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Voltage Follower Doesn't Work

Discussion in 'General Electronics Discussion' started by SpacePirate, Jan 26, 2017.

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  1. SpacePirate

    SpacePirate

    4
    1
    Jan 26, 2017
    I set up an extremely simple voltage follower circuit as shown in the image attached using a LF353N op amp on my breadboard. For my voltage source I'm using a 0.5V 500Hz sine wave. To power the op amp I'm using a 1.5V (AA) battery, with the + terminal attached to V+ and the - terminal attached to V-

    Shouldn't my output look exactly like my input? My oscilloscope picks up virtually nothing at the output pin and I'm not sure why. Do op amps only work with DC?

    Screen Shot 2017-01-26 at 1.22.27 PM.png
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    [QUOTE="SpacePirate, post: 1720774, member: 47220" For my voltage source I'm using a 0.5V 500Hz sine wave. To power the op amp I'm using a 1.5V (AA) battery, with the + terminal attached to V+ and the - terminal attached to V-
    View attachment 31581 [/QUOTE]
    Uh, I'd start eliminating possible causes with that!

    Chris
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi,
    welcome to EP

    these op-amps require a dual rail supply ... dunno if your single rail will work
    the rail voltage may also be too low .... use a couple of 9V batteries to give a dual ( split) rail supply


    no
     
  4. SpacePirate

    SpacePirate

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    1
    Jan 26, 2017
    Thanks for the quick reply! Could you elabortate a little bit on what you mean by single/dual rail and why my voltage might be too small? Beginner here.
     
  5. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Start with the data sheet. It will tell you the max and min power supply voltage range. The minimum is more than 1.5 V, so that is a lot of your problem right there.

    Separate from that is the fact that powering an opamp is a bit more complex than you think. Most "normal" opamps expect to see two separate power supply voltages, one positive with respect to ground and one negative. Note that usually there is no "ground" pin on an opamp, so figuring out how all of this works is going to take a little effort. Start with some introductory reading on opamp basics. This site has some.

    Normal opamps are powered by DC sources only, but can amplify both AC and DC signals.

    ak
     
  6. davenn

    davenn Moderator

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    here's a dual rail PSU using 9V batteries

    upload_2017-1-27_10-51-24.png


    AK answered that for you

    it ALWAYS pays to read the datasheet ;)
     
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  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you read the datasheet and look at the output voltage swing for (what from memory was) a +/-15V power supply, it says the max output is 13V (again, from memory).

    This means that whatever your power supply, you should not expect the output to be able to be driven to within 2V of the power supply rails.

    A 1.5V power supply is +/-0.75V and provides way less voltage than is required to have some margin for the signal once you subtract 2V from each rail.

    If you had a +/-3V supply (a total of 6V) then you could expect a peak to peak output of 2V.

    There are also limitations on the voltages which can appear at the input terminals. Some devices don't allow them to get too close to one or either supply rail, others allow small excursions beyond them. The may also be limitations on the difference between the voltage at input terminals.

    The are many variables and you really do need to read the datasheet.
     
  8. OBW0549

    OBW0549

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    Jul 5, 2016
    Another indication about supply voltage requirements comes from the data sheet graphs that show various performance parameters as a function of supply voltage: in all of them, the lowest supply voltage for which data are shown is ± 5 volts. That's a strong suggestion that the part cannot be expected to operate on supply voltages less than that.

    The LF353 is a very old part, and was designed back in an era when "standard" op amp power supplies were ± 15 volts. Low voltage op amps that can function on 5 volt single supplies, or lower, are a more modern development.

    Always read the data sheet before designing something. ALWAYS.
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    In addition to the minimum supply is 10V, the datasheet for the old LF353 shows that the inputs do not work if they are within about 2.5V from the negative supply that is 0V in your circuit. Therefore the opamp does not work properly unless the input is +2.5V or higher in your circuit if it has a +10V supply.
     
  10. SpacePirate

    SpacePirate

    4
    1
    Jan 26, 2017
    Wow thanks everybody I was not expecting such a large response on this site. Really appreciate all the explanations. I will definitely be more attentive to data sheets from now on!
     
