Vertical Deflection Circuit for a Cathode-Ray Tube Having a Vertical Image-Position Adjustment Circuit

This invention relates to a vertical deflection circuit and, more particularly, to a vertical deflection circuit having a vertical image-position adjustment circuit for vertically adjusting an image position on the screen of a cathode-ray tube (CRT).

An image display device with a CRT, such as a television receiver, is commonly provided with a circuit for vertical adjustment of the position of an image in order to compensate for an image-position deflection originating from deflection of a beam spot of the CRT, or for the purpose of properly correcting the image position as desired.

This image-position adjustment circuit typically has two vertical-output transistors coupled, for example, between both terminals of a DC voltage source in a push-pull manner. These transistors are driven by the output of a vertical drive circuit, and the node of transistors is coupled to a series circuit of a deflection coil, a capacitor and a resistor. A series circuit of a resistor, a variable resistor and another resistor is also coupled between both terminals of the DC voltage source. The node of the deflection coil and the capacitor is coupled to a slidable end of the variable resistor.

With the above arrangement, the output of the vertical-output transistors causes a deflection current to flow across the deflection coil of the image-position adjustment circuit. Further, the direction of the DC deflection current flowing through the deflection coil can be varied by changing the resistance of the variable resistor so that the position of an image on the CRT's screen is vertically adjusted.

In this image-position adjustment circuit, however a parabolic voltage is generated at the node of the deflection coil and the capacitor. This parabolic voltage varies the DC potential at the slidable end of the variable resistor and thus causes a parabolic ripple current to flow through the deflection coil, deforming a displayed image. The circuit disclosed in the Japanese Utility Model Publication No. 56-30043 is designed for a solution to this problem. However, the disclosed circuit cannot completely remove the aforementioned ripple component from the deflection current.

With the above situation in mind, this invention has been devised, and its object is to provide a vertical deflection circuit for a CRT having a vertical image-position adjustment circuit capable of completely eliminating a ripple current caused by a parabolic voltage so as to provide a deflection current with a high linearity.

To achieve this object, the vertical deflection circuit for a CRT having the image-position adjustment circuit of this invention comprises:

DC voltage source means having first and second terminals;

a series circuit of a vertical deflection coil and a capacitor;

a vertical output means for supplying a deflection current to the vertical deflection coil;

first and second transistors coupled together in a complementary arrangement, the transistors being connected between the first and second terminals of the DC voltage source means, a node of the first and second transistors being coupled to a node of the vertical deflection coil and the capacitor;

variable voltage generating means for generating a variable DC voltage; and

first and second constant voltage elements, respectively coupled in opposite directions between the variable voltage generating means and bases of the first and second transistors, whereby either the first or second transistor is activated in response to the variable DC voltage.

FIG. 1 is a circuit diagram of a vertical deflection circuit for a CRT having a vertical image-position adjustment circuit according to this invention;

FIGS. 2 and 3 are graphs showing the operational characteristics of the circuit of FIG. 1; and

FIGS. 4 to 7 are circuit diagrams each showing a vertical deflection circuit according to a different embodiment of this invention.

An embodiment of this invention will now be explained with reference to the accompanying drawings.

FIG. 1 is a circuit diagram of the first embodiment. Vertical-output transistors 3a and 3b are coupled between both terminals of a DC voltage source 1 in a push-pull arrangement, and they are driven by the output of a vertical drive circuit 2. The node of transistors 3a and 3b is coupled to a series circuit of a deflection coil 4, a capacitor 5 and a resistor 6 such that a vertical deflection current flows across deflection coil 4.

