# How to identify filter behaviour without memorizing different topologies

Is there a way to tell if a circuit is high or low pass without memorizing different topologies?

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Look at the extremes. DC and very high frequencies. For DC you can remove the capacitors and short the inductors. For high frequencies you can short the capacitors and remove the inductors. By looking at the resulting circuit it should be easy to tell whether low or high frequencies can pass.

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Ok thanks so would this be the correct logic, for example (f) at dc the capictor is removed so the signal will pass through the resistor at V_o and at high freqeuncies the capcictor is shorted so the signal will go through there and not the resistor at V_o – DragonDude Feb 25 at 20:11
Yes, exactly. So it only passes low frequencies. – Mario Feb 25 at 20:22
Nice, succinct answer. +1 – bitsmack Feb 25 at 20:23
You can also use this approach to identify bandpass/notch filters. – helloworld922 Feb 25 at 20:37

In case of passive configurations (R, L, C only) you can follow the steps as outlined in Mario`s answer. However, the situation is somewhat more complicated in case of active RC-topologies. In this case, you have to know how the feedback signals are processed (positive or negative feedback) and how the feedback signal is combined with the forward signal. In this context, sometimes you have to consider the phase relations for the typical frequency regions (very low, midband, very high).

Here is an example:

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Up to now - nobody has asked how to decide about the filter characteristics of the example circuit. Therefore, I answer by myself: Very low frequencies: Signals are blocked by the series capacitor. Very high frequencies: Full feedback via both capacitors. Result: very small output signals for low and high frequencies. Therefore: Bandpass. – LvW Mar 2 at 21:11