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?
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.
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.)