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I have used integrators in the feedback loop of standard inverting and noninverting amplifiers to eliminate DC off-set at the output, the circuit Im talking about is something like this

schematic

simulate this circuit – Schematic created using CircuitLab

I understand the function of the integrator, it samples the DC offset at the output and it starts ramping, the output of the integrator is feedback to the inverting input which gets added to the original offset but with opposite polarity, the integrator stops ramping (and stays there) when the offset at the output is zero.

Thats if we focus on DC, but what happens to AC?, imagine a 1KHz sine is inserted at the input, that means that the output of the integrator will be a cosine which is being fed back to the inverting input and added with the original signal. My question is: isnt this detrimental? I mean adding a cosine signal will affect the original AC signal (by a small amount), am I missing something?

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Yes, it will affect the AC signal by a small amount and that amount halves for every doubling of the AC frequency so, you design an integrator that is intended to remove DC so as to not cause a significant effect on the amplifier’s AC processing gain.

Look at R5 and C3; they form a low pass filter at about 0.2 Hz so, at 0.4 Hz the integrating effect is halved and at 0.8 Hz the effect has quartered. Do you follow where I am going with this?

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  • \$\begingroup\$ Yes, I see what you mean, so adding a dc servo is not completely transparent audio wise, granted that at audible frequencies the effect contribution will be insignificant. \$\endgroup\$ – S.s. Feb 9 '18 at 0:15
  • \$\begingroup\$ @S.s. I think you mean, "adding a dc servo is completely transparent audio wise, granted that at audible frequencies the effect contribution will be insignificant." If the contribution is insignificant, doesn't that mean the effect will also be insignificant? \$\endgroup\$ – WhatRoughBeast Feb 17 '19 at 4:05
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I though you understood this type of servo-loop, which is used on high performance audio amps to avoid the phase shift caused by capacitors. Your time constant appears to be .16 HZ, so any 'event' at or under a .16 HZ rate will be cancelled out by the loop.

Frequencies above .16 HZ are ignored, as the loop is stable. By the way, the TL072 series is much more quiet. It is designed for audio, while the TL082 is a more general purpose op-amp

NOTE: Just to be accurate, because you have a 2 pole design with C3/R5, and C2/R4, roll off is 6 dB per octave, or 70.7% of 70.7%, which is 50%, or 1/2 intensity per octave. It is not a sharp roll off but it is continuous. At 16 HZ their is nothing left of any DC component.

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  • \$\begingroup\$ How did you arrive to the 1.6Hz time constant, Im getting 0.16Hz, typo? \$\endgroup\$ – S.s. Feb 9 '18 at 0:10
  • \$\begingroup\$ Yep. Fixed them. I even have a look-up chart for that stuff. Mea Culpa. \$\endgroup\$ – Sparky256 Feb 9 '18 at 0:14
  • \$\begingroup\$ BTW im not using the TL082, its just that I selected whatever default part to draw the circuit in the simulator. \$\endgroup\$ – S.s. Feb 9 '18 at 0:18

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