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I'm trying to build this simple op-amp voltage follower circuit

enter image description here

But I keep getting distortions on the node out. I'm using a single voltage supply and the sine wave I send in is between +/-650mv, from ground reference that is. The distortions consists of noise like patterns on the raising an falling parts of the sine wave, while the top and bottom is okay. Any ideas of what it can be?

http://s18.postimage.org/u1gpbd6fb/DSC01219.jpg

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  • \$\begingroup\$ Instability seems likely. Add more decoupling (like 10uf) on the PSU. Also : what are you loading the output with? is it a capacitive load? \$\endgroup\$ – Brian Drummond Feb 12 '13 at 23:46
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    \$\begingroup\$ Post an oscilloscope screenshot, if you can. \$\endgroup\$ – Nick Alexeev Feb 13 '13 at 0:00
  • \$\begingroup\$ I could not login to my account before now, hence my reply came from another account as an answer. Sorry for that! I will provide you with the screenshot soon. \$\endgroup\$ – Red Feb 13 '13 at 11:20
  • \$\begingroup\$ Btw, thanks Nick for posting my circuit image in my question. :) \$\endgroup\$ – Red Feb 13 '13 at 11:22
  • \$\begingroup\$ Here is a picture of the curve (!Valid XHTML.). When I don't have any input, only feeding with supply voltage I get a sinus wave of about 40MHz. The same wave that gives the distortion of my signal seen in the image. \$\endgroup\$ – Red Feb 13 '13 at 18:30
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This certainly sounds like instability. Is that opamp "unity gain stable". If not, that's the problem, and that's likely what is happening. If so, you have to use a different opamp if you really need unity gain.

Depending on the opamp and what you really need this circuit to do, you could try giving it some gain. To do that, put a resistor between the output and the - input, then a resistor and capacitor in series from there to ground. The resistor and capacitor will form a high pass filter. Your input coupling already high pass filters with a rolloff of about 1.8 MHz, so you can set the other filter a octave or two below that so as to not matter.

Added in response to datasheet:

You realize this is a video amp, right? You might have mentioned that earlier. High speed amps like this tend to be a finnicky, and you have to pay special attention to layout. You want a nice ground plane, careful decoupling, etc. This is a case where you really need a proper PC board or use RF breadbording techniques, no solderless plug-in breadboard.

The datasheet suggests that this amp should be stable at unity gain, but doesn't come out and actually say so that I found from a quick skim. You would normally use this kind of amp with a gain of 2 at least, like they do in the example on page 13. At these frequencies, you have to consider transmission line effects, so you are usually down by a factor of two due to driving thru a resistor, which then forms a voltage divider with the characteristic impedance. They aren't really expecting anyone to use this amp at a gain of 1.

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    \$\begingroup\$ This is the op-amp I'm using: www1.elfa.se/data1/wwwroot/assets/datasheets/… . I'm going to try givign it some gain and see what happens. \$\endgroup\$ – user18943 Feb 13 '13 at 8:06
  • \$\begingroup\$ Okay, thanks for the input. I tried using it at a gain of 2 with two 1.2K resistors. What happened was that the output signal was exactly the same as the input, without any gain or the distortions that were present earlier. I tried to change the gain to more than 2, but the out signal was still identical with the input. Is there something obvious I'm missing or is the op amp broken? It feels like the signal goes right through it. \$\endgroup\$ – Red Feb 13 '13 at 18:27
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    \$\begingroup\$ @user: This is starting to sound like something isn't hooked up the way you think it is. Go thru the datasheet and check every pin, making sure you are looking at the picture of the right package. Of course don't load the output with anything until this is working. I have been assuming the output was unloaded. \$\endgroup\$ – Olin Lathrop Feb 13 '13 at 19:46
  • \$\begingroup\$ Is there anyway I can test the op amp with an even simpler circuit? Because I'm quite sure I've connected the right pins. I've gone through the circuit a couple of times to ensure that. However, I'm just using one part of my op amp, pins 4,5,6,7,8 that is. Thats okay right? \$\endgroup\$ – Red Feb 13 '13 at 20:02
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    \$\begingroup\$ @user: No, it's not if you are leaving the other part floating. Make sure the other part isn't slamming back and forth randomly causing noise. For unity gain stable amps, tying the output back to the - input can be good enough. Or make sure it is always pegged one way or another, like 1 MOhm to ground on one input and 1 MOhm to supply on the other. There are lots of ways. \$\endgroup\$ – Olin Lathrop Feb 13 '13 at 20:51

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