How to chnage a passive bandpass filter to an active opamp bandpass filter?

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• 29th December 2008, 16:01
darrenbkl
How to chnage a passive bandpass filter to an active opamp bandpass filter?
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.
• 29th December 2008, 16:42
LvW
first order +active lowpass filter
Quote:

Originally Posted by darrenbkl
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.

From the given values you can calculate the center frequency as well as the bandwidth resp. the quality factor of the filter. These parameters are the entrance values for designing an active Filter.
At this point you have the choice between several circuit alternatives - I donīt know how many, perhaps 10...15 .
The most popular are Sallen-Key (single pos. feedback), MFB (twofold neg. feedback), GIC-structure (with 2 opamps), KHN (3 opamps),....
• 29th December 2008, 17:03
FvM
passive bandpass filter gain of 0.5
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.
• 29th December 2008, 17:23
LvW
second order filter solved
Quote:

Originally Posted by FvM
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.

We should be careful with filter order.
A first order lowpass in series with a first order bandpass results in a bandpass which has the lowest possible order - and that is two ! (The denumerator is of second order).
However, the quality factor will be lower than 0.5 (only neg. real poles).
• 29th December 2008, 17:52
FvM
passive hum filter circuit
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.
• 29th December 2008, 19:32
LvW
circuit problem passive bandpass filter
Quote:

Originally Posted by FvM
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 Iīve 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 ?
• 29th December 2008, 19:45
FvM
filtr lm358
The problem would be meaningful with additional requirements, e. g. a higher stop band attenuation respectively faster roll-off.
• 30th December 2008, 05:44
xulfee
make llow pass filter and give it required gain from opamp,then make high pass filter along with opamp,then make series combination of these two circuits
• 30th December 2008, 15:41
darrenbkl
lm358 low input sensing resistor problem
emm,basically,the question is to cascade the LPF and the HPF to form a passive bandpass filter with all the specs given above.

THEN,"Modify your passic cascaded filter with active device to have a passband gain of 0dB with no phase inversion".

So the teacher has hint us to use op amp as the active device. THis is some problem based self learning stuff,so I have only few knowledge about this topic,probably won't go too far from active filter,most propably design the filter using 1st order op amp bandpass.

Thx for all replies : )

Quote:

Originally Posted by LvW
Quote:

Originally Posted by FvM
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 Iīve 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 ?

Ya,i think thats the thing i want to find : ) {Therefore, even if it makes no real practical sense, you can realize this bandpass as an active filter using only one opamp.}
• 30th December 2008, 16:56
LvW
multisim sallen key filter
Hi darrenbkl !

At the moment, I donīt know if you need further help.
You can use an active lowpass (1st order) in series with an active highpass (1st order) and adjust the gain to the wanted value. This gives you a 2nd order bandpass (but I doubt if this is what your teacher want to see).
Or you can design a second order bandpass in a classical one-opamp topology (as mentioned earlier in my first reply).

But remember: the minimum order of a bandpass function is TWO !
(Quote: ............,most propably design the filter using 1st order op amp bandpass. )
• 30th December 2008, 17:34
darrenbkl
mc34071 multisim
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?
• 30th December 2008, 17:50
LvW
tl071 tl081 difference -billed
Quote:

Originally Posted by darrenbkl
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?

There are some problematic regions:
1.) The value of 68 ohms is too small in comparison to the output resistance of your opamp; use at least (!) factor 10.
2.) A frequency of 50 kHz is too high for the 741 type opamp ; the open loop gain of the 741 at 10 kHz is app. only 40 dB.
3.) The slew rate of the 741 type is too small (0.5 V/usec) in comparison to your amplitude of several volts at 50 kHz .

Summary: Use another opamp with a GBW (transit frequency) of at least 10 MHz and a slew rate of at least 10V/usec.

