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Suppose I have a digital signal, or at least a signal that is gnd referenced. Like a NTSC video signal for instance like this:

enter image description here

Then I have a single supply opamp, and I want to amplify this by 2x say. I think that if I just input this to a simple opamp circuit with 2x gain that my signal will clip because it will be amplified around my virtual GND.

So then instead I have to add a DC offset probably equal to my virtual gnd voltage so I can amplify without clipping? I think I understand that to be true but maybe my understanding is wrong.

Then my last question is after I've done this and amplified the signal the output is going to be centered around my virtual gnd voltage. So how do I get my signal back to DC (assuming my receiver is not AC coupled and he expects levels referenced to 0V DC). I can't just subtract the DC offset, since my signal amplitude will vary I don't know the right amount to subtract.

I've read a little bit about DC restoration circuits, but I'm not sure if I'm on the right path or not.

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    \$\begingroup\$ If you use an op-amp with rail-to-rail input, you should be able to just use ground as the reference level. The sync level might not be precisely preserved but I expect that isn't critical to reproducing the image. \$\endgroup\$ – The Photon Mar 12 '15 at 16:52
  • \$\begingroup\$ If I am not understanding this incorrectly, one idea might be use a feedback with a Clamper circuit. \$\endgroup\$ – Sherby Mar 12 '15 at 16:56
  • \$\begingroup\$ @ThePhoton Ah so I didn't understand this about rail to rail opamps, but I'm learning. I just did a sim with a TI OPA344 and a sin wave and that did work. \$\endgroup\$ – confused Mar 12 '15 at 17:51
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Your signal is all positive, so you can amplify it around 0 to get what you want.

The issue is that not all opamps work with their inputs at 0, or can drive their output to 0. What you want is a rail to rail opamp, or at least one where the common mode range extends to the negative rail, and the output range also extends close to the negative rail.

Onother issue is the bandwidth of video. The opamp would need to have probably 20 MHz gain-bandwidth to be able to amplify reliably by 2, but more would be better. There are opamps specifically intended for video use.

If you do have to shift the DC level, you can get it back. What you show is actually biased up so that the sync tips are at 0. A "normal" composite video signal has negative sync tips with the front porch being 0. This is because the size of the sync tips can vary and are not guaranteed relative to the picture information. However, the front porch is specifically the black level.

Consider that TV signals received over the air don't have any inherent DC bias. The waveform was designed so that relatively simple analog circuitry could fix it's DC level after reception. This is done by detecting the sync tips, then clamping the waveform to 0 a short time after the trailing edge of the sync tip. This is called a DC restorer circuit, and you may be able to find ICs that do this for you. Back when composite video was more common, there were easily available ICs for detecting and stripping out vertical and horizontal sync, and DC restoring.

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