I'm limited to a single +5v rail and ground and so could anyone recommend an op amp which could swing atleast 0.3v below its negative rail and that would be able to amplify by atleast 2 (capable of -0.6v output). I will be using this to amplify a video signal which has a negative sync.

Would the LT1006 be able to do this?

Basically I will use the op amp to drive a 75ohm video line. So the + input will be the video line and the negative line will be connected to the output of the op amp

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    \$\begingroup\$ You want it to be able to drive its output below its negative rail? \$\endgroup\$ – The Photon Sep 3 '16 at 5:21
  • \$\begingroup\$ Correct that's what I want \$\endgroup\$ – Umar Mahmood Sep 3 '16 at 5:22
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    \$\begingroup\$ No such thing. You could look into using a charge pump chip to generate a negative supply rail for your op-amp. Or offset the whole output signal by +0.6 V so that it never has to go negative. \$\endgroup\$ – The Photon Sep 3 '16 at 5:24
  • \$\begingroup\$ And where will it get this voltage beyond its rail from? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 3 '16 at 5:24
  • \$\begingroup\$ RE LT1006, notice that even though the front page says "Output Swings to Ground while Sinking Current", the actual specs say the minimum output voltage is +6 mV (typical). \$\endgroup\$ – The Photon Sep 3 '16 at 5:27

Composite video signals are usually AC coupled, so you shouldn't need to pull the output below ground. The receiving end will restore the DC level (using the sync pulse as a reference) if it needs to.

At the input side the signal may go below ground - or not, depending on the source. To cover all possibilities you should terminate the input with 75Ω and AC couple the signal to your op amp. You can then apply whatever bias is required to keep the op amp's output voltage above ground.

If you cannot ensure that the AC coupled signal's peak voltages stay within the op amp's input and output ranges then you may need to restore the DC level. This can be done with a diode, or with a sync detector and transmission gate that clamps the video signal to the desired reference voltage during the sync period.

Here's a simple DC restore circuit using a diode. The second diode and resistor provide 0.6V of positive bias to compensate for the negative voltage across the clamp diode when it is conducting:-


simulate this circuit – Schematic created using CircuitLab

Would the LT1006 be able to do this?

The LT1006 has low bandwidth and limited output drive capability. You should choose an op amp which is designed for video applications, eg. LT6205.

  • \$\begingroup\$ +1(00) for explaining how to fix the problem rather than how to make the suggested idea work. \$\endgroup\$ – JRE Sep 3 '16 at 8:57

There is no op-amp that can drive its output below its negative supply rail.

Alternative approaches include using a charge pump chip to generate a negative supply rail for your op-amp or offsetting the whole output signal by +0.6 V so that it never has to go negative.

Edit: Bruce is quite right to point out that if your signal is DC balanced, there are even simpler ways to deal re-biasing it.


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