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  11. SpacePirate

    SpacePirate

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    Jan 26, 2017
    What's the point of the two caps if it's DC? Won't it just act like an open circuit at those parts? Also, as AnalogKid mentioned, this op-amp in particular does not use a ground pin, so would grounding in between the batteries make a real difference in this case?
     
  12. Audioguru

    Audioguru

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    An electronic circuit needs to have a power supply with a low impedance. The internal resistance of a battery increases as its voltage runs down then the supply is not a low impedance unless there is a parallel capacitor.
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Does drawing it like this help you? I omitted the 10uF caps for clarity. Their function is for supply rail filtering / decoupling.
    Chris
    upload_2017-1-27_20-57-59.png
     
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  14. Audioguru

    Audioguru

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    The circuit for most amplifiers always has a ground.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The ground if the reference for input and output signals. You need it.
     
  16. davenn

    davenn Moderator

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    still helps to smoothen variations mainly due to changes in current requirements by the circuit

    as Steve said, yes, it's needed .... circuit wont work without it
     
  17. kpatz

    kpatz

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    Feb 24, 2014
    Just to clarify on the ground thing: a ground in an electronic circuit is the reference point that all other voltages in the circuit are measured relative to. It doesn't mean it has to be connected to an earth ground.

    Most circuits that use a single power supply and all voltages are positive relative to ground, will use the negative terminal of the battery or power supply as the ground, and all voltages are positive relative to ground. It's also possible to make a circuit that uses the + side of the power supply as the "ground" and all voltages are negative relative to this. This is a lot less common though.

    But op amps work with voltages that can go both positive and negative relative to ground. Thus the need for a split/dual power source, with the ground "in the middle." Assuming 9V batteries are used, and they're fresh enough to be putting out 9 volts, the V+ pin on the op amp will be +9V relative to ground, and the V- pin -9V relative to ground. Voltage from V+ to V- (rails) will be 18 volts. Remember, voltage is always relative between 2 points, so the ground provides a common reference that all voltages in the circuit are relative to. If VG1 is putting out a sine wave at +/- 0.5V, that's its voltage relative to ground. Measuring VG1 relative to V- will give a different reading of 8.5V to 9.5 V. Measuring it relative to V+ would give you -8.5 to -9.5V.

    Now the LF353 can only handle inputs beyond 2.5V of the negative or positive rails, so with a +/-9V supply (18V total), it can handle inputs from -6.5V to +6.5V, and the output should mirror this in your voltage follower circuit (once again, relative to ground). More modern "rail-to-rail" op amps can handle inputs and outputs closer to the rails, like within 0.5V. Here's a crude diagram illustrating this:

    Code:
    V+ Rail (+9V)
            |
          +2.5V
            |
    Max Input Voltage (+6.5V)
            |
          +6.5V
            |
    Ground (0V)
            |
          -6.5V
            |
    Min Input Voltage (-6.5V)
            |
          -2.5V
            |
    V- Rail (-9V)

    There is a way to use an op amp such as this with a single power supply, but you still run into the "minimum 2.5V to the rail" rule. If you were to use a single 15V supply, with ground on the V- pin, it would work as long as your inputs are within the range 2.5V to 12.5V. Since the op amp doesn't have a "ground" pin, you can have "ground" be anything you want, though it's usually 1/2 the total supply voltage, as long as your inputs (and expected outputs) aren't too close to the rails.
     
  18. AnalogKid

    AnalogKid

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    Jun 10, 2015
    As AG said, the *circuit* for most amplifiers has a ground. That is not the same as an individual component like an opamp having an explicit ground *pin*. Most don't. However, some opamp circuits have one of the inputs connected directly to the circuit ground node. Note that while one of the power pins of the LM358 is labeled "GND" on the datasheet, it is not a ground pin. It is the negative power input pin of one of the very first devices designed for that pin to be connected directly to Ground. The 358 will operate just fine (ok, AG, *relatively* fine for its day) with dual supplies up to +/-18 Vdc.

    Ground (otherwise known as the circuit's reference potential) is both a physical connection defined and bounded by conventions, and a philosophical anchor for discussions of the circuit's signals and power.

    ak
     
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  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    AK, is that quote plagiarized or yours? In either case I love it! If yours I love it even more. ;)

    Chris
     
  20. AnalogKid

    AnalogKid

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    All mine, made it up on the spot, straight from Buckeye country.

    ak
     
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