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Video amplifier

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  • Nicolae Sfetcu
    Video amplifier From: iisakkil@gamma.hut.fi (Mika Iisakkila) Got some questions about video amps. I ve seen an NE592 used as a video buffer amp at the end of
    Message 1 of 2 , Nov 14, 2001

      Video amplifier

      From: iisakkil@... (Mika Iisakkila)

      " Got some questions about video amps. I've seen an NE592 used as a video buffer amp at the end of a 75 ohm line. Used so that the 75 ohm line could drive all kinds of neat processing stuff without affecting the signal (that's what a buffer is after all, right?) Now National Semiconductor makes an LM592 that's also a video amp. Do these two chips cross reference to eachother? "

      They are the same chip. Sources for NE/SE/LM/uA592 include TI, Harris, Philips (Signetics) and Motorola. Be aware that there are 8 and 14 pin versions of it, the difference being that the larger package has two additional gain control pins. It's not really an op-amp, so you can't use feedback to control the gain. Additionally, they're _fast_ circuits, so use a ground plane and ceramic bypassing caps as close as possible to the supply pins.

      " Also, is there a relatively simple video buffer amp I could make with discrete components? I really don't want capacitive coupling, since video has DC components. "

      The DC components in video are normally a non-issue. Most video equipment are AC coupled (at least the input), which is the reason why you can't get away without black level clamping if you plan to process the video signal. Nothing is said about the actual voltage levels of the video signal, they are just referenced to the black level which may float anywhere (well if I remember right, you're guaranteed to have less than 1W power dissipation in the terminating resistor with standard video...). A typical video input has a 75 ohm terminating resistor to ground and then the signal is fed to the input buffer via a ~50uF electrolytic cap.

      Anyway, here's a simple discrete video output stage. Can't get much simpler than this. Note that there's a serial matching resistor on the output, so you'll have to feed 2Vp-p video into the buffer to get the usual 1Vp-p into the equipment you're driving. This is the way it's usually done. Sorry for the crude transistors, but I hate doing ASCII graphics.

                   o +5v
               |   |
            1k R   |
               |   |c  BC108B
      2Vpp     |   |e      75ohm
      video    |e  +--------R-------> video out
      in >---b     |                  1Vpp @ 75 ohm
               |c  R 1k          +-->
         BC178B|   |             |
               +---+            ---
                   o -5v (yes, two-sided power supply, not ground)

      And while I'm at it, here's the input stage to go with it. It provides the 2x voltage gain you need to feed the output buffer above.

                                     o +5v
                    |    |           |
                3k8 R    R 680R      R 56R
                    |    |           |
                    |    |           |e BC178B
      Video in      |    +---------b
      1Vpp/75R   +  |    |c          |c    100n plastic
      >---+----||---+--b BC108B +-----||--------> to black level clamping
      | 47u | |e | 2Vp-p | | | R 220R R 75R R +-----------+ | |1k8 R 150R --- --- | --- (single power supply this time)

      The simplest black level clamp consists of a signal diode (1N4148) reverse-biased to ground from the output line of the input buffer above and a 4k7 resistor in parallel with it. That forces the sync tips to be at (gnd - threshold voltage of the diode), which shifts the black level of a 2x amplified video reasonably close to ground. Add that and you can connect the two circuits above together and see how they work. They should be very good as far as the signal quality goes (maybe not broadcast quality, but no visible signal degradation). Don't forget good power supply bypassing, use at least 220u of electrolytics and 100n ceramic caps near the transistors on both circuits (the output stage needs them on _both_ supplies).
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