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Inverting input greater than non-inverting input = positive voltage?

Discussion in 'General Electronics Discussion' started by 24Volts, Aug 17, 2013.

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  1. 24Volts

    24Volts

    164
    0
    Mar 21, 2010
    Hello,

    With the rails at +/- voltages, I was always under the assumption that when the voltage value at the inverting input is greater that the voltage value of the non-inverting input of an opamp, the output would be a negative voltage!

    OR

    With the rails, Vcc to positive voltage and VEE is to ground, I was always under the assumption that when the voltage value at the inverting input is greater that the voltage value of the non-inverting input of an opamp, the output would be zero!

    But in this particular circuit, the last assumption doesn't hold true. In reference to the circuit in attachment, why is it that the output is +1 VDC when the inverting input is greater that the non inverting input???

    Please view schematic. Isn't va = 2.0VDC and vb = 1.5VDC??? Hence the output should be zero no?

    I am using an lm324.

    All help appreciated!
     

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    Last edited: Aug 17, 2013
  2. john monks

    john monks

    693
    1
    Mar 9, 2012
    The circuit is functioning as it should. This is a linear circuit. It is using negative feedback from the output pin to the negative input. Output current is going through a resistor RF into va, the negative input causing the input to go down to 1.5 volts. In fact most operational amplifier circuits work this way. If you need further explanation I can try to simplify this further.
     
  3. 24Volts

    24Volts

    164
    0
    Mar 21, 2010
    Hi John Monks,

    yes but all maths aside, intuitively, shouldn't the output be 0 v since
    va will sense 2vdc and vb will sense 1.5vdc???

    I mean, if the inverting input is greater than the non inverting input,
    we should get a negative output right! But since the rails don't
    have negative but only 5 v to ground.... we should get 0v as the
    output ....

    confused
    help
     
  4. john monks

    john monks

    693
    1
    Mar 9, 2012
    No. You have 2 volts feeding a voltage divider RA and RF, both 10K resistors. Now let's assume that the output voltage goes to 0 volts as you first assumed. Now you have 2 10 K resistors in series with 2volts on one side an 0 volts on the other. This gives you 1volt between the resistors and the negative input of the operational amplifier. And you still have 1.5 volts on the positive input. This will tend to cause the output to go up towards 5 volts, your positive supply. OK the output starts going up but what happens before the output gets to 5 volts? The input voltages to the operational amplifier come close or cross each other. What happens is that the circuit reaches a state of equilibrium that is both input reaching 1.5 volts and the output of the operational amplifier keeping the input the same because of what we refer to as "negative feedback". If you still have trouble with this I will try a different approach.
     
  5. duke37

    duke37

    5,341
    753
    Jan 9, 2011
    Look at it another way.
    The 324 if operating correctly will have 1.5V on both inputs and will take no input current.

    With 2V input there will be 0.5V dropped across the 10k resistor.
    The same current goes through another 10k resistor so you get another 0.5V drop and an output of 1V.

    The 324 works to give 1V output voltage even with some external load.
     
  6. 24Volts

    24Volts

    164
    0
    Mar 21, 2010
    Exactly what I needed to know.... 1.5v on + and 1 v on -.

    Now it makes sense.

    thanks guys
    24v
     
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