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What is offset null 1 and offset null 2 in op-amp?

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wajahat

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hi everyone i am using op-amp 741 .I want to know what is this off set null 1 and 2 in the figure attached.
Any response in this regard would be helpful.
wajahat
 

I'm not entirely sure, but I think you should find a good explanation in the Sedra/Smith book.

If I remember correctly, you can use those two nodes to nullify the offset that you see at the output due to mismatches in the differential circuit. The output should be midway between VDD+ and VCC+.
You're supposed to insert a variable resistance between N1 and N2 and keep tuning it until Vout = 0V (if VDD = -VCC).
 

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    wajahat

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in my opinion, it's used to obtain a low input offset.
maybe, you can apply an external resistor shunt with the emitter resistor to achieve it.
 
liuyonggen_1 said:
in my opinion, it's used to obtain a low input offset.
maybe, you can apply an external resistor shunt with the emitter resistor to achieve it.
Click to expand...

You're right, adding a resistance between N1 and N2 is liking modifying the emitter resistances. You keep playing with the value of the external resistance until the output is balanced and minimize any offset at the output (which should be really small when referred back to the input).
 

You would use a trimpot with center tap to the negative
rail, to introduce your desired imbalance to the front end.
The higher the impedance, the finer / narrower the trim
range.
 

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