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I am building a simple op amp amplifier circuit using IC LF351. I m using this circuit at very low voltages about 100mV. however the gain does not remain the same at this voltage. Like the theoretical gain is 4.7 but at 300mV it shows a gain of 3. Why does this happen? How can i figure out the working input voltage range?

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You don't even say if it's an inverting or non-inverting amp. Schematic!! And don't forget to indicate the power supply. – stevenvh May 18 '12 at 11:33
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Without a schematic this is no question at all. It is difficult to tell what is being asked here. – Olin Lathrop May 18 '12 at 12:52
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I hope that by "operating this circuit at very low voltages of about 100mV" you are not referring to the supply voltage!!! That IC is not going to work on anything less than -5 to +5. About the gain changing; maybe you have some bias issue and/or are not measuring from the correct reference. Say we have gain 10 and we double the input voltage from 0.2 to 0.4V and the output voltage steps from 2 to 4. But if we measure that from, say, -1, it looks like 3 to 5. Then it looks as if the gain changed from 15 to 12.5. Plot a few pairs of in/out voltages and see if you get a straight line. – Kaz May 18 '12 at 22:14

closed as not a real question by Olin Lathrop, stevenvh, The Photon, Madmanguruman, Brian Carlton May 18 '12 at 16:30

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.

1 Answer

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You definitely have to show a schematic if you want to be helped properly.

But in the mean time I can make a guess. Are you using a single supply for the LF351? If so, the input voltage is too low. The LF351 is not a rail-to-rail input opamp. The datasheet shows a minimum \$\pm\$11V input range for a \$\pm\$15V supply, that's 4V from the rails!

If this is indeed the problem you may want to swap the LF351 for an LM7341. This is also a \$\pm\$15V opamp, but RRIO (Rail-to-Rail I/O). (Though if you use it with a single supply you probably won't need the wide power supply range.)

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If, with Vout=0.9 V (=Vin·G=(300 mV)·3), the output saturates, the design of the operating point worries me much more than the opamp used. – Telaclavo May 18 '12 at 12:57
There may also be an issue with offset esp if you assume 0 in gives 0 out in your calculations. – russ_hensel May 18 '12 at 15:38
@russ - probably, but without a schematic you can only speculate. The LF351 has an internally trimmed input offset, and additionally can be externally trimmed. – stevenvh May 18 '12 at 15:43

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