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What is Offset null in 1st and 5th pin in IC 741 (Op-Amp)? Why it is used, though it is not used in many circuits? Give me explanation regarding the offset null! Why offset voltage was formed in IC 741?

pinout diagram

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What they all say BUT do not use a 741. Use an LM324 or some even better more modern opamp. Offset voltages occur because the manufacturing processes are not perfect and there are small differences which combine to look like a single input voltage rror. They may be distributed through the IC but can be represented by and corrected by an offset voltage. –  Russell McMahon Jan 23 '13 at 14:51
    
I think that the question is a bit upside down. It'd be more insightful, I think, to ask why there would be no offset voltage. Once you pose the question that way, you can realize that perhaps physical objects can't ever be perfect, so it'd be rather unexpected to assume that one particular physical object - an operating amplifier - would be somehow exempt from the rules that affect everything else in Nature. As to why exactly an op-amp has input offset: the input circuitry isn't perfectly symmetric, in various ways. –  Kuba Ober Feb 6 at 14:37

3 Answers 3

up vote 14 down vote accepted

The datasheet gives an example.

offset null circuit

By adjusting the pot we can null any offset error. An offset error is when the inputs are exactly equal but the output isn't exactly zero. This error is also characterized by the datasheet:

electrical characteristics

It can be safely ignored in AC applications, where this offset will be ignored by the AC coupling. It becomes more important in DC applications, especially amplifiers, since this DC error will be amplified by the next stage.

This offset voltage exists because a real omp-amp can't be ideal. There will always be some unintended asymmetries between due to random variation in manufacturing. In all cases, there are op-amp designs that can minimize these errors, but usually at the expense of some other parameter, like cost.

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why off set voltage is produced in IC 741 ? –  AbiRAM02 Jan 23 '13 at 13:01
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@AbiRAM02 because nothing is perfect. An ideal op-amp has no offset because its gain is infinite, there are no leakage currents, and there's no undesired impedance anywhere. Any real op-amp will necessarily have these things, the 741 being no exception. –  Phil Frost Jan 23 '13 at 13:09
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@AbiRAM02 that question degrades to, "can you please explain to me everything about op-amps", but see edits for a partial answer. –  Phil Frost Jan 23 '13 at 13:23
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Just to clarify, it's not correct that an op-amp with infinite gain would as a result have no offset voltage; non-infinite gain and non-zero offset are independent deviations from the ideal op-amp model. An op-amp with non-zero offset and infinite gain, in a negative feedback configuration, will simply drive it's output to the point that the two inputs differ by the offset voltage, which is likely to cause an error at the output relative to the desired output. –  The Photon Feb 15 '13 at 3:55
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@ThePhoton: To clarify further, an infinite gain op amp may have input offset, output offset, or both. If an op amp has an input offset of X and output offset Y, the output will be voltage Y away from the point where the inputs would be voltage X away from each other. –  supercat Sep 11 '13 at 18:13

For circuits that accuracy and/or symmetry is critical, the offset null pins provide a means to cancel "internal" discrepancies. The best example I can give you is the old ohmmeter. You would short the leads and adjust the needle to read zero, then you knew your resistance readings would be as accurate as possible. With the 741, you provide the same voltage (signal) to the input pins and adjust the offset null to make sure the output is zero.

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as ics are manufactured with cascade amplifiers,a smal voltage is enough for its workng n as semiconductors are used the room temperature is enough to get extra voltage so while operating using opamp,offset nul is connected so as to make these voltages zero

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Correct. Anything new to add? –  Ignacio Vazquez-Abrams Sep 21 '13 at 19:27

protected by Dave Tweed Mar 1 at 13:05

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