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  DC Video Amplifier Construction

Introduction to Direct Coupled Video Amplifier Construction:

It consists of a driver and an output amplifier. Q1 works as a driver amplifier and Q2 like output amplifier. A supply of i 2 Volts is provided by resistors to base and collector of device transistor. A supply of 110 Volts is concerned to the collector of output transistor through a resistor and a coil. The base of it is provided bias supply from 12 Volts supply by variable resistors VR, and VRy VR, and VR, works as a contrast and cub contrast controls correspondingly.

Circuit Operation:

The driver amplifier amplifies the 2V (p-p) video signal to approximately 4V(p-p) and fed it as output. The LG circuit at the emitter works as a trap to block the 5.5MHz below /if to prevent sound bars in diagram.

10_Direct Coupled Video Amplifier Construction.jpg

After that the video signal is applied to the base of Q2 via VR, Q amplifies the signal to maximum and sends that signal to the cathode of the picture tube. Vertical and  Horizontals balancing pulses are delivered to the emitter of Q2 to cut off throughout retrace periods and so retrace lines are not to get better high frequency video signal response.

Peaking coil is employed to acquire better high frequency video signal response.

Beam current limiter is employed here. The brightness of picture tube becomes extremely high while the input video signal is not exist, in direct coupling method. Because of this, the life of picture tube is decreased. To control beam current Diode and 470K Resistor is required.

Contrast control VR1, ascertains the black, white and grey levels of picture and VR1 works as a sub contrast control. Preset is employed like the sub contrast control.

 MERITS:

1. Extremely good high frequency response

2. The black, white and grey levels of picture is kept to a wanted level

3. Protects the picture tube via controlling beam current.

DEMERITS:

1. Regulated EHT supply is required. Because, at height white levels been current get increases and the result of it EHT voltage decreases. Consecutively, balancing takes place. A well regulated EHT supply decreases this effect.

2. Beam current limiter should be employed. Since the picture tube is driven to a high brightness level if the input video signal is not present and it decreases the life of picture tube.

3. Because of the extremely low frequency of the amplifier, a momentary flutter of the regenerate picture is resulted because of the fluctuating RF signal gain. When the passing of aero planes. In spite of a proficient fast acting AGC, this effect can't be eliminated totally.

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