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I have 4 op-amps in cascade with a gain of 10 each to give a total gain of 10000. I noticed the circuit produces a DC offset which then gets amplified causing the amplified signal to clip asymmetrically.

I know op-amps have an offset pin but I have never used it before and I am looking for the simplest solution. I thought I might simply add a capacitor after each op-amp but I don't know how large it should be or if it would help. The signal I am amplifying is 40 kHz.

What is my best option?

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  • \$\begingroup\$ Some but not all op-amps have an offset pin. The dual DIP8/SO8 (e.g NE5532 and what not) normally don't have it because there's no room for it. \$\endgroup\$ – Fizz Sep 22 '15 at 19:20
  • \$\begingroup\$ Yeah just AC couple between stages with a cap and resistor to ground. Maybe 1 uF and 100 k ohm. (time constant =0.1s, freq ~100 Hz) You can pick the corner frequency of the high pass filter to fit your application. \$\endgroup\$ – George Herold Sep 22 '15 at 19:20
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    \$\begingroup\$ What op-amp are you using and what bandwidth is your signal? \$\endgroup\$ – Andy aka Sep 22 '15 at 20:14
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Of course you can try to compensate for the DC offset but why not eliminate the influence of DC offset in the first place ? You can do this if you do not need 10000 times gain at DC. You mention that your signal is 40 kHz, I conclude from that the DC value is irrelevant to you. Then I would just make amplifiers that have 10 x gain at 40 kHz but 1 x gain at DC !

Here's an example of how I would do that:

schematic

simulate this circuit – Schematic created using CircuitLab

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The equation for the capacitor is rather simple. The capacitor will interfere with frequencies lower than \$f = {1 \over {2\pi RC}}\$ where R is the input impedance of the next stage. Simply substitute the desired minimum frequency and impedance, solve for capacitance, and pick a larger value.

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  • \$\begingroup\$ @Ace: Also don't just add caps. Once your amp stages are AC-coupled you can run into funny problems if there's no DC path for the input bias currents. \$\endgroup\$ – Fizz Sep 22 '15 at 19:31
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If you are stuck with being only-able to replace the op-amps then....

Just use much better op-amps. You can get zero drift op-amps that have less than 10uV offset voltage - altogether that's 11.11mV at the output with regular DC coupling and all the offsets pointing the same way. 40kHz is a breeze for a lot of them - gain-bandwidth-product required is minimum 400kHz and this is also easily achievable. Check out TI, ADI and LT for offerings.

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