# How to calculate resistor values for certain filters?

Hello i am having trouble understanding how to calculate the resistance for the low pass filter. Mainly since i dont understand if i have to take in the other ressitors. When calculating can i just analyse them separately is the red lines show and break them up into sections? Also for the high pass, to calculate R do i have to take in the resistor 3 value or can i ignore that and focus on R1+R2 for Req? I need the 2 reistors so that the input voltage is dropped so that the output voltage 1 is less than a certain voltage as this voltage then feeds into an amp.

Is there a better way to implement the low pass after the highpass or not? Need the highpass to block dcbias and the voltage divider is used to keep voltage under a certain amount due to vout1 requiring a certain value.

Basically:Can i just find the values for the high pass as one section and ignore R3 for Req for that 1st section, then find the low pass filter components so that the output voltage of that can be fed to an amp.

For the low pass filter can i calculate it from using R1 and R2 or not. If so please show how you did this.

• this is not a LPF. it is a band-pass filter (BPF). you have essentially 3 independent parameters of the BPF frequency response: $\omega_0, H(j\omega_0),$ and $Q$. those are three constraints and there are five unknown circuit parameters. if you add to this the input impedance at the resonant frequency $\omega_0$, they you have four constraints meaning one of the five circuit parameters can essentially be arbitrarily chosen (within a reasonable range of what is practical). – robert bristow-johnson May 12 '17 at 21:06

The two parts of the filter interact to some degree depending on the values. If you added an op-amp buffer between the two halves they would not interact. But the interaction is probably not important here.

Since the high pass filter is to block DC typically the impedance of C1:

$X_{C_1} << R_1 + (R_2||R_3)$

if so then you can ignore C1 for the low pass (assume it behaves close enough to a short) and the -3dB cutoff will be at approximately:

$f_c = \frac{1}{2\pi (R_3 + (R_1||R_2)) C_2}$

Note that this assumes low impedance for the source Vin. If it has a source impedance Rs then just add that to R1.

• I initially had an opamp in between them, but my lecturer said that for our given use of it, it has to be before the amp – Student May 13 '17 at 0:32
• Also so when calculating both filters, to find both cutoff frequencies I have to include the 3 resistor s – Student May 13 '17 at 0:35
• Calculate Xc2 at the desired cutoff frequency for the high pass and see if you need to consider R3. – Spehro Pefhany May 13 '17 at 1:12
• how would i know if i need to consider R3? and so when calculating fc would it be 1/(2pi*(R1+R2)*C1)? – Student May 13 '17 at 1:17