Discrete Op Amp circuit does not work properly

Discussion in 'General Electronics Chat' started by Robin Mitchell, Jan 17, 2015.

  1. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
    732
    199
    Hi all,

    The circuit shown below is what I have constructed
    [​IMG]
    Now firstly, the -Supply is connected to ground with a supply voltage of around 16V. Now I have replaced the BC549 with both a BC548 AND a 2N3904. I have tested this circuit in LTSpice with different transistors and it works as expected.

    So why in real life does it not? If I connect the output to the inverting input I should have a unity gain buffer. But what I get instead is a unity gain buffer that matches the input up to 1.8V or so but after that point up to some higher voltage the output is constant. Then when the input approaches the supply voltage the output quickly shoots to meet the current input voltage. Its like there is a range of values for which the output just stays fixed. Any reason why? Does this circuit only work in theory or am I just mad?


    All the best,
    Robin
     
  2. #12

    Expert

    Nov 30, 2010
    16,268
    6,780
    I don't see what's wrong, and I'm good at these. Of all the competent people here, I wouldn't expect you to make a mistake in assembly, but that's all I have.

    I see a KI source of about 8 to 10 ma, sufficient current in the differential pair, your Miller cap, and correct logic. Maybe if you look into the frequency range with a scope and see if it's oscillating? A lot of perfectly good looking designs are not, "unity gain stable".
     
    Last edited: Jan 17, 2015
  3. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
    732
    199
    I built this on a PCB twice and one in breadboard with all the same results. I bought some chinese transistors (BC559 and 2N3904) and I am thinking that they could be the issue. Having said that a transistor is a transistor and all my other circuits have worked fine :/

    It works in LTSpice AND the author has said it works but I just cant understand???

    I will build it yet again tomorrow but this time using a split supply to see if its a single rail issue.

    Also, thanks for the confidence in my building skills!

    All the best,
    Robin
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    When the circuit is not operating properly, measure all the node voltages and report them here and/or compare them with the simulation for the same conditions.
    That should help tell what is wrong.
     
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