Lexifare26

Joined Feb 22, 2022
1
Hi there,
I was just wondering how much if any effect the loading effect would have on the frequency response of a band-pass filter made of two cascaded RC low and high pass filters. Circuit below

LvW

Joined Jun 13, 2013
1,650
"---if any..." ?
Of course, any load (resistive, capacitive) will alter the frequency response of the passive filter.

Papabravo

Joined Feb 24, 2006
20,168
You could do a simulation to view the effects. This is probably easier than doing the calculations by hand. It looks like this:

You could compare this to a response with opamp followers between the two RC sections

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LvW

Joined Jun 13, 2013
1,650
I suppose you mean a possible loading effect at the output node of the bandpass circuit, right?
In this case, the most simple way is to look at the transfer function:

H(s)=sR2C2/[1+s(R1C1+R2C2+R1C2)+s²R1R2C1C2].

When the load is resitive (RL) you simply must replace R2 bei R2||RL.

crutschow

Joined Mar 14, 2008
32,090
If loading is a concern, then you could add a high-impedance buffer (such as a follower op amp) at the filter output.

Ian0

Joined Aug 7, 2020
7,527
if you’re going to use an op-amp, then make a Sallen-and-Key filter, and it will have a low impedance output.

MrAl

Joined Jun 17, 2014
10,110
Hi there,
I was just wondering how much if any effect the loading effect would have on the frequency response of a band-pass filter made of two cascaded RC low and high pass filters. Circuit belowView attachment 261298
There will be a loading effect but it may or may not be significant. It depends on the impedances of the circuit and the input impedance of the CRO.

For a low impedance circuit and high impedance CRO the effect will be minimal. The percent effect will depend on the ratio of the impedances.
With a high impedance circuit and low impedance CRO, the effect will be maximum. Still it depends on the ratio of the impedances.

For example, if the filter has an output capacitor of 1000uf and the scope an input capacitance of 10pf, the effect should be almost nothing. To find out for sure you would do a circuit analysis with the equivalent input circuit of the CRO attached to the circuit. It's not that hard to do but you should know a bit about AC circuit analysis.