A series circuit of a resistor 7, a variable resistor 8 and another resistor 9 is coupled between the terminals of DC voltage source 1. This series circuit is coupled in parallel to a series circuit of a resistor 11, the emitter-collector path of a PNP transistor 10a, the collector-emitter path of an NPN transistor 10b and a resistor 12. Transistor 10a has its base coupled to the cathode of a Zener diode 13 whose anode is coupled to a slidable end of variable resistor 8. Transistor 10b has its base coupled to the anode of a Zener diode 14 whose cathode is also coupled to the slidable end of variable resistor 8. A resistor 15 is coupled between one terminal of DC voltage source 1 and the base of transistor 10a, and a resistor 16 is coupled between the other terminal of DC voltage source 1 and the base of transistor 10b.

Transistors 10a and 10b are coupled together in a complementary arrangement and constitute a constant current circuit together with Zener diodes 13 and 14 which are respectively coupled to the bases of the transistors. A current flowing through transistors 10a and 10b is dependent on the position of the slidable end of variable resistor 8.

The operation of the circuit shown in FIG. 1 will now be explained in conjunction with FIG. 2.

Suppose that in FIG. 1 the slidable end of variable resistor 8 is moved toward point Q. Then, the voltage across resistor 15 increases so that a collector current I.sub.1 of transistor 10a increases as shown in FIG. 2. The voltage across resistor 16, however, decreases, thus increasing a collector current I.sub.2 of transistor 10b toward zero in the positive-potential direction (see FIG. 2). On the other hand, when the slidable end of variable resistor 8 slides toward point R in FIG. 1, the collector current I.sub.1 of transistor 10a decreases and the collector current I.sub.2 of transistor 10b increases in the negative-potential direction. Therefore, the current difference I.sub.3 between I.sub.1 and I.sub.2 has the characteristic as indicated by the dotted line shown in FIG. 2.

As should be clear from FIG. 2, varying the resistance of variable resistor 8 changes the current difference I.sub.3 between collector currents I.sub.1 and I.sub.2. To be specific, when the position of the slidable end of variable resistor 8 is moved toward point Q, Zener diode 13 becomes conductive and Zener diode 14 becomes nonconductive. As a result, collector current I.sub.1 corresponding to the slid position flows through transistor 10a. Similarly when the slidable end of variable resistor 8 slides toward point R, Zener diode 14 becomes conductive and Zener diode 13 becomes nonconductive, thus causing collector current I.sub.2 corresponding to the slid position to flow through transistor 10b. The difference current I.sub.3 between I.sub.1 and I.sub.2 flows across deflection coil 4, thus adjusting the position of an image on the screen of a CRT, and when the slidable end of variable resistor 8 is positioned at the center, both the Zener diodes 13 and 14 are conductive so that currents flow through both of transistors 10a and 10b. Consequently, the position of an image on the CRT's screen can be smoothly changed.

By properly selecting the characteristics of Zener diodes 13 and 14 and/or variable resistor 8, it is possible to make both the transistors 10a and 10b nonconductive when the slidable end of variable resistor 8 is at the center as shown in FIG. 3. In this case, it is also possible to make the image position stay unchanged when the slidable end of variable resistor 8 is within a predetermined range around the center point. This would reduce a power dissipation of the image-position adjustment circuit. Also, according to the image-position adjustment circuit of this embodiment, currents I.sub.1 and I.sub.2 can respectively flow through transistors 10a and 10b without adverse influence of a parabolic voltage component on the collectors thereof by causing these transistors 10a and 10b to act as a constant current circuit.

The second embodiment of this invention will now be explained in conjunction with FIG. 4.

The circuit of FIG. 4 differs from the circuit of FIG. 1 in the following points:

(1) Another DC voltage source 17, which generates a voltage lower than that of voltage source 1, is provided.

(2) The series circuit of resistor 7, variable resistor 8 and resistor 9 is coupled between both terminals of DC voltage source 17.

(3) The slidable end of variable resistor 8 is coupled to the anode of Zener diode 13 and the cathode of Zener diode 14, and the cathode of the former diode and the anode of the latter diode are respectively coupled to the bases of transistors 18a and 18b.

