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High Speed Low Power Deflection Amplifier

IP.com Disclosure Number: IPCOM000078427D
Original Publication Date: 1973-Jan-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Bogdan, CA: AUTHOR

Abstract

A random positioning cathode-ray tube (CRT) display requires small linear changes in deflection current for character writing, along with large changes in current for positioning. The time it takes to position the beam to write a character or symbol is considered to be wasted time. The present circuit minimizes this position time without a substantial increase in power dissipation. The time saving allows for a larger number of random characters and symbols to be displayed during one refreshed cycle. This is particularly important when trying to display enough meaningful data during the vertical retrace time of a raster scan, where again the time is limited.

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High Speed Low Power Deflection Amplifier

A random positioning cathode-ray tube (CRT) display requires small linear changes in deflection current for character writing, along with large changes in current for positioning. The time it takes to position the beam to write a character or symbol is considered to be wasted time. The present circuit minimizes this position time without a substantial increase in power dissipation. The time saving allows for a larger number of random characters and symbols to be displayed during one refreshed cycle. This is particularly important when trying to display enough meaningful data during the vertical retrace time of a raster scan, where again the time is limited.

The present deflection amplifier 10 provides this minimized position time, while being able to maintain any position on the CRT indefinitely.

The deflection amplifier 10 includes a preamplifier 1 with plus and minus voltage clamps 2, 3, a power amplifier 4, and a deflection yoke 5, connected as a feedback amplifier. In the normal mode of operation, negative feedback is used to maintain the accuracy and linearity of the deflection system and is obtained by sampling the yoke current. The deflection amplifier converts the input voltage waveform to an equivalent current through the yoke for a ramp or small signal input.

The integrated circuit operational amplifier 6 in the front end of the preamplifier 1 provides good common mode rejection on the inputs and power supplies. Local feedback from the emitter of transistor Q1 provides a stable open-loop gain for preamplifier 1. Additional gain is provided by transistors Q1, Q2, and Q3. Transistors Q2 and Q3 are substantially constant-current sources which feed the load resistor R17 and are controlled by transistor Q1.

Q2, Q3 prevent the transistors in the preamplifier from going into saturation storage and allow the output to be independent of power supply variation. In addition, Q2, Q3 allow an output signal swing which is approximately equal to the supply voltages, when the plus and minus voltage clamps 2, 3 are turned off. Voltage clamps 2, 3 are normally turned on in this linear operation and limit the output swing to +/- 20 volts. Transistors Q4 and Q5 serve as an input buffer to power amplifier 4.

Power amplifier 4 is a class AB amplifier connected in a compound NPN and PNP complementary connection. Both transistors Q8, Q9 in the output stage operate at low current, e.g. 10 to 20 mA, with the yoke current set at zero. This prevents both transistors Q8, Q9 from being cut off when the input signal goes through zero. The complementary connection provides a simple method of obtaining a push-pull output. For example, when a positive signal is applied to the inpu...