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How to avoid DC bias on output?

  1. Apr 10, 2013 #1
    I designed the BJT amplifier below. After substituting in for the values of the actual resistances measured in lab, I reran the simulation. The hand calculation I derived for the gain was nearly spot on with what I was seeing in Multisim.

    Now, when in lab I get approximately the same value I should. The only issue is that I seem to have some sort of increasing DC bias on the output. The output will first start out at around 2.11V and slowly creep up until around 3V.

    What can I do to avoid this problem? I thought perhaps by adding a larger capacitance, but it did not change much.

  2. jcsd
  3. Apr 10, 2013 #2

    jim hardy

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    Sounds like C2 is installed backward.

    At 200 uf it has to be electrolytic.....
    Its + side should go to the node of Q1 & R2 (your point 12), its - side to R4 (your point 3).
  4. Apr 10, 2013 #3


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    if C2 was backward and passed DC for a good bit, it's likely fried. then get a new C2 and put it in correctly.
  5. Apr 10, 2013 #4


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    The size of the resistors in the circuit will limit the leakage current to less than a milliamp, so it's unlikely you destroyed it.

    But after you connect it the right way round, it might take a little while before the capacitance and leakage current settle down to stable values (the chemistry going on inside the capacitor needs to get back to normal). If your measurements still seem to be drifting, just leave it running for an hour and then see what it's doing.
  6. Apr 10, 2013 #5
    C2 and R4 have a time constant of around 4 seconds. You will have to wait for several seconds for DC voltages to settle (and change)
    What is R4? (20000ohms?)
  7. Apr 10, 2013 #6


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    yeah, you're right. 20K ain't much of a load.

    i've seen electrolytics literally explode when stuck in backwards. it ain't purty.
  8. Apr 11, 2013 #7
    I am always unsure when it comes to wiring up electrolytic capacitors. How can you be certain which way to orient them?
  9. Apr 12, 2013 #8


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    They have markings on them that indicate the positive and/or negative terminals




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  10. Apr 12, 2013 #9
    If you are working with DC then it's easy. What about when you implement them in AC design? Or AC biased with DC as in the BJT amplifier above?
    Last edited: Apr 12, 2013
  11. Apr 12, 2013 #10


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    When in doubt, a multimeter will confirm the DC polarity.
  12. Apr 12, 2013 #11

    jim hardy

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    Observe on your schematic that, in absence of signal, point 12 will be positive about five or six volts (Vsupply minus Icollector R2)

    and since the capacitor should block DC, point 3 will be zero.

    So the cap should go + side to 12.

    When installed backward , electrolytic capacitors don't block DC at all well. They'll 'leak' in the milliamps range. They eventually lose capacitance and some chemistrys like solid tantalum will even catch fire. (I was worried there might be one in my pacemaker that got recalled.....)
    When installed forward they do a good but not perfect job - DC leakage might be as much as a microamp ..

    look to cornell dubelier website for their capacitor tutorials. Like everything else they are a world unto themselves...
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