Video Line Driver

This circuit is a video line driver specifically intended for use with a single-ended power supply. As a matter of fact, the synchronised outputs of a line driver for composite-video signals go negative with respect to ground. In order to be able to process these negative signals in a circuit powered from a single-ended supply, it is necessary to AC-couple the input of the opamp as well as level-shift the signal in the positive direction.

Circuit diagram :

Video Line Driver Circuit Diagram

The input is terminated into a 75 Ω resistor (R1). From here, the signal passes through AC-coupling capacitor C2 and is applied to potential divider R2-R3, which provides the necessary DC-offset. The shift into the positive direction amounts to +1.7 V, with the values shown in the schematic. To avoid any misunderstandings we should add that this value is fairly critical. Deviating from the values shown can lead to distortion in the complementary input stage of the opamp that has been used here, and this of course, has to be avoided.

PCB-Layout :

Video Line Driver PCB-Layout

Because we provided the circuit with its own voltage regulator circuit (IC2), just about any mains adapter will suffice for the power supply. The current consumption is less than 20 mA. The construction of the line driver using the accompanying printed circuit board layout is no more than a simple, routine job.

Parts List :

Resistors:
R1,R7 = 75Ω
R2...R4 = 4kΩ7
R5,R6 = 1kΩ

Capacitors:
C1,C4,C5,C7C10,C12 =
100nF
C2 = 47µF 16V radial
C3,C11 = 10µF 6 V radial
C6 = 220µF 6 V radial
C8 = 1000 µF 6V radial
C9 = 100µF 16V radial

Semiconductors:
D1 = 1N4001
IC1 = OPA353UA
IC2 = 78L05

Miscellaneous:
PC1-PC6 = PCB solder pin
Case, e.g., Hammond type
1590A

Copyright : Elektor