Op-amp voltage followers/buffer problem.

Discussion in 'The Projects Forum' started by JingleJoe, Nov 26, 2011.

  1. JingleJoe

    Thread Starter Member

    Jul 23, 2011
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    I'm having alot of trouble making the standard op-amp voltage follower/ buffer work.
    I've had them working before absolutely fine, but a few weeks ago they just stopped working for me :confused: Do they hibernate in the winter or something? I suspected I had a bad chip so changed it, but to no avail.
    I really have no idea what could be wrong with them. My wiring is fine as far as I can tell.

    I'll elaborate a bit more so that one may be able to help me figure out whats up with my op-amps;

    I'm using this circuit
    [​IMG]

    Observed problem: The output goes to about half Vs and stays there.

    Tech info
    Vs: 10V
    chip: LM324 (I know it's not very good but it's all I've got and should work for this)
    input: sawtooth wave, about 1.5V P-P, biased to about half Vs.
     
    Last edited: Nov 26, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, if you are using those pin numbers with the LM324, that's your problem.

    Here's the pin numbers and functions:

    [​IMG]

    The pins for 4-channel opamps are fairly standard, but always check the datasheet to make certain; you can get bitten if you make assumptions.
     
  3. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    Oh, not at all! I didn't even see those tiny numbers :rolleyes:
    I go by the schematics where I can, not numbers, my circuit is wired up correctly going by the schematic.

    I changed the image to avoid any further confusion.
     
  4. Yako

    New Member

    Nov 24, 2011
    245
    2
    I don't agree with that. LM324 is pretty good.

    Much like the LM339, the production of them was HUGE, so they are cheap because there is an abundance of them. They fall into armature hands and earn a bad name.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    This is why we need to have complete and accurate schematics of the circuit in question.

    At this point, without further evidence, I'll have to suspect wiring (such as solder balls or whiskers, stray strands of wires, etc) or excess heat applied when soldering, PCB not etched all the way, too much heat used when soldering resulting in damage to the IC, or something similar.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The LM324 beats the heck out of a 741. It's old and slow, but still - it can sense and output down to the negative rail, so it's very handy for single supply use. If you understand its' limitations, it is still a useful opamp.

    You don't have to use a Ferrari to pick up the groceries.
     
  7. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    Armature? Do you mean amateur?
    The LM324 chip has been fine for my circuits in the past, I'd recommend it, however I hear that it's not very good with high frequencies among other things.


    It's on breadboard and I didn't even draw a schemtic because I know that simple voltage follower off by heart ;) I thought perhaps there may be some common culprit for non-functioning op amp buffers or something obvious to someone more experienced than I.
     
  8. Yako

    New Member

    Nov 24, 2011
    245
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    What sort of projects have you made using the LM324 man?
     
  9. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    Oh all kinds of things, mostly simple amplifiers, buffers and comparators though.
    I've used them in everything from a small robot to a sound machine which used a 555 timer too.

    ... Just wish I knew why they've suddenly stopped working when used as a buffer :confused:
     
  10. Yako

    New Member

    Nov 24, 2011
    245
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    Can you be specific about your complaints with this IC?
     
  11. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    I don't have any (apart from this voltage follower thing) but on half the websites I read people only have bad things to say about it; like how it has bad frequency response above 5kHz or that it's output is very distorted. I haven't used it for anything that required critical accuracy or low distortion so I have not had any of these problems.
     
  12. Yako

    New Member

    Nov 24, 2011
    245
    2
    Don't believe everything that you read. Why don't you hook up a signal generator to a simple CRO and find out for yourself.
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Please post the complete schematic showing how it is biased.
    Show the input coupling capacitor value.
    Show the frequency of the triangle wave.
    What is the load on the LM324 opamp?
     
  14. Yako

    New Member

    Nov 24, 2011
    245
    2
    Someone knows what they're talking about there.
     
  15. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Look the problems of the lousy old LM324 quad opamp and its LM358 dual opamp sister:
    It is low power so its output transistors do not have enough bias current which causes horrible crossover distortion.
    Its low power slows it down a lot.
    It is noisy (hiss).
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    That's all well and good, but the voltage follower is still not working.

    Until we get a full schematic - and most preferably a photo of the breadboard - it's going to be difficult to make any progress on resolving the issue.

    No matter how poor the frequency response of the LMx24 series is, in a voltage follower configuration the output should still resemble the input.
     
  17. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    I had input a saw/ramp wave acctually, but have and will use this circuit for triangle waves too.

    The wave in question is the charge and discharge of a capacitor in a schmitt trigger oscillator utilizing a 4093 NAND gate with built in schmitt triggers on the inputs (Sgt wookie told me about this one ages ago acctually!;)) This circuit has an added transistor to allow for voltage control of the frequency.
    See attachment for circuit diagram.
    The simulation doesn't work quite right (oops just noticed that's because my transistor is backwards!) but it works in practice, I probed around with my o-scope and it functioned correctly after the voltage follower was added, but I get no output from the voltage follower.

    P.S. the supply in the diagram reads 5 volts but I used 10.
     
  18. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The LM324 can take inputs down to the negative rail, but has a restriction that the common-mode voltage has to be below V+ -2V. Could that have something to do with your problem? See datasheet: http://www.national.com/ds/LM/LM124.pdf
     
  19. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    It is a horrible circuit:
    1) Its transistor is upside-down.
    2) The 4093 quickly charges the capacitor through the diode but nothing discharges the capacitor except the backwards transistor acting like a diode.

    Maybe you should make the voltage-controlled oscillator shown in the LM324 datasheet. It has square and triangle outputs. Adding a diode and resistor can convert the triangle into a sawtooth waveform if you want.
     
  20. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    I did point that out, I only made the schematic because you asked for it, otherwise I knew what I was doing and put everything together correctly.
    Yup, thats about right.
    Looking at that circuit and many similar, they seem to impliment a schmitt trigger made from an op amp, which this "horrible" circuit does but with a logic gate with a built in schmitt trigger, ordinarily people seem to like to use the 40106 for this kind of oscillator but I only had some 4093's to hand.
    I don't see whats so horrible, it's simple and effective if you ask me :p

    Yes I'm pretty certain it was within the limits; my wave being around half Vs (supply of 10V, half = 5) with a peak of about 1.5, makes a maximum of 5.75 a lower end of 4.25 (correct me if I'm wrong)
     
    Last edited: Nov 26, 2011
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