Harmonic Distortion

Discussion in 'General Electronics Chat' started by Mr. Hiyasaki, May 7, 2008.

  1. Mr. Hiyasaki

    Thread Starter New Member

    Mar 24, 2008
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    I have a question about Harmonic Distortion.

    I designed a three stage bjt amplifier for a class project and to simplify my calculations I grounded the emitter on the second stage. This had the expected affect of killing my gain on the first stage and I accounted for that. However, I also thought that doing so would add a lot of harmonic distortion. When I simulated it and constructed it in lab I ended up with a THD of about 0.5%. I can't think of any reason why this is so low. I was wondering if you had any suggestions on what I could investigate to figure out what is keeping the harmonic distortion down.
     
    Last edited: May 7, 2008
  2. Wendy

    Moderator

    Mar 24, 2008
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    Standard advice, post the schematic. Otherwise it is speculation.
     
  3. Mr. Hiyasaki

    Thread Starter New Member

    Mar 24, 2008
    3
    0
    I edited my post in include a pdf of the schematic, the sch file and a netlist for the circuit
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your transistors are not biased correctly. The different hFE of each transistor and lack of negative feedback will cause many transistors to be either saturated or cutoff. The temperature will also affect their biasing.

    I can't see the value of the base resistor for Q1 because the numbers are crammed together. The voltage gain of Q1 is only about 3.

    The values of the coupling capacitors are rediculously high.
     
  5. Mr. Hiyasaki

    Thread Starter New Member

    Mar 24, 2008
    3
    0
    Audioguru, I was thinking the same thing. However, I ended up building the circuit anyway and they were operating in active mode despite the terrible biasing.

    As for why my coupling caps are huge is because when I constructed the circuit those were the smallest caps available to me and given the time constraint I was faced with, I was unable to go out and buy something reasonable. The assignment was due on Monday, but I never could explain why my THD was so low and it has kinda been bugging me.

    The base resistor is 2.4MEG

    The design specifications required a gain of 50 and an input impedence > 200k with a harmonic distortion < -30dB driving a 50Ohm load. I was able to meet all the the specifications with this terrible design, its just that originally going into it I had anticipated a whole lot of harmonic distortion because the emitter on Q2 is grounded. However when I measured the harmonic distortion it was 0.42%. I was wondering why my assumptions were wrong in this case.





    I think figured it out. The Q2n2222 is very happy with a small input signal. The spec called for a 2.5mV input signal at the source, and since my first stage actually attenuates I am only pumping in a little over 2 mV into the second stage, so despite the emitter being connected directly to ground the transistor is still able to operate linearly because of the extremely small input signal. Grant it this design isn't practical at all as the first stage is completely worthless in the whole scheme of things. I figured this out when I increased the input signal to a higher amplitude and noticed that the distortion became pretty obvious very quickly.
     
    Last edited: May 7, 2008
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