# Question about LP, HP and BP filters.

Discussion in 'General Electronics Chat' started by Kango, Sep 23, 2007.

1. ### Kango Thread Starter Active Member

Sep 22, 2007
30
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If you don't have a data sheet for a certain circuit how do you found out whether there is a LP, HP or BP filter? How shall you analyse the circuit on the matter?

2. ### ixisuprflyixi Active Member

Sep 16, 2007
52
1
To identify a passive LP, HP or BP filter it would help to be familiar with what their typical configuration is. as such you may start in the E-book volumes which are great. http://www.allaboutcircuits.com/vol_2/chpt_8/2.html start there and just work your way through. Also try googling the filter types. If you have a more specific question that these resources dont cover feel free to ask again.

3. ### Dave Retired Moderator

Nov 17, 2003
6,960
147
The question is quite broad. Do you have a circuit in mind that we could look at and provide some pointers?

Dave

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Bandpass filters can be tricky. In general for the other types, if the series pass element is a capacitor, it is going to be an HP filter. If the capacitors shunt the signal to ground, then it will be an LP filter. BP filters use HP and LP filtering, so they are somewhat harder to figure out.

5. ### Kango Thread Starter Active Member

Sep 22, 2007
30
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Thanks for the good information!

I appreciate it.

6. ### Dave Retired Moderator

Nov 17, 2003
6,960
147
When looking at LP and HP filters with resistive and capacitive components you need to remember the reactance equation for a capacitor:

Xc = 1/2$\pi$fC

The important variable here is the frequency (f) which defines the frequency response of the filter.

When the frequency is low, the denominator of the above equation approaches zero and the reactance (Xc) is approaches infinity - conceptually this means the resistance is high.

When the frequency is high, the denominator of the above equation approaches infinity and the reactance (Xc) is approaches zero - conceptually this means the resistance of the capacitor is low.

So think about this with your circuits. Look at the first circuit for example, the input to the op amp feeds a capacitor - when the frequency is low the reactance of the capacitor is high compared to the resistor and the capacitor presents a large resistance acting as a band-stop. As the frequency increases the reactance decreases in comparison to the resistance and the capacitor presents a smaller resistance acting as a band pass to the input of the filter. Therefore this is a high-pass filter. If the capacitor and resistor were the other way around then the arguments would be the exact opposite and it would be a low pass filter.

Using the above logic you should be able to analyse your other filter arrangements.

Dave

7. ### Kango Thread Starter Active Member

Sep 22, 2007
30
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Very nice information Dave, i think I got it know .