# Band-pass Filter Topology help

I am trying to design a pre-amplifer and just starting basic.

Using a low pass filter and a high pass filter with already determine cut off frequencies.

The problem is that I thought it doesn't matter how you cascade the filters as its super positioned (Both linear), however when I put the LPF then the HPF my cut off frequencies become all whack. When I put the HPF then the LPF it works perfectly, whats going on? I am pretty sure there's something simply I am not taking to account, making the low resistor and cap values to ground.

Low-pass filter:

High-pass filter:

LPF-> HPF Topology

HPF-> LPF Topology

Circuit:

Any help would be appreciated!

• Can you show the circuit? It looks like the I/O impedances interfere. – a concerned citizen Jun 13 '18 at 19:21
• Yeah, of course sorry about that, – Pllsz Jun 13 '18 at 19:33
• I think you're right like 100%, I put a voltage follower in between the two and it worked. Maybe further explain how the impedances are affecting one another? – Pllsz Jun 13 '18 at 19:35
• You are not allowed to consider and design each part (LP resp. HP) separately because the second one always act as a load for the first one. Instead, use a buffer amplifier in between. – LvW Jun 13 '18 at 20:13
• Yeah that's another solution – Pllsz Jun 13 '18 at 20:16

When I put the HPF then the LPF it works perfectly, whats going on?

Whenever you passively cascade RC filter stages like the ones you have, it's important to have the stage with the lower impedances first in the chain. So, you say it works when you have the high-pass first and it's clear why. Your HP stage uses a 1 uF capacitor and a 180 ohm resistor and this has a much lower input and output impedances compared to the LP filter using a 7.5 k resistor and 680 pF.

The rule of thumb for cascading these types of circuit is that you make the resistor in the first stage at least ten times lower in value than the resistor in the 2nd stage.

• Comments are not for extended discussion; this conversation has been moved to chat. – clabacchio Jun 15 '18 at 7:10

Transfer function of each stage in any passive filter is a ratio divider function of load /(load + source ) impedance (f).

However for very high order RC or LC filters to simulate Bessel Functions there are better mathematical matrix methods to design and many simulators to verify and/or design.

But source impedance is a critical part of any transfer function. As well so is Input and output impedance (f) defined by scattering parameters called s11, s22 for in out on a 2 port and s21, s12 for forward and reverse transfer function.

• How would one find the I/O impedance ? Thevenin's theorm? – Pllsz Jun 13 '18 at 20:11