Browse Prior Art Database

Video Amplifier for CRT Display

IP.com Disclosure Number: IPCOM000034524D
Original Publication Date: 1989-Mar-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 3 page(s) / 70K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR

Abstract

CRT displays require an analog amplifier having high bandwidth, DC controlled gain, good linearity and low component count. A video amplifier with these properties is disclosed using an available NPN transistor module with accurately matched transistor pairs. It provides 45 V or more video output up to video clock rates in excess of 50 MHz, making it suitable for monochrome or colour displays. Fig. 1 shows the basic video amplifier circuit using transistors in an IC array module type CA3046. The video signal is terminated at the input by R1 and AC coupled into the base of the emitter follower Q1. Transistor Q2 is turned on by a flyback pulse every cycle to clamp the voltage across C1.

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Video Amplifier for CRT Display

CRT displays require an analog amplifier having high bandwidth, DC controlled gain, good linearity and low component count. A video amplifier with these properties is disclosed using an available NPN transistor module with accurately matched transistor pairs. It provides 45 V or more video output up to video clock rates in excess of 50 MHz, making it suitable for monochrome or colour displays. Fig. 1 shows the basic video amplifier circuit using transistors in an IC array module type CA3046. The video signal is terminated at the input by R1 and AC coupled into the base of the emitter follower Q1. Transistor Q2 is turned on by a flyback pulse every cycle to clamp the voltage across C1. As the only current flowing into C1 is the base current of Q1, which is very small, there is little change in the voltage across C1 and hence no video smearing or droop is caused, as is often the case when DC restored AC clamping is used. A self-test facility is achieved by a resistor R2 and diode D1 network that

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generates a raster signal if the self-test line is unconnected. Otherwise, this is connected to ground, which reverses diode D1 and prevents the injection of the raster signal. The signal at the emitter of Q1 is fed into the base of Q3 via a potential divider R3 and R4 which raises the DC level by about 0.2 V; this is to overcome tolerance differences in Vbe between Q1 and Q3 and, also, to allow a quiescent current to flow in the output stage so that there is good linearity at the output over the entire range of video and contrast levels. As shown in Fig. 1, the amount of quiescent current is preset; this leads to a variable amount of DC offset at the collector of Q7. This is satisfactory for a low voltage swing monochrome amplifier; however, for a larger voltage swing color amplifier the quiescent current can be controlled, if necessary, by using a black level feedback system, as shown in Fig. 2, to control the standing quiescent circuit. This uses a zener diode to monitor the peak voltage at the collector of Q7, and, via negative feedback applied through the spare IC array transistor Q8, alters the amount of DC offset applied to the base of Q3. Q3 forms a current source as in a...