# How to calculate resistors and capacitors values for analog RC filters?

How to calculate resistors and capacitors values for band-pass RC filters based on LM348 operational amplifier? In this example, 3.2KHz, 4.3KHz are standard center frequencies, that are individually adjustable down to 1.2KHz and 1.8KHz. I need calculate values of R40, R38, R54, R52, C29, C28, C27, C26 values for my custom center frequencies, for example 1.8KHz, 2.4KHz

Below is Filter frequency response

Full schematic:

• All those components have values in your circuit so what is it you really want to know? You might also want to state the source of the circuit and tell us where the points "PH" connect i.e. a complete circuit diagram is required. Commented May 4, 2015 at 18:42
• @Andy aka You don't understand my question. No, I have no values for those components. I posted example of components connectivity, and those values are for different frequencies, not mine. I need find R and C values for my own frequencies. Commented May 4, 2015 at 19:25
• And YOU haven't answered the other points I raised Commented May 4, 2015 at 19:46
• Yes, I missed that. Now corrected. Commented May 4, 2015 at 20:04

Each of the filters in the examples you posted are awkwardly drawn Multiple Feedback topology band pass filters. When designing a filter, you need to decide on:

• Type (bandpass, high pass, low pass)
• Center frequency
• Order (how quickly the filter rolls off)
• Shape (butterworth, Chebyshev, Bessel, etc)
• Topology (how the filter is physically implemented)

If you're comfortable with the math of scaling and transforming via the prototype filter, you're good to go now.

Otherwise, you can use a filter design tool. One that I've used in the past is from Analog devices: the Analog Filter Wizard. This tool will let you design a filter using all of the above considerations.

• As I'm not an author of design, I don't know what considerations been used when he selects the type of filter, I just want to change center frequency used in these filters. So these is band-pass Butterworth filters, and what order are filters? Commented May 4, 2015 at 22:14
• All stages are "simple" second-order bandpass filters (for second order there is no distinction between Butterworth and other possible approximations).
– LvW
Commented May 5, 2015 at 7:01

At first you must identify the topology of the filter circuit. As it can be seen the whole circuit consist of various filter stages connected simply in parallel (the first diagram). So it is sufficient to analyze one of the stages only. And it is relatively easy to see that the topology of each stage resembles a bridged T-network. Thus, each bandpass stage can be analysed based on a the set formulas for the well-known "Multi-feedback bandpass filters (MFB)" to be found in each filter book.

Please note that you have forgotten to show the power supply voltages (single/dual ?). I suppose you are using single supply because of the bias circuitry connected to the non-inv. opamp input terminals, correct?

• Yes, +12V AC single power supply. Commented May 4, 2015 at 21:28

It requires very complicated calculations If you want to calculate it yourself, but I don't think what you want is that. What I suggest you is that you use a CAD software such as PSpice or Proteus (even Matlab if you are familiar with Simulink), assemble your circuit there and define some initial values for your circuit. Then find the correct combination trying and failing. This is the practical method.

Another method is that you can find the transfer function of the system with the variables C26, C27, C28 etc. and then use an iterative numerical method to solve the system for these parameters. You have two separate outputs, thus the calculation would not be that difficult. You can calculate them separately.

Final note: You probably will not be able to get a specific answer from here because of the complexity and the effort that requires to solve the problem.