0
\$\begingroup\$

I was trying to do a project on LT spice for the frequency response for Sallen-Key low Pass filter. But when I tried the Following circuit, I am getting a roll off 20db / decade instead of 40db/decade. Even I am getting a lower cut off frequency than expected. I had done this with 1 nF capacitor and circuit was behaving as expected. Please help, I want an explanation for this deviation.Circuit Diagram

gain vs frequency plot

\$\endgroup\$
2
  • \$\begingroup\$ What is your target corner frequency? \$\endgroup\$ – The Photon Nov 5 '20 at 6:14
  • 1
    \$\begingroup\$ Looks like a possible school project/assignment and you scrolled to the end of the opamp list and picked the OP777. If so, I suggest using the UniversalOpamp2 for simulations like these, as outlined here: electronics.stackexchange.com/questions/529510/… \$\endgroup\$ – Ste Kulov Nov 5 '20 at 6:51
3
\$\begingroup\$

First of all in your circuit you are using 1pF capacitors? this is too little for any practical circuit, even the parasitic capacitance of a PCB is higher. Second, with 1pF and 10Kohm resistors your cutoff frequency is 15.92MHz, the opamp you are using has a GBP of 0.7MHz, can you see a problem with this? with the specified gain the -3dB point of the opamp is around 270KHz, and if the opamp uses a single pole compensation the roll off will be 20dB/Dec, so what you are seeing is the opamp rolling off the gain long before it reaches the 15.92MHz cut-off frequency. With a 1nF cap the cut-off frequency is 15.92KHz, well within the bandwidth of the opamp.

\$\endgroup\$
3
\$\begingroup\$

The OP777 only has a gain-bandwidth product of about 0.7 MHz, but you are trying to use it in a circuit with gain of about 2.5 and cut-off frequency above 1 MHz.

enter image description here

The circuit response you are seeing doesn't even depend on capacitors. You'd see roughly the same if you removed the capacitors and just built a gain-of-2.5 non-inverting amplifier. (But in the real world, unlike the simulator, the cut-off frequency might vary from part to part)

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.