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I am trying to implement the following circuit in LTspice for studying the frequency response of the 2nd order Sallen-Key low pass filter for a project.

  • Why am I getting a negative peak in the frequency response curve at higher frequencies?
  • Why is the maximum gain not occurring at the theoretical undamped natural frequency?

enter image description here

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  • \$\begingroup\$ Your opamp is too slow, check the gain-bandwidth product (BWP) of the opamp you decided to use in the simulation. \$\endgroup\$
    – G36
    Nov 9, 2020 at 14:13
  • \$\begingroup\$ It's a lab assignment I have to use that only. I just needed an explanation of why that is happening. It's a new topic for me. I have to comment on the nature of the plot. That is why I was asking. It would be beneficial if you could elaborate on your answer \$\endgroup\$
    – kodekuzuri
    Nov 9, 2020 at 14:17
  • \$\begingroup\$ If you use a differet opamp you'll see a similar zero, but higher, which means it's something about the transfer function. \$\endgroup\$ Nov 9, 2020 at 15:01
  • \$\begingroup\$ Thanks for that. I wanted to know what causes that "negative peak." \$\endgroup\$
    – kodekuzuri
    Nov 9, 2020 at 15:12
  • \$\begingroup\$ Try read this ti.com/lit/an/slyt306/… \$\endgroup\$
    – G36
    Nov 9, 2020 at 15:20

1 Answer 1

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At above 100 kHz you can begin to write off the op-amp's output as playing much of a role in producing the output signal. This is because it runs out of open-loop gain and, in effect becomes just a passive resistor load at the output node: -

enter image description here

At 1 MHz the open-loop gain is unity and the op-amp just doesn't deliver the goods any more but it's still having a go up to about 200 kHz but it's fighting against the 1 nF capacitor (C1) that connects to the input via R1.

So, as frequency gets higher, C1's impedance gets lower and the OP777's output impedance becomes, in effect, just a resistor to ground with no significant signal to contribute. At this point, any signal seen at the output is due to C1 putting it there via the input resistor R1. The dip in the attenuation at around 200 kHz is just a nuance of one thing conspiring against another thing. For instance if you chose the frequency setting capacitors to be ten times lower and the frequency setting resistors ten times higher then you'd see a better attenuation characteristic from 200 kHz but you'll still get a plateau as the op-amp runs out of steam (maybe 100 kHz higher). If you don't believe me, try it.

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