power supply for opamp!!!

Discussion in 'The Projects Forum' started by onlyvinod56, Nov 7, 2008.

  1. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008
    Im having a doubt regarding the power supply of opamp.
    Im using LM324 as a comparator. The circuit is shown in the attachment. (please dont consider the values of the input voltages. randomly i draw the figure)
    Both the inputs are connected through pots.The voltage at the input terminal(sinusoidal voltage) is observed as a rectfied sinusoidal in CRO.
    I used the power supply as +12v & 0v.
    In this forum i observed that in some circuits, the opamps are connected to +15v & -15v. What happens if +15v & 0v is given? Shall i get that rectified voltage?
    Is it mandatory to use +v & -v instead of +v & 0v ?
  2. Audioguru


    Dec 20, 2007
    The opamp has a voltage gain of 200,000 at low frequencies. Its max allowed negative input voltage is only 0V for a 741 opamp and -0.3V for an LM324 quad opamp.
    A lousy old 741 opamp won't work when its supply is only 5V.

    Because its voltage gain is so high and because your opamp circvuit does not have negative feedback then if the input of the opamp is not destroyed by having an input voltage too negative then the output at low frequencies will be a max level rectangular-wave.

    If you add negative feedback then a negative voltage to an input resistor will cause the output to go positive and have the same waveshape for half a cycle as the input.
    then the (-) input of the opamp will stay at 0V and not be destroyed.

    Your load resistor is only 5.1 ohms which is a dead short to the output of an opamp that drives a minimum of 2000 ohms.
  3. Audioguru


    Dec 20, 2007
    I just noticed that the half-wave signal is the input and not the output.

    But your circuit inverts. Its output is trying to go negative when the input goes positive. It can't go negative without having a negative supply in addition to the positive supply.
  4. SgtWookie


    Jul 17, 2007
    Adding to Audioguru's suggestions...
    R2 and R3 are wired as rheostats rather than potentiometers. A 741 opamp has a rather high input impedance, so your rheostats are having very little effect on the actual voltage levels that the inputs are seeing.

    The good thing is that your R3 may have saved the input of the opamp from certain destruction by limiting the current when the sinewave went below 0 volts.