# Filter help - calculating fc for this circuit

#### duff650

Joined Mar 15, 2019
38

I would appreciate some help understanding this circuit.

Am I correct in saying that R1 and C2 constitute a low pass filter and C1 and R3 are a high pass filter? struggling to comprehend whats going on, oi know its a band pass I have simulated the circuit with a 0.5V AC 1-10kHz testbench, here are the results

I got these values using 1/2*pi*RC for what I thought were the low and high pass filter, I need a low pass fc of 15Hz and a high pass of 75Hz with 11 gain, I know the gain is correct but can someone show me how to calculate the filter specs?

The cursors show the -3dB points and they are nowhere near what I expected, any help appreciated!

EDIT: I know the high pass before is there, ive calculated this previously

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#### crutschow

Joined Mar 14, 2008
24,313
You are putting an AC signal that goes plus and minus into a single-supply op amp that can only go positive.
Add a negative supply or bias the input at 1/2 the supply voltage of the op amp for proper operation.

Last edited:

#### Jony130

Joined Feb 17, 2009
5,145

#### MrAl

Joined Jun 17, 2014
7,109
View attachment 173220

I would appreciate some help understanding this circuit.

Am I correct in saying that R1 and C2 constitute a low pass filter and C1 and R3 are a high pass filter? struggling to comprehend whats going on, oi know its a band pass I have simulated the circuit with a 0.5V AC 1-10kHz testbench, here are the results

View attachment 173219

I got these values using 1/2*pi*RC for what I thought were the low and high pass filter, I need a low pass fc of 15Hz and a high pass of 75Hz with 11 gain, I know the gain is correct but can someone show me how to calculate the filter specs?

The cursors show the -3dB points and they are nowhere near what I expected, any help appreciated!

EDIT: I know the high pass before is there, ive calculated this previously
View attachment 173221

Hi,

There is no C2 in the circuit.
As others have said, you need a negative supply too.

Given you have that negative supply too, you can analyze this circuit by substituting 4 impedances for the different circuit parts. You can then compute the output, then substitute actual values back in, and you get your response. That's about the simplest way to do this. Try again and then i'll show you how to do this, in part.

#### SLK001

Joined Nov 29, 2011
1,543
The fc (cutoff frequency) of a filter is defined as a point -3dB from the center frequency (power) or -6dB voltage. Use that relationship and calculate two fc's, or f1 and f2.

#### LvW

Joined Jun 13, 2013
904
The fc (cutoff frequency) of a filter is defined as a point -3dB from the center frequency (power) or -6dB voltage. Use that relationship and calculate two fc's, or f1 and f2.

You wrote "....-6dB voltage....".
I think, this is not correct. The classical cut-off is at a frequency where the voltage (magnitude) is 3 dB down .

#### bertus

Joined Apr 5, 2008
20,317
Hello,

The circuit still will not work correctly.
You are using a single supply circuit without offset.
Have a look at the attached PDF to correct it.

Bertus

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