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The pre-distorted signal is applied to the input transistor of a differential amplifier comprised of Q702, Q703 and constant current source Q704. The current supplied by 0704 is split almost equally between Q702 and Q703 resulting in similar no signal collector voltages within 5% of the supply level. Very heavy negative feedback is applied to the base of Q703 from across R720, a non- inductive current sensing resistor. The voltage across which supplies an accurate representation of yoke cur- rent and forces the amplifier to correct any distortion pre- sent. The signal present at the collector of Q702 is current amplified by 0705 and this in turn is used to drive Q706 which then drives the output transistors Q708 and Q709 forcing current through the x-winding of the yoke and pro- ducing beam deflection. The output transistors are current driven with the actual driver transistor being Q706 and its constant current source being Q707. D708 is used to hold the bases of Q708 and Q709 separate by 0.6 of a volt. This results in output transistors that are biased class B. Because of push pull arrangement of output transistors, and the |
cancelling effect of such a stage on any common ripple component considerable amount of power supply rippie can be tolerated. Any small amount of crossover distor- tion that may result from class B operation is removed by means of the very heavy negative feedback present. C704 is used to maintain a low impedance path across D708. Q704 is a constant current source with fixed bias voltage maintained by D706 and D707 with R711 allowing current to flow through them from -ve rail to ground. Q707 works exactly the same way as Q704 except R716 has been added to dissipate more power where total cur- rent flowing through both Q707 and R716 is constant. R712 is a suppressing resistor to prevent spontaneous oscillation of Q707. C701 is an H.F. bypass. R721 is a resistor critically selected to damp the yoke and minimize any tendency for ringing. In case of a failure of an output device or an extreme ex- cursion of the electron beam, F700 will open and prevent damage to the yoke or sense resistor as well as remove all loading from Q708 and Q709, thus protecting those from potential failure also. |