JPH08223439A - Dynamic focus circuit - Google Patents

Abstract

PURPOSE: To prevent waveform distortion of a parabolic voltage of a horizontal period and a vertical period superimposed on a DC focus voltage of a dynamic focus voltage due to power supply fluctuation. CONSTITUTION: A DC operating point of a required bias voltage is set by a voltage from a 1st power supply B1 and a voltage from a 2nd power supply B2 obtained by rectifying a pulse voltage generated by a flyback transformer. The circuit is made up of a 1st amplifier circuit (operational amplifier 1 or the like) amplifying parabolic voltages Vp of a horizontal period and a vertical period based on a DC operating point and 2nd amplifier circuits TR1, TR2 connecting to the 1st amplifier circuit 1 in terms of DC, amplifying the parabolic voltage from the 1st amplifier circuit 1 by using the 2nd power supply B2 as a power supply and superimposing the amplified parabolic voltage onto the DC focus voltage via a capacitor. The DC operating point of the 1st amplifier circuit 1 is changed according to voltage fluctuation of the 2nd power supply B2 to compensate the fluctuation of the DC operating point attended with the fluctuation in the power supply B2 of the emitter of the TR1 being an output terminal.

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 prevention of waveform distortion due to power supply fluctuation of parabolic voltage of horizontal period and vertical period superimposed on DC focus voltage.

[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 in FIG. 2, the amplification of the parabolic voltage is performed by an operational amplifier circuit (opamp 1) as a preamplifier 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 voltage is applied from a power source B1 via a resistor to determine the DC operating point. Also, the inverting input terminal (-) of the amplifier 1
A parabola voltage Vp is input to the input terminal and amplifies it to a required level around the DC operating point. As shown in FIG. 3, the parabolic voltage has a horizontal period (1H) and a vertical period (1V), and the input polarity to the inverting input terminal (−) is the same as that in FIG.
The parabolic voltage amplified by the operational amplifier 1 is sent to the output circuit composed of TR1 and TR2 and further amplified. The output circuit receives power supply from the second power supply B2 obtained by rectifying the pulse voltage (horizontal period) generated in the flyback transformer (FBT) 2. 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. CRT
The voltage applied to the No. 3 focus electrodes is as shown in FIG.

[0004]

As described above, since the second power source B2 used in the output circuit requires a high voltage, it can be obtained by rectifying the pulses of the flyback transformer.
This pulse voltage fluctuates when the luminance of the image changes or when the horizontal frequency of the image signal changes in a multi-scan display device or the like, and therefore the rectified voltage B2 changes. When this voltage decreases, the parabolic voltage collapses, resulting in waveform distortion and degrading focus quality. The circuit of FIG. 2 has such drawbacks. The present invention is intended to improve such a drawback of the conventional circuit, and to provide a dimic focus circuit that does not cause distortion in the parabolic voltage even when voltage fluctuation occurs in the power supply obtained from the flyback transformer. The purpose is to provide.

[0005]

According to the present invention, a direct current of a required bias voltage is obtained by a voltage from a first power supply and a voltage from a second power supply obtained by rectifying a pulse voltage generated by a flyback transformer. An operating point is set, and a first amplifying circuit that amplifies the input parabolic voltage of a horizontal period and a vertical period according to the same operating point of the direct current is connected to the first amplifying circuit in a direct current manner, and the second amplifying circuit is connected to the second amplifying circuit. And a second amplifying circuit for amplifying the parabolic voltage from the first amplifying circuit and superimposing the amplified parabolic voltage on the DC focus voltage via a capacitor. It is intended to provide a dimic focus circuit in which the DC operating point of the circuit is changed according to the voltage fluctuation of the second power supply.

[0006]

When the first amplifier circuit is composed of an operational amplifier,
The DC operating point is set to the required bias by the first power source and the second power source (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 voltage of the second power supply drops, the voltage at the positive phase input terminal also drops, and the DC voltage at the output terminal also drops due to the positive phase input. This decrease acts to raise the operating point of the directly connected output circuit. Therefore, the output circuit is the second
This will compensate for those whose operating point is going to be lowered due to the voltage drop of the power source. As a result, the collapse of the parabolic 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 2 which constitute an output circuit (cascade connection) for parabolic amplification, a flyback transformer (hereinafter referred to as “FBT”) that outputs high voltage, a DC focus voltage, a second power supply pulse, and the like. Note that 3 is a cathode ray tube (CRT).

