bandpass filter question

[sweet_nth]

ok,, this might sound stupid, but i have the following situation,,,

i want to design a bandpass filter, and im using a general purpose op amp,
it looks just like the filter here
http://www.fulcrum.ru/Read/CDROMs/NS/OA-28.pdf#search='bandpass%20filter%20op%20amp'

the problem is when i power up the op amp in the filter, the output is always the same,, no matter what the input is(if any) !! its just giving my voltage a gain,, and nothing else, its not acting like a filter, my input is an audio signal, and im using the same power supply for the op amp, and the audio amplifier (input)..

what am i missing??? why isnt my circuit acting like a filter?? im using lm748 op amp..

 

[actra]

r u sure from ur calculations of Rs and Cs

 

[sweet_nth]

yes, im sure,, as i simulated the filter using a software and it gave me the correct results!!

but as i said, it doesnt matter, as im getting a constant output regardless of input, even if there is no input,,, (i have the power supply connected to the op amp from the V+ pin tho)...

its my first time to build a filter, ive dont them on papers a thousand times,, but when this very simple one didnt work, i was confused, as it workes on theory, i dont know what im doing wrong, i tried 3 different op amps... and i always get a constant output...

 

[Borber]

We can not see what you have done to analyze the problem. But it seems you have stability problem in your filter. It oscillates?

 

[artem]

ptu your schematics (real not simulated) here .

 

[mmohsen]

i donot know where is the problem i had built filters many times and it works
so try to use LM358 or try to check your connections again as it is the most popular fault in the practical work ,
i wish it works ,..

** you have a nice logo.

mmohsen

 

[elbadry]

I think there may be a problem with stability. You can check that by grounding the input and observing the output voltage, if there is oscillation then the circuit is unstable...