Common Emitter Amplifier Design Help

Discussion in 'The Projects Forum' started by just_gaze, Jan 4, 2016.

  1. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    Hi all,

    I am designing a simple common emitter amplifier for an audio application. The circuit consists of a PN2222a transistor, base resistor, and collector resistor. The first attachment shows my calculations for the component values, along with voltages and currents measured using both a SPICE simulation and circuit build. The build is shown in the second attachment. Vin should be 68.3mVrms, 20Hz. It got cut off in the picture.

    The measurements from the circuit do not match up with the simulation. For example, collector voltage is 8.56 V, where it is 6.23 V simulated. There is only a .14 V drop across the collector resistor. It should be much higher than that, and I cannot figure out why it is so low. Also, DC collector current is only marginally larger than base current. Output voltage is 0 Vrms. I measured the DC voltages without any AC going into the circuit, which I believe is the correct way to do it.

    I understand that Vc should be about half of Vcc to allow for voltage swing, and also that Ic = Ib x Hfe. For a NPN to be in forward active mode, Ve < Vb < Vc. I do not think the circuit I built is indeed in forward active mode, because it does not seem like any significant amount of current it being passed through to the collector.

    Can anyone find a mistake in my calculations or circuit build?
     
  2. sailorjoe

    Member

    Jun 4, 2013
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    First look, it seems like you're bias currents aren't yet in the linear range for the transistor. Now, I haven't checked the datasheet, but you only have a half volt across the base-emitter junction. Shoot for more like 700 mvolts minimum. Your quiescent current of only 1 ma seems too low as well. I'd start by redoing the calculations with perhaps 5-10 ma. Even better, put yourself into the middle of the linear region and split the voltage between the collector resistor and the transistor collector to emitter, by selecting the right collector resistor and base current. Lastly, Hfe can vary a lot from one transistor to the next, so plan for a minimum value. Is 200 the min for 2N2222? You're seeing how simulators use idealized models of semiconductors, and under some conditions don't model very well. I think your transistor is at or near cutoff, a nonlinear region of operation that is hard to model. Plus, your simulator simply doesn't model all the intrinsic parameters of your specific transistor, so results will be different. Actually, this is a great lesson in learning the limits of modeling, and how the results have to be taken with a certain skepticism until verified with a real circuit.

    Incidentally, have you measured your battery voltage? As it drops, your transistor drops out of forward bias. Another thing your simulator won't show.
     
    Last edited: Jan 4, 2016
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  3. Bordodynov

    Active Member

    May 20, 2015
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    You do not write uA. I think you are reversed between the emitter and collector.
     
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  4. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    Thanks for the help. It turns out I had the emitter and collector reversed! It's actually hilarious considering how much time I have spent pulling out my hair over this. Now I have the transistor in linear mode, with emitter grounded, base at 0.7 V, and collector at 3.6 V. Yet, the DC current only raises from .56mA at the base, to .62mA at collector. So, it's basically at unity gain, which is not what I am shooting for...
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    Your bias scheme makes the collector current and therefore the collector voltage dependent on the current gain of the transistor. Current gain can vary greatly from transistor-to-transistor and with temperature. That would explain the difference between expected and measured collector voltage.
     
  6. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    But a current gain of 1? There must be something wrong with my biasing, because gain for the PN2222a is minimum 50.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    You circuit is too simple. Its bias point and AC gain depends upon the transistor gain, and it will have high distortion without an emitter resistor to give some negative feedback.
    A configuration such as this is much better.
     
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  8. sailorjoe

    Member

    Jun 4, 2013
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    I don't quite understand what you're measuring. How did you change the base current? Did you raise it from its quiescent state X and increase it by 0.56ma? Did you raise it from zero? That's a big delta on base current.
     
  9. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    The configuration you attached is one that I have seen most often. Right now, I am just trying to get any signal at all to pass through the transistor. I'm just using the simplest configuration possible to make it easier on me, but that will be the next step.
     
  10. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    I really just used trial and error (bad practice, I know), when selecting the two resistors, in order to apply the correct voltages to the transistor for forward active. I ended up with emitter at .5V (even though it's connected straight to ground...), base at 1.2V, and collector at 4V. My AC input voltage actually amplified by a factor of 10, undistorted. Yet, DC currents are .56mA at base, and .62mA at collector. I don't see how this is possible. Maybe it's a measurement error? My AC voltage is amplifying.
     
  11. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    Furthermore, how do I find the correct quiescent current, since this is the first thing you need to know when figuring everything else out? I do not see anything referring to quiescent current on the datasheet (attached). So, it is the current coming out of the collector when the circuit is not powered? Does this mean you just plug in a transistor and measure it dry, to find quiescent?https://www.fairchildsemi.com/datasheets/PN/PN2222A.pdf
     
  12. crutschow

    Expert

    Mar 14, 2008
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    The quiescent current is not specified in the transistor data sheet since the quiescent current is mainly a function (or should be) of the circuit design, not the transistor, for a well-designed circuit.
    For small signal AC amps the quiescent current is usually in the neighborhood of a mA or so per stage.
     
  13. sailorjoe

    Member

    Jun 4, 2013
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    If you're measuring .5 volts at the emitter, and the emitter is at ground, then you aren't measuring what you think you are. So all your other readings are suspect until you get that one right.
     
  14. sailorjoe

    Member

    Jun 4, 2013
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    Quiescent current is what you get through the transistor due to DC biasing, so it can't be in the datasheet, it depends on your circuit. If you want to try something, replace your resistors with variable resistors and adjust the, and see what happens.
     
  15. Colin55

    Member

    Aug 27, 2015
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    Your circuit is wrong.
    You should use a SELF-BIASED CE stage with 2M2 and 4k7 and see if the collector voltage is about 4v.
    See talkingelectronics.com website under THE TRANSISTOR AMPLIFIER to see how to design this type of stage as no-one here seems to know anything.
     
  16. just_gaze

    Thread Starter New Member

    Jan 3, 2016
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    I have not yet implemented negative feedback into my circuit, but everything I have read about circuits has said it is equally advisable to place a feedback resistor between emitter and ground, or between base and collector. I'm not sure self-biasing is the only way to go.
     
  17. Colin55

    Member

    Aug 27, 2015
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    "I'm not sure self-biasing is the only way to go."
    Well, don't bother me any more.
     
  18. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Colin,

    Only 60 posts here and I already have enough of your material to make a "best of Colin montage". Thank you.
    By the way, if you are ever in Toronto, I would love to introduce you to someone with a similar sunny personality.
     
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  19. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Can you post an updated schematic with the resistor values you used to get these measurements?
     
  20. crutschow

    Expert

    Mar 14, 2008
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    That includes yourself I presume. :rolleyes:
     
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