Next, the operation of the present invention will be described. A voltage is applied to the positive phase input (+) terminal of the operational amplifier 1 by the first power source B1 and the second power source B2 via the resistor R7 to set the DC operating point of the required bias voltage. The parabola voltage is input to the inverting input terminal (-) of the same operational amplifier. Therefore,
The output phase is inverted with respect to the input phase. In addition, the parabolic voltage is the same as the above, as shown in FIG.
And a vertical period (1V). The output of the operational amplifier 1 is connected to the second transistor TR2 via the resistor R4.
To the base of. That is, the DC component is transmitted together with the parabola voltage which has been inverted and amplified. The second power source B2 is applied to the collector of the first transistor TR1 connected to the same TR2 as shown in the figure. 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 original pulse voltage of the power supply B2 changes when the luminance of the video signal changes, or when the horizontal frequency changes when the video signal having a different horizontal frequency is used as the input source like a multi-scan monitor. . Therefore, B2 also changes.
Now, if the second power source B2 drops due to the above causes, TR1
The DC voltage of the emitter also drops. That is, the point a in FIG. 3 is lowered and approaches the ground potential. When this drop is large, it finally reaches a saturation voltage and begins to collapse from the same point.

On the other hand, the positive phase input terminal (+) of the operational amplifier 1 also drops due to the decrease in the second power source B2, and its output terminal also decreases due to the positive phase. Due to this decrease, the base voltage of TR2 connected directly also decreases, and this decrease acts to increase the emitter voltage of TR1 to compensate for the decrease in the output circuit itself. That is, the fluctuation of the emitter voltage of TR1 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. As described above, this circuit is used by feeding back the fluctuation of the power source to compensate the fluctuation of the operating point. This feedback amount is determined by comprehensively considering fluctuations in B2, fluctuations in TR1 emitter voltage due to fluctuations in B2, the gain of the operational amplifier 1, and the bias voltage to the positive phase input terminal (+). Set by the peripheral constants of each amplifier circuit such as R9. In addition, VR1 in the figure is a volume for adjusting the focus voltage.

[0011]

As described above, according to the present invention, in the dynamic focus circuit often used in the cathode ray tube display device, the phenomenon of parabolic wave collapse caused by the fluctuation of the power source obtained from the flyback transformer is prevented. To be done. This power fluctuation is due to the brightness change of the video signal,
Or, it is caused by the change of the horizontal frequency due to the video signal switching when the video signal with different horizontal frequency is used as the input source like the multi-scan monitor. However, even if the power supply changes due to these factors, the deterioration of the focus quality is prevented. Therefore, image quality deterioration is prevented. As described above, the present invention can mainly contribute to improving the performance of the multi-scan monitor.

[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 D1 Diode 2 Flyback transformer 3 CRT

Claims (3)

[Claims] 1. A second voltage obtained by rectifying a voltage from a first power supply and a pulse voltage generated by a flyback transformer.
A DC operating point of a required bias voltage is set by the voltage from the power supply of the first power source, and a first amplifying circuit that amplifies the input parabola voltage of the horizontal period and the vertical period according to the DC operating point is provided. A second circuit which is connected to the amplifier circuit in a direct current manner, amplifies the parabolic voltage from the first amplifier circuit using the second power source as a power source, and superimposes the amplified parabolic voltage on the DC focus voltage via a capacitor. A dynamic focus circuit configured by an amplifier circuit, wherein a DC operating point of the first amplifier circuit is changed according to a voltage fluctuation of the second power supply. 2. A voltage from the first power supply and a voltage from a second power supply are applied to a positive phase input terminal of the first amplifier circuit, and the parabolic voltage is input to an inverting input terminal. The dynamic focus circuit according to claim 1, wherein the dynamic focus circuit is configured by an operational amplifier circuit. 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 type first transistor for superimposing a parabolic voltage on a focus voltage via a first capacitor, an anode terminal was connected to an emitter of the first transistor, and a cathode terminal was connected to a base of the first transistor. A diode, a second capacitor having one end connected to the base of the first transistor, a second resistor having one end connected to the base of the first transistor, and one end connected to the emitter of the first transistor. Done 3rd
Of the second capacitor, the other ends of the second resistor and the third resistor are connected to the collector, and the parabolic voltage from the first amplifier circuit is input to the base via the fourth resistor. 2. The emitter is composed of a series circuit composed of a fifth resistor and a third capacitor, and an NPN type second transistor grounded via each of the sixth resistors. Dynamic focus circuit.