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Vertical Deflection Driver Circuit

IP.com Disclosure Number: IPCOM000088962D
Original Publication Date: 1977-Aug-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 4 page(s) / 69K

Publishing Venue

IBM

Related People

Mortelmans, J: AUTHOR

Abstract

This vertical deflection amplifier is a low cost, low power, high performance circuit. It features two driver transistors that are each active for about one-half the forward sweep time and a dual-yoke damping circuit that provides for a very short retrace time.

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Vertical Deflection Driver Circuit

This vertical deflection amplifier is a low cost, low power, high performance circuit. It features two driver transistors that are each active for about one-half the forward sweep time and a dual-yoke damping circuit that provides for a very short retrace time.

The vertical section 10 of the yoke is connected to dual damping components 12, 14 and 16. One driver transistor 18 is active in the first half, and another transistor 20 is active in the second half of the raster. An operational amplifier (OPAMP) 22 has applied at its input terminals the yoke current feedback voltage across a resistor 24 through an integrating capacitor 26. A small DC voltage, obtained by adjustment of a potentiometer 28, is applied to adjust the peak amplitude of the yoke current sawtooth. The amplitude of a short positive pulse 30 (Fig. 2a) is obtained by means of a potentiometer 31 to control the centering.

Although the OPAMP 22 does not produce complementary output signals necessary to implement the first feature of this circuit, these complementary signals are obtained by loading the OPAMP 22 with a 510-ohm resistor 32 so that complementary currents occur at the power supply inputs to the bases of the transistors 18 and 20. The values of resistors 34 and 36 are so chosen as to provide for only a small time interval where both drivers are active, thus avoiding a discontinuity at the center of the raster. These values also satisfy the following relationship: VBE20,1s = (I OPAMPp,m - IB20,18).R34,36, thereby preventing the drivers from being fully cut off. This direct generation of the bidirectional yoke current uses theoretically only one-seventh of the power of the more conventional circuit where a unidirectional sawtooth is balanced against a constant current (of half the sawtooth amplitude). This approach also avoids the use of a large and costly inductor found in many deflection amplifiers, through which the constant current is applied to the yoke. A high voltage diode 38 prevents reverse operation of the transistor 18 when the negative flyback pulse occurs across the yoke.

Fast retrace time is obtained by activating the capacitor 14 in the first phase of retrace with a diode 42, and eliminating it in the second phase with the diode
38. The damping resistor 16 is similarly inactivated during the first phase of retrace which starts when the transistor 20 is cut off and the yoke current decreases quickly (Fig. 2b), generating a sinewave-like negative voltage pulse across the inductor 10 (Fig. 2c). The duration time Tfp = Pi Square Root L(10) x (C(12) + C(14)) and amplitude of this flyback pulse and the value of the yoke current at the time Tip are related and depend on the values of the capacitors 12 and 14 and the resistor 16. The value of the resistor 16 is chosen so that the inductor 10 in parallel with the capacitor 12 defines a second order system (wherein the capacitor 14 is inactive), which is critically d...