(4) The collector of transistor 18a is coupled to the base of transistor 10b, while the collector of transistor 18b is coupled to the base of transistor 10a.

(5) Resistors 19 and 20 are coupled between the DC voltage source 17 and the emitter and base of transistor 18a, respectively, and resistors 21 and 22 are coupled between DC voltage source 17 and the emitter and base of transistor 18b, respectively.

The Zener diodes 13 and 14 of the circuit of FIG. 4 can be of a rated voltage lower than that of the Zener diodes 13 and 14 of the circuit of FIG. 1. Therefore, the temperature coefficients of these diodes in the circuit of FIG. 4 can be reduced, thus reducing the temperature drifts. For example, when Zener diodes 13 and 14 have a rated voltage of about five volts, the temperature drifts can be cancelled out by the temperature coefficients of the base-emitter voltages (V.sub.BE) of transistors 18a and 18b. Also, when Zener diodes with a rated voltage of seven volts are used for diodes 13 and 14, the temperature drifts can be cancelled out by connecting an additional diode in series to each of diodes 13 and 14.

FIG. 5 shows another embodiment of this invention, in which the locations of deflection coil 4 and capacitor 5 of the first and second embodiments are reversed. In addition, another DC voltage source 23 is connected in series to DC voltage source 1, and the emitter and base of transistor 10b are coupled to the minus terminal of DC voltage source 23 respectively through resistors 12 and 16.

In the circuit of FIG. 5, the collector current I.sub.1 of transistor 10a flows from the plus terminal of DC voltage source 1 to the minus terminal thereof through resistor 11, the emitter-collector path of transistor 10a, deflection coil 4 and resistor 6. The collector current I.sub.2 of transistor 10b, on the other hand, flows from the plus terminal of DC voltage source 23 to the minus terminal thereof through resistor 6, deflection coil 4, the collector-emitter path of transistor 10b and resistor 12.

FIG. 6 shows a modification of the circuit of FIG. 5, which is designed, like the circuit of FIG. 4, so as to reduce the temperature coefficients of Zener diodes 13 and 14. The circuit of FIG. 6 differs from the circuit of FIG. 5 in the following points:

(1) In addition to DC voltage source 1, a DC voltage source 17 whose voltage is lower than voltage source 1 is provided.

(2) The series circuit of resistor 7, variable resistor 8 and resistor 9 is coupled between both terminals of DC voltage source 17.

(3) The slidable end of variable resistor 8 is coupled to the anode of Zener diode 13 and the cathode of Zener diode 14, and the cathode of the former diode and the anode of the latter diode are respectively coupled to the bases of transistors 18a and 18b.

(4) The collector of transistor 18a is coupled to the base of transistor 10b, while the collector of transistor 18b is coupled to the base of transistor 10a.

(5) Resistors 19 and 20 are coupled between the DC voltage source 17 and the emitter and base of transistor 18a, respectively, and resistors 21 and 22 are coupled between DC voltage source 17 and the emitter and base of transistor 18b, respectively.

The Zener diodes 13 and 14 of the circuit of FIG. 6 can be of a rated voltage lower than that of the Zener diodes 13 and 14 of the circuit of FIG. 5. Therefore, the temperature coefficients of these diodes in the circuit of FIG. 6 can be reduced, thus reducing the temperature drifts.

As explained above, since the vertical deflection circuit for a CRT having the vertical image-position adjustment circuit of this invention uses a constant current circuit to make a DC deflection current flow through a deflection coil to adjust the position of an image on the screen of a CRT, the DC current is free of ripple components which would otherwise deform the image. Accordingly, the deflection current has a high linearity and the amount of adjustment can be set as desired.

This invention is not limited to the aforementioned embodiments. For example, Zener diodes 13 and 14 may be replaced with other constant voltage elements, such as varistors 13' and 14' used in the circuit of FIG. 7. The circuit of this invention may be otherwise modified in various manners without departing from the scope of the invention.