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darrenbkl said:I m studying basic filter circuit.Was given a task to modify a passive bandpass filter to an active op amp bandpass filter,the passive filter spec is given below:
Low pass filter - R(68 ohm), C(0.047u F) , fc=49.8kHz
High pass filter - R(3.3k ohm), c(0.047u F) , fc=1.03kHz
anyone can teach me how to convert it ?Thx.
FvM said:The passive filter is apparently a first order filter. You didn't tell any specifications resulting in a higher filter order. So a first order active filter would be the obvious and almost trivial solution.
FvM said:The given passive filter is a series circuit of a first order lowpass and higpass. Although you may describe it as a bandpass, this won't make sense to my opinion. So I prefer to see two individual filters with real poles. Replacing it by an active filter doesn't change anything.
LvW said:FvM said:The given passive filter is a series circuit of a first order lowpass and higpass. Although you may describe it as a bandpass, this won't make sense to my opinion. So I prefer to see two individual filters with real poles. Replacing it by an active filter doesn't change anything.
You are right - I have produced a typing error (A first order lowpass in series with a first order bandpass results in a bandpass); of course the lowpass is in series with a highpass.
However, as far as I´ve understood the original question from darrenbkl the idea behind the problem is a kind of academical task (...Was given a task to modify a passive bandpass filter....) with the aim to learn something about filter properties.
Therefore, even if it makes no real practical sense, you can realize this bandpass as an active filter using only one opamp.
Hello darrenbkl , is it true that someone gave you the task to transfer a passive structure into an active one ?
darrenbkl said:I have done some simulation in NI multisim,by cascading an 1st order LPF with a 2nd oder HPF. When i obeserve the output with bode plotter,something strange happen!! The bode curve look strange at high frequency,and the phase between input and output also looks strange. What could be the problem?
Audioguru said:Use a Sallen and Key second order highpass filter to cut rumble and hum followed by a Sallen and Key second order lowpass filter to cut hiss.
Use a better opamp like a TL071 single, TL072 dual or TL074 quad. They are low noise, have a wide bandwidth and cost the same as a lousy old 741 opamp.
The lousy old 741 opamp should curl up and die.
The lousy old LM324 and LM358 too.
May they rest in pieces.
Basically an ignorant and inappropriate statement, to my opinion. I'm frequently using these parts for general purpose DC and low frequency applications, where they fit the specification.The lousy old LM324 and LM358 too.
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