BartlebyScrivener
Member level 5
I have tried to design a third order active filter with a cut off freq of 20kHz. Using 10 kOhm resistors, I make that the capacitors should be 795pF using 2*pi*f = 1/CR
I made a folded cascode amplifier, with an output buffer stage
![Screen Shot 2013-12-20 at 18.46.47.png Screen Shot 2013-12-20 at 18.46.47.png](https://www.edaboard.com/data/attachments/39/39281-1a9daf43e95297f7048dbba222d142a0.jpg)
then added a first order filter to the output
![Screen Shot 2013-12-20 at 18.47.27.png Screen Shot 2013-12-20 at 18.47.27.png](https://www.edaboard.com/data/attachments/39/39280-3bd40c38c444ad6b13345595f5a80660.jpg)
then added the first order to the input, and the cap to the output.
![Screen Shot 2013-12-20 at 18.47.13.png Screen Shot 2013-12-20 at 18.47.13.png](https://www.edaboard.com/data/attachments/39/39279-df3ef3e6ef854473a086e25720058082.jpg)
But I seem to get a first order response when I do an ac sweep? Also, the -3dB point is around 1kHz instead of 20kHz.
![Screen Shot 2013-12-20 at 17.27.23.png Screen Shot 2013-12-20 at 17.27.23.png](https://www.edaboard.com/data/attachments/39/39273-eb121657055ba65b70ff9578c8ec8a3f.jpg)
What am I doing wrong?! Thanks.
I made a folded cascode amplifier, with an output buffer stage
![Screen Shot 2013-12-20 at 18.46.47.png Screen Shot 2013-12-20 at 18.46.47.png](https://www.edaboard.com/data/attachments/39/39281-1a9daf43e95297f7048dbba222d142a0.jpg)
then added a first order filter to the output
![Screen Shot 2013-12-20 at 18.47.27.png Screen Shot 2013-12-20 at 18.47.27.png](https://www.edaboard.com/data/attachments/39/39280-3bd40c38c444ad6b13345595f5a80660.jpg)
then added the first order to the input, and the cap to the output.
![Screen Shot 2013-12-20 at 18.47.13.png Screen Shot 2013-12-20 at 18.47.13.png](https://www.edaboard.com/data/attachments/39/39279-df3ef3e6ef854473a086e25720058082.jpg)
But I seem to get a first order response when I do an ac sweep? Also, the -3dB point is around 1kHz instead of 20kHz.
![Screen Shot 2013-12-20 at 17.27.23.png Screen Shot 2013-12-20 at 17.27.23.png](https://www.edaboard.com/data/attachments/39/39273-eb121657055ba65b70ff9578c8ec8a3f.jpg)
What am I doing wrong?! Thanks.
Attachments
Last edited: