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I am using a ua741 op-amp. My question is, how can I use a potentiometer to null the output offset voltage?

I believe if I'm correct, that output offset voltage means the output voltage provided by the op-amp even though no input voltage is connected. But I don't get how you can use a resistor to null the offset voltage.

Richard
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    STOP USING A CRAP 741 OP AMP! Seriously, that part is completely old and dead. – Connor Wolf Feb 15 '13 at 04:32
  • @ConnorWolf The 741, because of its lower part count and bigger transistors, reduces thermal and shot noise. The traditional current mirrors also cause the peaks to distort more softly, resulting in tighter midrange and higher fidelity bass in audio applications. Of course, it's not as good as tubes. – Phil Frost Feb 15 '13 at 16:12

1 Answers1

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In an ideal case, if both inputs of an uA741 were connected to Ground, the output should be at precisely ground potential. However, due to practical constraints in actual behavior of the IC, this does not happen, the output typically is "offset" by some voltage, either positive or negative relative to Ground.

This output offset voltage is the voltage seen at its output, when the inputs are both connected to ground (not "no input voltage is connected"): It is important to keep in mind that a dual-supply op-amp like the uA741 operates on a dual power supply, +V and -V, and ground is midway between these.

The offset null pins allow for nulling this offset. This is thus an adjustment input mechanism provided by the IC designers: Moving the wiper of the potentiometer changes the ratio of currents from each of the offset-null pins to -Vdd. This ratio is used to inject a corresponding adjustment to the output voltage.

Specific to the question, thus: It is not resistors that are used to directly null any offset, but a potentiometer or a pair of resistors, to set a ratio of currents that works as internal input for the required adjustment.

Also, see another question here, What is “offset null” in IC 741?, for a good explanation approaching the same question from another side.

Community
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Anindo Ghosh
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