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Simulating Multiple Feedback Low Pass filter in LTSPICE!

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sean87:
Hi all, again!

I am trying to simulate the follwoing circuit in LTSpice IV. What I am looking for is overall performance in frequency domain.



I downloaded and readed a lot of LTSpice guides, but none of them were speaking about how to do for this kind of circuit. If you know how-to, I would really appriciate yuor help! Maybe it is stupid but I do not know what voltage source (AC, DC, sine or ...) I should use and I do not know which simulation type will be what I need.

I have attached the schematic design, if you want to look.

jahonen:
Unless you want to use your own model for the opamp, I suggest you use suitable LT opamp in simulation. Or if you want just theoretical result, then substitute E (voltage controlled voltage source) component for the opamp and give it a very big gain (something like 10^12 or similar).

Another thing, delete the f's in capacitor values. LTSpice might complain about them.

Regards,
Janne

Bored@Work:
Treat it like you want to measure a real circuit. You decide what you want to measure. From there it follows what input signal(s) you need to apply, what type of simulation you want to run, and for how long or over what range (not all applicable).

sean87:
Thanks for caring guys :P

I want to see how is the frequency responce. Actually I am trying to design a filter and I amfollowing this guide. I managed to do the calculations and it seems to be like what I need it to be.

The problem now is simulating the circuit in LTSPICE! Luckly I have found this site that plots the frequency responce, and it is like this:



But when I am trying to simulate this circuit by myself using LTSPICE, I get something like this:


And here is the schematic and setting for the circuit I designed in LTSPICE as well as simulation commands:


The opamp, I right clicked and changed "Aol" to 100M, that is 100,000,000
There was another option in the opamp called GBW which is set by default to 10Meg

For voltage source I just put 1 in AC Amplitude

Can you please tell me what I am doing wrong? how can I get a similar result to the plot generated by that website?

Thanks!

jahonen:
Try changing your simulation command to ".ac dec 500 1 1meg" and remove "F"'s in capacitance values. Further, you are missing a connection dot in crossing of R1-R2-R3-C2.

Regards,
Janne

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