Op Amp Help!

Discussion in 'General Electronics Chat' started by bixbytj, Aug 14, 2014.

  1. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    Hi all!
    I am trying to test some simple op amp circuits before I ask my instrumental analysis students to build them in class and I am a little out of my depth. I took a fairly extensive analog electronics course in undergrad, but that was more than 10 years ago and I am a little rusty. My first attempt here was with a straightforward level detector and the output does not seem to be amplified, or in fact influenced by the power supplies in any way. Let me summarize what I've got:

    1. None of our power supplies have negative voltage so I got some LT1054s to make a voltage inverter. That worked on the first try (woohoo!) and now I have reliable +/- power for the op amp.
    2. I am using LM741 op amps, a function generator, and an oscilloscope. I am confident that my signals into the inverting and noninverting inputs are what I think they are. I set up a diode to check the signal in from the function generator and I'm just comparing to ground.

    There is not much to this circuit so I am at a loss for troubleshooting. When I monitor the output of the chip given the input of a square wave from the function generator when the chip is not powered, I get a spike on the leading edge of each pulse, but it is not Vsat and it is short-lived. Then when I turn on the power supply, there is some mess that eventually dies down to what I have without the power on. Changing the magnitude of the rails has no effect on the output. This is best case scenario, worst case scenario I start to smell burning chip and then there is no output at all. I've probably fried 6-7 chips now. The current from the power supply is less than 0.05 A, and I've tried a variety of voltage magnitudes for the square wave (also tried sin wave).

    Help!
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Any electrical device has two responses. Transient response. Steady state response.

    The transient spikes indicate transient response of the device. The transient response is how device reacts to sudden changes. Like transitions in the square waves, device suddenly switches from high to low (or from low to high), this sudden change causes device to act (transient response), once the device settles the transient response is gone and device enters steady state with its own steady state response.

    Usually the spikes are removed with smoothing capacitor.


    As far as proper operation of chips, normal people get information about proper operation conditions from datasheet. This way they don't burn their equipment. Join the normal people!
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Sounds like the op amp is operating open loop. That won't work since the open-loop gain of an op amp is typically well over 10,000. You need a design with feedback, such as this simple inverting circuit. The gain is -(Rf / Ri).

    [​IMG]
     
  4. shteii01

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    In that case: Gain=-(Rf/R1)
     
  5. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    As far as I know from the spec sheets, I am within operating conditions given the current and the voltage requirements.

    A simple comparator should not need feedback. Yes, it works in open loop, but since the output can't match or exceed the rails it should simply peg at plus or minus saturation (even though the AOL is ~200,000x). This is in every single one of my 4 op amp/ electronics/ instrumental analysis books.
    [​IMG]
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If you apply the signal with no power to the chip, it may blow the input. Add a 10k resistor in series with the output from the generator to help protect the op amp input.
     
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  7. nigelwright7557

    Senior Member

    May 10, 2008
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    You shouldnt be applying voltages with no power applied.
    This will latch up the 741 and probably destroy it.

    Why are you using an op-amp as a comparator ?
    They don't do the job as well as a comparator.

    You also need to decouple the power supply close to the 741 or you will get all sorts of odd glitches.
     
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  8. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    The course I'm teaching is in Instrumental Analysis (for Chemistry majors). I am having them build a comparator as the simplest example of what you can do with an op amp. A warm-up exercise, if you will. I also was hoping to add a bit of noise to the signal so we can work with hysteresis as well.

    I will definitely try this again without the input until the power is applied, but what do you mean about the power supply being decoupled?
     
  9. to3metalcan

    Member

    Jul 20, 2014
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    Run some capacitors from the op-amp's power-pins to ground...this keeps the induction of the wires/traces from "stalling" current on its way to the pins, which is important in high-speed operation where the op amp is swinging as close to the power rails as it can. .1uF is the usual value; a larger electrolytic (10-22uF) can also be added in parallel.
     
  10. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    OK. I spent some more time with this this morning and I made sure to:
    1) decouple the rails. This was already done as the voltage inverter I built required this anyway.
    2) use a resistor on the input from the signal source.
    3) not apply the source signal until the chip was powered (with a new chip... several new chips).
    I also double checked the spec sheet to make sure my input voltages and currents were within the tolerances (they are all WELL within the tolerances) and tried a straightforward 4x amplifier with feedback loop. I did this multiple times (my prof always said, when in doubt, rip it out!).
    I get NOTHING. No signal from any chip in any configuration. Is it possible that I got a whole batch of 741s that are defective? I did buy them on ebay. Are there any diagnostics I can run on the chips themselves to be sure that they are not faulty?
     
  11. crutschow

    Expert

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    What are the voltages of the two power supplies?

    Is the common for the two supplies connected to the common for the signal generator and whatever measurement instruments you are using? Everything must have a common ground connection.
     
  12. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    I am using one power supply with a +V in a voltage inverting circuit using an LT1054 chip. (That circuit has a 2uF capacitor between +V and ground and a 100uF capacitor from the -V output to ground, as suggested in the spec sheet for the LT1054.) The ground from the +V power supply is the ground I reference in all cases; for the LM1054, the UA741, and also all leads from the function generator and oscilloscope (BNC to red + black, black always connected to that common ground).
     
  13. shteii01

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    Feb 19, 2010
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    Very cute.

    crutchshow is asking for effing numbers, things like: 9 volts, 10 volts, 15 volts, 20 volts, 100 volts.

    I think I am done here. OP, good luck.
     
  14. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    OH I'M SORRY. I MISSED THE FIRST QUESTION AND ANSWERED THE SECOND.


    To answer the first question, I'm working with +12 or 15 V, reasonable values, and the inverter has a little loss, maybe 2-5%, so the negative supply is about -14.5V for a +15V. The function generator is 5V PTP, though I have tried much smaller voltages, ~10mV PTP. These are all well within spec: supply should be between +/-22V, input between +/-15V.
     
  15. MikeML

    AAC Fanatic!

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    741s are about the worst possible opamp you could select for this project...
     
  16. crutschow

    Expert

    Mar 14, 2008
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    Well, if it is connected up properly as you say (did you double check all the connections to the chip and verify the chip pin-out?), than it would appear you have bad op amps, even though I would think than unlikely. But can't think of anything else.
     
  17. crutschow

    Expert

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    True for this or any project. But it's cheap and for this demo it should be perfectly adequate. ;)
     
  18. bixbytj

    Thread Starter New Member

    Aug 14, 2014
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    It really IS only a demo, I'm not trying to build any type of circuit that will actually be incorporated into a device. The 741 is used a lot in teaching labs.

    I ordered a new batch from TI, hopefully that fixes my problem. But I guess it's time to come up with an alternative lesson plan in case I can't get it running.

    Thanks for your help! The advice was really spot on. I will let you know if the new chips help, or if I ever figure out what the problem is otherwise!
     
  19. shteii01

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    Didn't you say on one occasion that 741 needs 15, or was it 18 volts, to actually work?
     
  20. crutschow

    Expert

    Mar 14, 2008
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    It's specified at ±15V (which was the standard op amp supply voltage at the time the 741 was designed) but will likely work down to ±10V or less with possible reduced performance.
     
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