor) of the transistor. The fluctuation of the center voltage (midpoint voltage) can be suppressed. As a result, the phenomenon of parabolic wave collapse is prevented. Therefore, deterioration of image quality is prevented without deterioration of focus quality. As described above, the present invention can contribute to improving the performance of a display device using a cathode ray tube.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a main part of an embodiment of a dynamic focus circuit according to the present invention.

FIG. 2 is a main part circuit diagram showing an example of a conventional dynamic focus circuit.

FIG. 3 is a parabola voltage waveform diagram for explaining dynamic focus.

[Explanation of symbols]

 1 operational amplifier B1 first power supply B2 second power supply TR1 first transistor TR2 second transistor R1 first resistance R2 second resistance R3 third resistance R4 fourth resistance R5 fifth resistance R6 sixth Resistor C1 1st capacitor C2 2nd capacitor C3 3rd capacitor 2 Flyback transformer 3 CRT

Claims (3)

[Claims] 1. A first amplifier circuit, which applies a required DC bias voltage from a first power supply and amplifies an input parabolic voltage that changes in a horizontal cycle and a vertical cycle, and is connected to the first amplifier circuit in a direct current manner. Then, the power from the second power supply is received, the parabola voltage from the first amplifier circuit is amplified, the amplified parabola voltage is superimposed on the DC focus voltage via the capacitor, and the output operation center voltage is And a second amplifier circuit which is fed back so as to be superimposed on the DC bias voltage of the first amplifier circuit via the DC amplifier, and the DC operating point of the first amplifier circuit is the same as that of the second amplifier circuit. A dynamic focus circuit characterized in that it is changed according to a change in output operation center voltage. 2. The first amplifier circuit is inverted by applying a voltage from the first power supply and an output operation center voltage from a resistor from the second amplifier circuit to a positive phase input terminal of the first amplifier circuit. The dynamic focus circuit according to claim 1, wherein the dynamic focus circuit comprises an operational amplifier circuit to which the parabolic voltage is input at an input end. 3. In the second amplifier circuit, a voltage from the second power source is applied to a collector, a voltage is applied to the base from the second power source through a first resistor, and a voltage is applied from an emitter to the second amplifier circuit. An NPN first transistor that superimposes a parabola voltage on a focus voltage via a first capacitor, a second capacitor whose one end is connected to the base of the first transistor, and one end of which is the base of the first transistor. Connected to the second resistor, a third resistor having one end connected to the emitter of the first transistor, and a collector connected to the other ends of the second capacitor, the second resistor and the third resistor. The parabolic voltage from the first amplifier circuit is input to the base via the fourth resistor, the emitter is connected in series with the fifth resistor and the third capacitor, and the sixth resistor is connected. NPN grounded through each
2. The dynamic focus circuit according to claim 1, wherein the dynamic focus circuit comprises a second transistor.