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Does anyone know what's the input offset voltage (oscillated 2.5V) to the op-amp in the circuit for? From my understanding, the input offset voltage needs to be extremely small so that output DC offset can be reduced. But for this circuit, the input offset voltage is big, therefore, it is making the output offset voltage high. Hope someone can help me!

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    \$\begingroup\$ think about this ..... what should the output be when there is no input? \$\endgroup\$ – jsotola Nov 23 '18 at 9:14
  • \$\begingroup\$ Do you mean R3 and R4? \$\endgroup\$ – JRE Nov 23 '18 at 9:30
  • \$\begingroup\$ If you could explain what's R3 and R4 for, that would be great too! \$\endgroup\$ – Taka Nov 23 '18 at 9:50
  • \$\begingroup\$ Like my question would be how can the voltage oscillate from the two resistors? \$\endgroup\$ – Taka Nov 23 '18 at 9:59
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Can you explain how DC offset at the output is an issue, because I don't think it is DC offset nor that it is an issue.

This is a microphone amplifier, microphones do not work for DC. There are even capacitors (C1 and c3) present to block the DC.

You can do the same at the output: simply connect a capacitor (of 22 uF for example) in series with the output: hey presto, your DC offset is gone.

This circuit uses a single 5 V supply. To do so the inputs of the opamp need a proper "working point" and that is set at 2.5 V by R3 and R4. The consequence of this (the single supply) is that the output will also have a constant 2.5 V DC voltage. But as mentioned, for audio signals this is a non-issue as a simple AC coupling (capacitor in series) eliminates that DC voltage.

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  • \$\begingroup\$ Thanks for comment Bimpelrekkie! I totally understand your point! But by any chance do you know what's the input offset voltage to the op-amp in the circuit for? \$\endgroup\$ – Taka Nov 23 '18 at 9:19
  • \$\begingroup\$ That 2.5 V "working point" voltage is not an offset, it is more a "bias voltage". Offset is the word we use to describe (small) non-intentional DC voltages due to an opamp not being ideal for example. \$\endgroup\$ – Bimpelrekkie Nov 23 '18 at 9:49

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