# types of filters and properties

#### templeavenue

Joined Dec 2, 2004
1
Im just taking an electrical (ppb introductory) course and cracking for test. Im really confused by frequency response sections(with filters). I dont know how to approach such problems, where it asked me to find out the frequency response of the circuit (given in a regular circuit or op-amp) and identify the type of filter. There are different types of frequency (low pass, high-pass, band-pass. Etc.), but I dont know how to determine it (neither graphing or some other method). In the text book, it mentions that when the load is in series with a resistor and the load is in parallel with capacitor, it gives low-pass. Also, when use capacitor and inductor, it gives bandpass filter). I cant see what filter it is just looking at a circuit. Also, I want to know how to compute omega-not wo, the cut-off frequency. In the book, it used (1/RC) for cut-off frequency for H(jw) = 1 / ( 1+jwCR). Is the the same for any type of circuit(which I think not). To my understanding, it has to do with the denominator. But how about when w is in both numerator and denominator? Also, I want to know more about dB when graphing. Is the slope always (+20 or -20 dB), no matter what? Any help will be appreciate. (Or any link that explains very well). Thanks in advance

#### mozikluv

Joined Jan 22, 2004
1,435
hi

i would like to refer you to my post about filters. just click on my user name and then click on "find members topic"

#### xmen205

Joined Oct 31, 2004
13
hey dude,filters is one of THE most simplest topic which i have come across.
first about low pass filter(LPF) .we connect the capacitance to slot no 3 of opamp.the capacitance is connectd in parallel with the source resistance and the ac voltage source.
secondly for a high pass filter,we connect the capacitor in series with the voltacge source and connect the resistance parallel to them.

#### phibes

Joined Dec 20, 2004
1
Originally posted by xmen205@Dec 7 2004, 06:56 AM
hey dude,filters is one of THE most simplest topic which i have come across.
first about low pass filter(LPF) .we connect the capacitance to slot no 3 of opamp.the capacitance is connectd in parallel with the source resistance and the ac voltage source.
secondly for a high pass filter,we connect the capacitor in series with the voltacge source and connect the resistance parallel to them.
[post=3993]Quoted post[/post]​
Dude, sounds like you know filters!
Can you tell me how you get from jwRC/(1+jwRC) to 1/(1-jwRC)?
Both expressions are equivalent in defining a single-pole series RC highpass-filter.

#### Brandon

Joined Dec 14, 2004
306
Originally posted by phibes@Dec 20 2004, 04:13 PM
Dude, sounds like you know filters!
Can you tell me how you get from jwRC/(1+jwRC) to 1/(1-jwRC)?
Both expressions are equivalent in defining a single-pole series RC highpass-filter.
[post=4223]Quoted post[/post]​
I don't think that is correct. I believe this is the right route. Someone correct me if I am wrong. Haven't done analog filters in a while. Only digital ones.

=jwRC/(1+jwRC)
=(jwRC/jwRC)/(1/jwRC+jwRC/jwRC)
=1/(1/jwRC+1)
=1/(1-j/wRC)
=1/(1-jw0/w)

where w0=1/RC (The cutoff frequency)
w is the frequency of the signal

From the above you can plot a frequency curve fairly easily. From this transfer function we have a high pass filter. At w->infinity, w0/w ->0 giving a gain of 1.

When w=w0, we have 0.707 as the gain (1/sqrt(2)), i.e. the -3db point. Also called the half power point since 0.707^2 = .5 the power.

when w<w0, the gain tapers off at -20db/decade.