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Passive band-pass filter simulation on LTspice. Given: cutoff frequencies Flow= 10 kHz and Fhigh= 22 kHz. The run isn't getting me -3 dB between the pass.

I calculated R and C and put them in the simulation, but when I click run the values are wrong.

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It must have the values of cut-off frequencies at -3 dB.

What mistake did I make?

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    \$\begingroup\$ @JessieRaya Two things: 1) the -3 dB point is relative to the passband, and 2) you are using an unbuffered RC filter, which means you'll need to account for the loading effect (the transfer function differs than the buffered one). Also, don't forget that you used AC 10. \$\endgroup\$ Sep 3, 2022 at 8:58
  • \$\begingroup\$ I didn't understand what i did wrong \$\endgroup\$ Sep 3, 2022 at 9:04

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No, you won't get a -3 dB point with your filter because the low-pass and high-pass cut-off frequencies are too close to each other. Given that your AC signal is 10 volts, I'd expect a properly designed band-pass filter to peak at about 20 dBV on the output but, yours peaks at about 16.5 dBV hence, it doesn't quite reach 17 dBV (the -3 dB point).

There are also impedance interactions between the low-pass and high-pass filter circuits and, this might muddle things up a little.

What mistake did I make?

Try plotting low-pass and high-pass circuits individually on the same graph and you should be able to see what I mean. You could also experiment with spreading the low-pass and high-pass cut-off frequencies and you will see that the band-pass section will inevitably get closer to 20 dBV on the output.

In future, use an AC signal strength of 1 to make it easier on the numbers.

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