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Voltage follower

  1. Mar 5, 2012 #1
    hi all,

    I am trying to design a voltage follower circuit. The input is quite low, around 0.8 - 0.9V.

    I tried an OP, whose power supply is +/-16V, then connected the output with inverting-input, and connected the non-inverting-input with my input (0.8 - 0.9V). But the output does not follow the input, and actually the output is around 1.4+V.

    The circuit does not work well. I guess it is due to the input is too low. Am I right? So what OP should I use then? Any comment is welcome.

    Thanks a lot.
     
  2. jcsd
  3. Mar 6, 2012 #2

    vk6kro

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    That is a classic circuit and usually works OK. Small signals don't matter.

    Some opamps require some DC path to ground from the input pins, so you could try a 100 K resistor from the non inverting input to ground. (The common of your two supplies could be called a ground.)
    This is specified in the data sheets of the opamp or you can just look at the schematic diagram of the opamp's insides and look for open circuited bases on the input transistors. These have to have DC current flowing into them for the transistors to work.

    Another test is to measure the input and output simultaneously. If they are the same, then the opamp is working properly. In your case, if the input is actually 1.4 volts when it should be 0.9 volts, then you need to work out why this is the case.
    Electrolytic and Tantalum capacitors are pretty leaky and this can disturb high impedance circuits.

    It would also be worth looking at the output with an oscilloscope. What measures as a small DC voltage on a DC meter could actually be quite a large AC voltage that happens to average as a small DC voltage
     
  4. Mar 6, 2012 #3

    jim hardy

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    You said +/- 16 volts.

    Are there three leads on the supply, + - and common?

    or just two?

    Most opamps need "headroom" between signal and supply rails. 1.4 volts sounds like 741's common mode limit.

    Try a LM324.
     
  5. Mar 6, 2012 #4

    psparky

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    I can picture a voltage follower op amp.....V+ = V-......I get that. Vin = Vout.

    The obvious question is why do you need the voltage follower? Why not just hook your original voltage source to your load?

    Is the op-amp in this case perhaps allowing more power while maintaining the same voltage? In other words...is the op amp capable of delivering more amps in this case?
     
  6. Mar 6, 2012 #5

    vk6kro

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    Also, check the pins of the opamp chip to see if it has offset control.

    This is simple to use and may give the effect you are having if it is not connected.
     
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