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I am making a Sallen-Key topology second order low pass filter with 10Hx cutoff frequency:

enter image description here

I am using the LM248 op amp. The resistor and capacitor values are specified in the drawing. However, when I input a waveform of 5Hz with 100mV amplitude(and a power supply voltage of 3V), the resulting output from my circuit is a straight line at -1 V:

enter image description here

The interesting observation was that when I raised my power supply to 4V, the output became a straight line at -2V, when I increased it to 5V, output became a straight line at -3V and so on. So the difference between the output voltage and power supply differed by 2.

I have checked my breadboard circuit four times and all the connections are right so I am very confused as to what is happening.

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    \$\begingroup\$ I've never used an LM248, but from the datasheet, it's not likely to run well on a 3 V supply. I'd shoot for +/- 10 or +/- 12 as minimum supplies to use with this part. Also, it's a quad part and you only show one op-amp being used. What are you doing with the other 3? \$\endgroup\$ – The Photon Feb 4 '19 at 4:19
  • \$\begingroup\$ The other three are just not being involved at all \$\endgroup\$ – user510 Feb 4 '19 at 4:30
  • \$\begingroup\$ but if you leave them totally unconnected, they could oscillate and muck up the performance of the 1st one. \$\endgroup\$ – The Photon Feb 4 '19 at 4:31
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    \$\begingroup\$ The usual thing is connect them as voltage followers with input connected to ground (or mid-supply in single-supply applications). \$\endgroup\$ – The Photon Feb 4 '19 at 4:47
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    \$\begingroup\$ See How to disable a single op-amp in dual packaging? for more details. \$\endgroup\$ – The Photon Feb 4 '19 at 4:57
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You have your OP-amp backwards.

You are giving it positive feedback instead of negative. It’s simply saturated.

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  • \$\begingroup\$ But I got this circuit from a textbook. I am also seeing some of the same circuits online for second order sallen key. Could you please let me know how to correct my error \$\endgroup\$ – user510 Feb 4 '19 at 4:30
  • \$\begingroup\$ @user510 Either you transcribed it wrong or the textbook made a mistake. Do an image search for Sallen-Key, and you will see the error. \$\endgroup\$ – Edgar Brown Feb 4 '19 at 4:33
  • \$\begingroup\$ yes you are right \$\endgroup\$ – user510 Feb 4 '19 at 4:34

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