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Diode Modulator Drive Amplifier

IP.com Disclosure Number: IPCOM000049829D
Original Publication Date: 1982-Jul-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR

Abstract

A conventional amplifier (Fig. 1) for driving a cathode ray tube (CRT) line deflection diode modulator causes high dissipation in the output devices with poor noise immunity. In the novel circuit (Figs. 2 and 3), a resistor in series with the output provides an RC network with the diode modulator capacitor. This prevents the feedback of the high frequency signals derived across the capacitor as the feedback is taken before the resistor. To compensate for phase and amplitude changes of the amplified signals, a compensation network in the feedback loop extends the useful range of the amplifier from 2 kHz to 10 kHz (-3 dB.) This also has the important advantage of reducing high frequency (HF) noise generated at the output being fed back to the input which causes excessive power dissipation.

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Diode Modulator Drive Amplifier

A conventional amplifier (Fig. 1) for driving a cathode ray tube (CRT) line deflection diode modulator causes high dissipation in the output devices with poor noise immunity. In the novel circuit (Figs. 2 and 3), a resistor in series with the output provides an RC network with the diode modulator capacitor. This prevents the feedback of the high frequency signals derived across the capacitor as the feedback is taken before the resistor. To compensate for phase and amplitude changes of the amplified signals, a compensation network in the feedback loop extends the useful range of the amplifier from 2 kHz to 10 kHz (-3 dB.) This also has the important advantage of reducing high frequency (HF) noise generated at the output being fed back to the input which causes excessive power dissipation.

Fig. 1 shows a conventional amplifier for e CRT line deflection circuit but which suffers from poor output noise immunity, limited slew rate, character tilt/skip response, temperature dependance, poor jitter rejection and poor line tearing/EHT tracking.

Many of these problems arise due to the type of load presented to the output stage. The current through the modulation induction L(M) is approximating a ramping current with an average DC offset which is proportional in a non-linear manner to the voltage applied to it. The 25 (mu) f capacitor has been chosen to smooth out the AC component. If the DC offset is plotted as a function of applied voltage, the current has been found by experiment to obey the following law to a good approximation over the voltage range 0 to 40 volts. (see original) where I is the current in L(M), V(DM) is the voltage applied to L(M),

k is a constant of value -0.079,

C is an integration constant of value -0.0073, and

Vo is a constant of value 42 volts.

Beam current has an effect upon the value of k, decreasing it to -0.077 when high beam currents are used.

It has been noticed that where V(DM)(see original) 18 volts, the modulation of the scan current becomes correspondingly non-lineer, making this region useless as a normal operation point.

As has been mentioned a large capacitor is employed to smooth the AC component of the current. A line rate parabola would thus be seen across the capacitor due to the integrated ramping current if it were not for the efforts of the modulation amplifier to keep its output at the level defined by its inputs. Thus the feedback resistor R(EB) feeds any high frequency noise at the output directly to the input of the amplifier. Given enough gain, the amplifier would alternatively sink or supply enough current to render the output capacitor redundant. However this would be very power consuming. To overcome this, the output devices are current limited in the conventional circuit, but this is a disadvantage as this limits the slew rate of the amplifier depending upon the voltage at its

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