Description:
Expandable NTSC Video Distribution Amplifier
Many times, such as in the case
of retail displays or video presentations, a single video source is
required to drive multiple monitors. To maintain the NTSC signals
required 1 Vp-p at 75 ohms, a distribution amplifier becomes necessary.
Commercial versions of this are available but are quite expensive. This
design accomplishes the same goal using a handful of inexpensive and
readily available parts. The NTSC input is terminated via R17 to 75
ohms. Potentiometer VR1 provides output level adjustment. Amplifier
stages consisting of Q1 and Q2 provide buffering and voltage
amplification. This signal is fed to Q3, which provides a non-inverting
signal to all output stages consisting of Q4 through Q6. Up to ten of
output stages can be added to provided as many outputs as necessary. VR1
can then be adjusted to provide the proper level to the monitors. The
output stages are configured as emitter-follower amplifiers which
maintain low output impedance. The output resistors R4, 7, and15
establish a 75 ohm transmission line when used with RG type video cable.
In addition, an open or shorted condition on any output will not affect
operation of the other outputs. A 12-volt DC source, which should be
well filtered to eliminate noise in the video outputs, powers the
circuit. Layout is not critical, breadboard construction if fine. As
always, proper grounding techniques should be employed.
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