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  #1  
Old 03-11-2012, 03:20 PM
fiatuno fiatuno is offline
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Default LM358 single supply band pass filter

Dear members,
I have a problem designing a non inverting single supply band pass filter, with the use of an LM358. I have worked the calculations for the gain and they seem to be right, Then I worked out the calculations from my school book as attached in one of the attachments and simulated on proteus something like in the below attachment, I can't seem to see what is going wrong with my simulation, since I need an bandpass filter for a 150mV ac signal to be amplified, letting frequencies between 200Hz-20kHz to pass. The outputs can't seem to be what I need as I need around 25.66dB in gain. Any help would be greatly appreciated as I can't seem to find a way out, Thanks in advance.
Regards
Fiat Uno
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File Type: jpg lm358filter.jpg (9.9 KB, 56 views)
File Type: jpg BOOK.jpg (18.4 KB, 44 views)

Last edited by fiatuno; 03-11-2012 at 03:31 PM.
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  #2  
Old 03-11-2012, 05:53 PM
fiatuno fiatuno is offline
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Default Single Supply non-inverting Amplifier LM358 big problem

Am I doing the right configuration or is there any other one as I seem to be at a standstill and really need to solve this problem if someone could give me some help. Thanks in advance
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  #3  
Old 03-11-2012, 07:06 PM
Jony130 Jony130 is offline
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Here you have a tip
http://forum.allaboutcircuits.com/sh...47&postcount=4
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fiatuno (04-04-2012)
  #4  
Old 03-11-2012, 07:56 PM
Audioguru Audioguru is offline
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Check your math.

Your input capacitor of 796pF into 100k reduces frequencies below 2010Hz.
Your feedback grounding capacitor of 723nF reduces frequencies below 201Hz.
Your 80pf output capacitor feeding the 100k load reduces frequencies below 20kHz.
The lousy old LM358 reduces frequencies above 1kHz to 20khz depending on the output level.

Since the values of two of your capacitors are too small to pass most of the frequencies you need then you have no gain. Since you are using an extremely old and poor quality opamp then you also have no high frequency gain.
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  #5  
Old 03-11-2012, 08:40 PM
fiatuno fiatuno is offline
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Default Single supply non inverting amplifier with bandpass filter to Jony130

Quote:
Originally Posted by Jony130 View Post
First of all I would like to thank you for your reply, but that one I have already used it as I had to design a bandpass filter using an op amp with dual supply, but now I need to do the same thing with single supply and I opted for the LM358. This configuration this time doesn't seem to work just by substituting the ic and just introducing the bias to the input of the non-inverting input. Thanks anyways.
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  #6  
Old 03-11-2012, 08:50 PM
fiatuno fiatuno is offline
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Default Single supply non inverting amplifier with bandpass filter to Audioguru

Quote:
Originally Posted by Audioguru View Post
Check your math.

Your input capacitor of 796pF into 100k reduces frequencies below 2010Hz.
Your feedback grounding capacitor of 723nF reduces frequencies below 201Hz.
Your 80pf output capacitor feeding the 100k load reduces frequencies below 20kHz.
The lousy old LM358 reduces frequencies above 1kHz to 20khz depending on the output level.

Since the values of two of your capacitors are too small to pass most of the frequencies you need then you have no gain. Since you are using an extremely old and poor quality opamp then you also have no high frequency gain.
So what would you suggest as an opamp that I can use?
What is wrong then with my calculations as I double checked the equations I have done and seem to match. Just to confirm if I am on the right track please as shown in the attachment above:
f1 was used with 200Hz R2=200k C1 came to be 796pF
f2 was used with 20kHz RL=100k C2 came to be 80pF
f3 was used with 200Hz R1=1.1k C3 came to be 723nF

Thanks in advance.

Regards,
Fiatuno
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  #7  
Old 03-11-2012, 10:50 PM
Audioguru Audioguru is offline
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Quote:
Originally Posted by fiatuno View Post
So what would you suggest as an opamp that I can use?
I use and many stereo manufacturers use the TL071 single, TL072 dual and TL074 quad low noise, low distortion and wideband inexpensive opamps.
The lousy old LM358 is very noisy (hissss), has horrible crossover distortion and has troulble above only 2kHz.

Quote:
What is wrong then with my calculations as I double checked the equations I have done and seem to match. Just to confirm if I am on the right track please as shown in the attachment above:
f1 was used with 200Hz R2=200k C1 came to be 796pF
Two 200k resistors make 100k, not 200k.
1 divided by (2 x pi x 100k x 200Hz)= 8000pf, not 800pF.

Quote:
f2 was used with 20kHz RL=100k C2 came to be 80pF
Your 80pF capacitor feeding 100k at the output cuts low frequencies, not high frequencies. The lousy old opamp cuts mid frequencies and cuts high frequencies.

Quote:
f3 was used with 200Hz R1=1.1k C3 came to be 723nF
Correct.
EXCEPT, Now you have two RC networks cutting low frequencies below 200Hz so their total cutoff frequency is 400Hz, not 200Hz.
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  #8  
Old 03-11-2012, 11:15 PM
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KJ6EAD KJ6EAD is offline
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Have you tried using Texas Instrument's FilterPro application to check your work?
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  #9  
Old 03-12-2012, 10:42 AM
fiatuno fiatuno is offline
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Quote:
Originally Posted by KJ6EAD View Post
Have you tried using Texas Instrument's FilterPro application to check your work?
Yes I did try to use it already, but it seems to have a different configuration and doesn't cater for single supply as long as I know. I wish to edit the design attached from the book to make it workable. Thanks anyways KJ6EAD
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  #10  
Old 03-12-2012, 10:49 AM
fiatuno fiatuno is offline
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Quote:
Originally Posted by Audioguru View Post
I use and many stereo manufacturers use the TL071 single, TL072 dual and TL074 quad low noise, low distortion and wideband inexpensive opamps.
The lousy old LM358 is very noisy (hissss), has horrible crossover distortion and has troulble above only 2kHz.


Two 200k resistors make 100k, not 200k.
1 divided by (2 x pi x 100k x 200Hz)= 8000pf, not 800pF.


Your 80pF capacitor feeding 100k at the output cuts low frequencies, not high frequencies. The lousy old opamp cuts mid frequencies and cuts high frequencies.


Correct.
EXCEPT, Now you have two RC networks cutting low frequencies below 200Hz so their total cutoff frequency is 400Hz, not 200Hz.
Hi Audioguru,

First of all thanks for the TL071 as I matched some information from the datasheets and showed that it is a much better solution and also for f1 I have understood your reasoning and I agree.

But for f2 since I used the 80pF capacitor which is feeding the 100k at the output, why is it cutting low frequencies if I need it to cut higher frequencies which are greater than 20kHz. Do I need to exchange the position between the capacitor and the resistor? I have tried it already and the output is more like a square wave which has max amplitude at 3.5 and min amplitude at 2.5.

For the f3 why is it that I have 400Hz, which parts are doing such thing.

Thanks for your reply and for your help,
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