1. Field of the Invention
The present invention relates generally to a pin cushion distortion correction circuit for a television receiver, and more particularly is directed to a pin cushion distortion correction circuit for a television receiver for preventing the formation of a distortion from being produced at the center portion of a video screen.
2. Description of the Prior Art
There has been proposed in the art a pin cushion distortion correction circuit for a television receiver, as shown in FIG. 1. In the prior art shown in FIG. 1, an input terminal 1, which is supplied with a horizontal driving signal, is connected to the gate electrode of a gate-controlled switch GCS 2, which forms a horizontal output circuit. The cathode electrode of the gate-controlled switch GCS 2 is grounded, and the anode electrode thereof is connected through a horizontal output transformer 3 to a voltage terminal 4, which is supplied with a positive DC voltage. The anode is also and grounded through a parallel circuit consisting of a damper diode 5 and a tuning capacitor 6. The anode electrode of the gate-controlled switch GCS 2 is further grounded through a series connection of a horizontal deflection coil 7, a secondary winding 8b of a saturable reactor 8 for the correction of left and right pin cushion distortions, and a capacitor 9 for S-letter correction. A vertical output signal input terminal 10 is connected to one end of a series connection of a vertical deflection coil 11 and a capacitor 12 the other end of which is grounded. At the connection point between the vertical deflection coil 11 and the capacitor 12, there is obtained a parabolic wave signal in synchronism with the vertical synchronizing signal. This connection point is connected through a series connection of a capacitor 13 and a resistor 14 to the base electrode of an NPN-type transistor 15. The base electrode of the transistor 15 is grounded through a resistor 16 and also connected through a resistor 16a to the voltage terminal 4 which is supplied with a positive DC voltage to be supplied with a predetermined bias voltage. The emitter electrode of the transistor 15 is grounded through a resistor 17 and its collector electrode is connected through a primary winding 8a of the saturable reactor 8 to the voltage terminal 4.
The inductance-current characteristic of the saturable reactor 8 used in the prior art pin cushion distortion correction circuit of FIG. 1 is shown by a curve a in FIG. 2 in which the abscissa represents the current I.sub.V and the ordinate the inductance L.sub.H, respectively. From FIG. 2, it will be apparent that as the current I.sub.V increases, the slope of curve a gradually flattens. The voltage V applied to the base electrode of the transistor 15 is shown in FIG. 4A in which the abscissa represents the time T and the ordinate the voltage V, respectively. Accordingly, the parabolic current applied to the primary winding 8a of the saturable reactor 8 becomes as shown in FIG. 2 by a curve b. Thus, it often occurs that the correction for the left and right pin cushion distortions by the parabolic current can not be achieved sufficiently and correctly at the center of the video screen.