Op-Amp non-inverting low-pass filter

Discussion in 'General Electronics Chat' started by eyesee, Jun 7, 2014.

  1. eyesee

    Thread Starter Active Member

    Oct 19, 2013
    Why would a low-pass non-inverting filter circuit not work when operating from a single supply rail as opposed to a dual-rail supply?

    The Op-Amp is a TL081.

    The signal being filtered is a pulse waveform and the output required is a positive (high) in response to a positive input.
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Every real life "active device" to work properly as an amplifier supplied from a single source need proper bias circuit.
    When you use BJT as a CE amplifier, you use a voltage divider to bias the active device somewhere in the "linear region".
    In case of single supply op amp you have to do the same think. You need to bias the op amp somewhere in the middle of his "linear region". So if you proper bias your op amp, your low-pass filter will work just fine.
  3. crutschow


    Mar 14, 2008
    A standard op amp cannot work down to 0V at the input and output (as shown in the data sheet input common-mode rating and the output voltage-swing value). That's why you need to either bias the input above ground, or use a dual supply.

    An alternate is to use an op amp designed for "single supply" or "rail-rail" operation. A single-supply device input and output will typically work down to 0V on the input and output, but not necessarily to the positive rail. The rail-rail type will typically work between 0V and the positive rail for both input and output.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    Here is an amplifier I designed for my grandson (Guitar player). It is powered from a USB port (5V) and uses a rail to rail op-amp. Notice the voltage divider R8/R9. It creates a reference voltage that is 1/2 of the supply voltage. This causes the op-amp to float at 1/2 of the supply voltage. Now the output of the op-amp can swing +/- 2.5 volts around that reference voltage.
    This would work for your TL081.


    Edit: Your TL081 can only drive to within 1.5 volts of the power supply voltage. So, a 5 volt supply would limit your op-amps output to +/- 1 volt.
    Last edited: Jun 7, 2014
  5. atferrari

    AAC Fanatic!

    Jan 6, 2004
    Why do you name those GND points as TPX? Does it make real sense? At first glance I was confused.

    In a second thougth, would you call it "common"?
  6. Lestraveled

    Well-Known Member

    May 19, 2014
    "TP" is just a generic symbol for a pad on a PCB design. They are connection points to hard wire to on the PCB.
    atferrari likes this.