By the way: Your HP is of FIRST order !
• 30th December 2008, 22:56
FvM
high pass filter using lm358
To my opinion, the problem is basically solved, if you adjust the impedance level according to OP requirements, a factor of 100 resulting in 6.8k/470pF would be most simple. The slew rate/output voltage restrictions of 741 OP are an issue of course, but without an AC voltage specification, it's O.K. anyway, I think. A modern OP with higher slew rate, e. g. TL081 would be better, as said.
• 31st December 2008, 01:29
Audioguru
lm358 as low pass filter circuit
Why do teachers still use the lousy old 741 opamp? On another forum somebody asked why doesn't the lousy old 741 opamp just die.

The lousy old 741 opamp is 40 years old this year. It should retire and go away.
• 31st December 2008, 03:45
darrenbkl
first order low pass filter problems
Emm..then how should I design it?
• 31st December 2008, 04:00
Audioguru
single supply active-filter tl072
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.
• 31st December 2008, 11:00
LvW
single suply tl 072
Quote:

Originally Posted by Audioguru
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.

1.) As far as I remember, the original task was to create a BP of second order. Your recommendation is a BP of 4th order.
2.) I can give you a reason why teachers and authors sometimes use the old and - as you prefer to say: "lousy" - 741 type opamp for demonstration resp. teaching purposes in the filtering area. Consider the following example:
All - letīs say 20 different - active filter circuits with a 2nd order Tschebycheff (1 dB) low pass response do exactly the same, if the opamp is ideal. You can see some differences only if you assume non-ideal properties for passive and active parts. This is one of the most important arguments for or against some filter topologies, because some structures react more sensitive than other structures on these non-ideal properties.
Now. since app. 40 years the 741 opamp is used very, very often as a reference amplifier when new circuit ideas are proposed and compared with older solutions.
This may be one good reason not to let the 741 "rest in peace".
Donīt be afraid, nobody will use the 741 for professional new designs. But because of the mentioned reason it still can serve as good "working horse" for educational/academic purposes.
• 31st December 2008, 11:59
FvM
low-pass filter tl072
Quote:

The lousy old LM324 and LM358 too.
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.

I'm apparently not the only one, cause my favorite distributor has large quantities on stack, even considerable amounts of LM741. Personally, I didn't use the latter since 25 or 30 years, but's it's still an active product and used in quantities, although I see this one rather as a veteran OP.

It's good anyway, to point out the limitations of the said parts and help electronic beginners to learn about possible alternatives.
• 31st December 2008, 16:27
Audioguru
mc34071 multisim problem
For many years I have replaced the lousy old LM358 and LM324 opamps with MC33171 single, MC33172 dual and MC33174 quad opamps. They have the same low supply current, the same minimum 3.0V supply voltage, the same or lower input bias current and the same input common-mode range to the negative supply (so they can use a single supply without biasing) and the output goes down to almost the negative supply.

The Motorola opamps (ON Semi now) do not have the high crossover distortion of the National Semi ones and the Motorola opamps have a much higher slew rate allowing a high level output response up to 35kHz. The National Semi opamps have difficulty reaching only 2kHz when the output must swing 20V p-p or more.

The lousy old 741 opamp has difficulty reaching only 9kHz at high levels. The OP wants an active lowpass filter at 50kHz.

I use the MC34071 single, MC34072 dual and MC34074 quad opamps as normal supply current opamps that have a minimum supply voltage of 3.0V, inputs that have a common-mode range down to the negative supply and a good slew rate so the output reaches 100kHz at high levels.

I use TL071, TL072 and TL074 opamps for line-level audio and OPA134, OPA2134 and OPA4134 opamps for low levels. The OPA opamps have distortion of only 0.00008% when the gain is low and a high output level response up to 250kHz.
• 31st December 2008, 16:50
FvM
first order +bandpass +filter +opamp
That's a very reasonable part selection. The crossover-distortions of the LM358/LM324 class B output stage are in fact the most problematic feature of this parts, making it unsuitable for any audio application, unless you change the operation mode to pure class A by a load resistor.
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