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Q point and distortion

  1. May 26, 2012 #1
    What does it mean? Why should it the that signal be symmetrically across the Q point. I really don't have any idea about what's going on.

    What is actually a Q point? How do we get distortion?
  2. jcsd
  3. May 27, 2012 #2


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    Staff: Mentor

    Q stands for "quiescent". It is the operating point at which we choose to bias the transistor. A good general choice would be where it allows the output to be driven as far in one direction (say, more positive) as in the other direction (viz., less positive, or negative) before distortion sets in.

    Distortion? What happens if the operating point is driven too far along the load line?
  4. May 27, 2012 #3
    Distortion also happens when the transfer function from input to output of the device is not linear.
  5. May 27, 2012 #4


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    Science Advisor

    If you connect up a simple NPN amplifier and vary the input voltage, this is what you get:

    http://dl.dropbox.com/u/4222062/NPN%20bias%20point.PNG [Broken]

    Input voltage shown along the horizontal axis. Output (collector) voltage shown vertically.

    So, if you arranged for the input voltage to be at the orange line (just below 0.76 volts input) then the output would be about 5 volts.
    Can you see that if you varied the input between the red lines the output would vary between 3 volts and 7 volts? And smaller shifts would result in smaller output. So, you could get relatively undistorted output.
    This means the output is the same shape as the input, but bigger.

    Notice that the output change could be up to 4 volts (peak to peak) but this is being produced from an input of about 0.02 volts peak to peak. So we are getting a voltage gain of about 200.

    Notice, too, that this won't happen unless we arrange for a voltage of just below 0.76 volts to be on the base before we apply a signal. So, the circuit will need to be a little more complicated than the one shown above, but not much more. Just a resistor or two.
    Last edited by a moderator: May 6, 2017
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