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I'm looking to build an amplifier circuit to amplify a small DC voltage signal (between 0-15mV) to the range of 0-1.3V. I tried messing around with a standard non-inverting op-amp amplifier circuit but have found that the gain is not consistent in this range, it increases as the input voltage increases until the input voltage reaches 20mV then stays steady until the output hits the rail (as expected).

My question is are there any good circuits that can be used to amplify DC voltages with a consistent gain at this level of input. I need to be able to math my way back to the original signal (from a photo-diode) so consistent gain is important. Also this is for a class so I need a circuit I can build with components, not one chip that solves the problem.

Thanks!

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    \$\begingroup\$ Almost without doubt you are not using the op-amp correctly. Do you have a circuit of what you tried? \$\endgroup\$ – Andy aka Mar 24 '13 at 23:12
  • \$\begingroup\$ What model OpAmp did you try? What were the voltages on the OpAmp supply rails? \$\endgroup\$ – Nick Alexeev Mar 24 '13 at 23:12
  • \$\begingroup\$ The circuit is the non-inverting amplifier configuration shown here: en.wikipedia.org/wiki/Operational_amplifier_applications The op-amp model is OPA333 and the op-amp was being driven with 5V on the V+ rail and ground on the V- rail. \$\endgroup\$ – Pat Mar 25 '13 at 0:43
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    \$\begingroup\$ Seeing your complete schematic would be better. For example: what resistor values are you using? Do you have decoupling caps in place? Do you have any bandwidth limitation in the feedback loop? (i.e. a capacitor across Rf) Have you checked the signal on a scope or with a multimeter? \$\endgroup\$ – Oli Glaser Mar 25 '13 at 2:12
  • \$\begingroup\$ Also, when you say you need a circuit you can build with components, do you mean ultimately you don't want to use just one opamp? (do you need to use transistors only or something similar?) \$\endgroup\$ – Oli Glaser Mar 25 '13 at 2:14
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You want a DC gain of 86 for positive voltages relative to ground. Yes, a opamp can do this. However, the common mode range of the opamp must extend to ground if you don't have a negative supply available. Check the datasheet carefully.

Also consider the error from the input offset voltage. With only 15 mV input, 1 mV of offset could be significant. You can get around this if you don't need DC operation, but it sounds like you do.

If you have a way of dealing with the offset downstream, like subtracting out a calibration value measured with a known zero input, then you can add a little more to make sure everything is aways positive after the opamp's offset is added. A voltage divider that moves the input up just a little towards the supply might be all you need.

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  • \$\begingroup\$ I was actually thinking of doing something similar to this so now I will definitely try it. \$\endgroup\$ – Pat Mar 25 '13 at 18:33

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