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I am working on an audio amplifier for fun. The OP-AMP I chose is NE5534. I think I figured out most of the circuit, but the pin 5 and 8 confuses me.

Pin 5 is "External offset voltage adjustment" pin (output) Pin 8 is "External offset voltage adjustment" pin (input)

I haven't had the chance to understand this from the datasheet

It seems optional, so I am tempted to leave unconnected. However, perhaps I should break out the pin to a throughhole or a pad.

Since I am unsure what kind of capacitor/resistor is required, I don't know if I can use a generic 0804 type place holder for it.

Can someone please enlighten me?

[update] Ok, I found the description for the two pins:

The input offset voltage of operational amplifiers (op amps) arises from unavoidable mismatches in the differential input stage of the op-amp circuit caused by mismatched transistor pairs, collector currents, currentgain betas (β), collector or emitter resistors, and so on. The input offset pins allow the designer to adjust for these mismatches by external circuitry. These input mismatches can be adjusted by putting resistors or a potentiometer between the inputs as shown in Figure 10. A potentiometer can be used to fine tune the circuit during testing or for applications which require precision offset control. More information about designing using the input-offset pins, see Offset Voltage of Operational Amplifiers (SLOA045). enter image description here

After reading the description, I must admit I am lost. Can you please explain what "mismatches in the differential input stage" is?

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  • \$\begingroup\$ note that for most of the applications it is not necessary to make an ofset compensation. \$\endgroup\$ – LvW Aug 13 '15 at 8:15
  • \$\begingroup\$ If 5 or 10mV of DC offset doesn't matter, and you don't have high gain at DC, you can usually ignore the offset adjustment. \$\endgroup\$ – Brian Drummond Aug 13 '15 at 10:03
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Can you please explain what "mismatches in the differential input stage" is?

All op-amps have an input voltage error term. It's just like a small millivolt battery being placed in series with one of the inputs. If you build an amplifier with a gain of ten, the output voltage from the op-amp becomes offset from where you would expect it to be by 10 x the mV offset I just mentioned.

It is specifically this static error that the offset-voltage nulling circuit intends to eradicate. But, also remember that the offset voltage that is trying to be nulled, will drift with temperature (and time) and these drifting effects cannot be "cancelled" without re-positioning the potentiometer in your circuit.

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  • \$\begingroup\$ Great, thank you for your answer. It was easy enough for me to understand. So one way to find the value for this offset-voltage would be ground both inputs, and measure the amplified output? \$\endgroup\$ – Adam Lee Aug 13 '15 at 8:36
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    \$\begingroup\$ It's never as easy as you think - build resistors around your op-amp to produce a gain of (say) 100 and tie the input to 0V then measure the output. You can use an inverting or non-inverting amplifier configuration. \$\endgroup\$ – Andy aka Aug 13 '15 at 8:40

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