How does the sallen key filter work?

Thread Starter

Nanaiah

Joined Mar 16, 2017
25
I need to design a sallen key filter such that the output of the filter is a pulsating DC or ripple output when compared to the input. I tried implementing the below circuit but the output is not satisfactory. I am getting a sine wave instead of a pulsating dc or a ripple.
Frequency at the input of the filter is 20Khz.
Note: The input of the filter is the result of multiplication of sine and square wave.
Blue wave form - input of filter
Green wave form - output of the filter
and the desired ripple output image is also posted below.
upload_2017-4-8_16-3-44.png

Desired output:
upload_2017-4-8_16-6-6.png
 

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Papabravo

Joined Feb 24, 2006
17,554
A Sallen-Key is supposed to produce the output you are getting. It is NOT supposed to produce the output you want. To produce the output you want from a sinewave input you use a rectifier and a filter capacitor. I don't know any other way.
 

Thread Starter

Nanaiah

Joined Mar 16, 2017
25
A Sallen-Key is supposed to produce the output you are getting. It is NOT supposed to produce the output you want. To produce the output you want from a sinewave input you use a rectifier and a filter capacitor. I don't know any other way.
Thank you for your reply. I wanted to make sure whether the output I am getting was wrong or not. So you mean to say that my output is perfectly right?
If i use a rectifier i need to use 2 inputs for the diodes. But i have just 1 input. how do i go about connecting them? should i ground the other input of the diode?
 
Last edited:

Papabravo

Joined Feb 24, 2006
17,554
Use Google to find:
  1. Full wave rectifier
  2. Bridge rectifier
Either circuit will provide an AC signal with positive peaks only. Adding a capacitor to this output will provide the signal you seek.
 
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