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  Video Stages

Introduction to Video Stages:

This stage is used among Tunes and Video detector stages. Video IF (38.9 MHz) and sound IF (33.AMHz) signals are supply like the input for this stage. Usually three stage IF amplifiers are employed. Thus, AGC voltage to first IF stage and the complete gain is controlled. This stage is considered like a transformer coupled amplifier. Wave trap circuits are employed in the input of this stage.

FUNCTIONS:

1. It amplifies video IF signal to a wanted level.

2. But sound IF signal is amplified to a5% of highest gain.

3. This type raises the selectivity and sensitivity of the TV receiver.

Basic video IF amplifier circuit is displayed in the diagram, required signals are selected and the not wanted signals are discarded through the wave trap circuit. The input signals are delivered by shielded cable to prevent stray capacitance. These amplifiers employs stagger tuning. In this technique, three coils are tuned to dissimilar response. The amplified IF signal is delivered to the video detector stage.

VIDEO DETECTOR:

1544_Video Stages Homework Help 1.jpg

The major task of this circuit is to filter the carrier waves from the video IF signal and divides the composite video signal. Diode D is connected in series with the secondary of last IFT C, L, C2  L2 C, i created like two II filters.

CIRCUIT OPERATION

Diode D conducts just throughout the negative half cycle of the video IF signals from the secondary of the last IFT. The carrier waves in the negative half cycle filtered by the two n filter circuits. So, composite video signal is gets separated and its voltage is approximately 2 V (p-p).

Other function is also performed in the diode D. i.e. video carrier wave (38.9MHz) and sound carrier wave (33.4MHz) is mixed here. When mixing occur the output is acquired as the variation between the two- that is 5.5MHz as intermarried sound trap circuit and delivered to the sound IF amplifier.

VIDEO AMPLIFIER

It consists of a device and an output amplifier.

FUNCTIONS

1. Video signal is amplified to a highest level and about 80V (p-p) level signal is delivered to the cathode of picture tube.

2. It turns off throughout the retrace periods to blank retrace lines

There are 4 methods of coupling a video amplifier to the cathode of picture tube.

The four methods expressed as follows:

1. Direct coupling

2. Partial dc coupling

3. AC coupling

4. AC coupling with dc retraction

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