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I am using an OPA241PA op amp to amplify a variety of voltages (around 0.6-3.3V input to about 4-23V output). If I have understood the datasheet correctly, this amp should be capable of accomplishing my goal. The schematic is as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

When 3.3V is input, the amp behaves as I believe it should and the output is about 23V. However, when the input is 0.9V, the output is around 15-16V; a gain of almost triple the intended. How is it possible that my gain changes when my input voltage changes?

EDIT: I have since found that when the 0.9V input is supplied, there is ~2.6V developed on the negative terminal of the amp. I don't know how this voltage could be present.

Link to datasheet http://www.ti.com/lit/ds/symlink/opa251.pdf


Thanks for the input, I've concluded that the problem is with my source, not the op amp circuit. Once the source was replaced the amp worked correctly.

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  • \$\begingroup\$ The simulation (from the link you have provided) gives me correct results. The output voltage at 0.9V is approximately 6.7V. I have varied the input voltage from 0.6 to 3.3V with a step size of 0.1V. Can you please share your test conditions. Or are you asking about a practical scenario?. \$\endgroup\$ – Jinu Jun 20 '16 at 11:24
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    \$\begingroup\$ Maybe you have one of the resistor values wrong and what you see when you apply 3.3V is the op-amp end-stopping against the positive rail (23V actual and not 24V). \$\endgroup\$ – Andy aka Jun 20 '16 at 11:27
  • \$\begingroup\$ What is the output actually driving? Do you have a load there at all? Does sticking a 10k or 1k resistor to ground as a dummy load change anything? \$\endgroup\$ – pjc50 Jun 20 '16 at 12:32
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    \$\begingroup\$ First, best guess. You have not connected the power supply ground to the signal ground. You may think you have, but you haven't. All 3 of your schematic ground points must be physically tied together. Just because the power supply - has a ground symbol on the chassis, and the signal generator the same, they are not connected until you connect them. \$\endgroup\$ – WhatRoughBeast Jun 20 '16 at 13:24
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You didn't supply a link to the datasheet, so this is only a guess.

Check the common mode input range of the opamp and see if it includes 600 mV. If not, then what you see should be no surprise.

Check the supply voltage range. Can it really handle 24 V.

Another possibility is that this particular opamp is broken. Probably not, but trying a second one is a good idea if you've checked the above two points and everything is within spec.

You should also have a bypass cap across the opamp power pins.

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