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Clipping in a transistor amplifier

  1. Sep 21, 2014 #1
    I need to understand how does an amplified signal gets clipped depending upon the biasing voltage level? It says that upon insufficient biasing, the output voltage becomes constant, but how?
    Please if you attach this link:-
    http://www.electronics-tutorials.ws/amplifier/amp_4.html
    I mention you that I've read it but didn't understand. Please help...
     
  2. jcsd
  3. Sep 21, 2014 #2
    The output of the last stage of amplification is bounded by the upper and lower power supply limits. If the input level is so large, the amplification is so great or the bias of any of the stages has shifted the output closer to one power supply rail than the other, there is always the possibility that part of the signal tries to exceed the power supply limits. When that happens part of the signal is cut off or clipped causing distortion as shown in the diagrams.
     
  4. Sep 21, 2014 #3
    But I wnat to ask that why it gets clipped? The inner mechanism,,,we obviously don't have a clipper set there...
     
  5. Sep 21, 2014 #4

    sophiecentaur

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    The "inner mechanism" does "obviously" involve a clipping mechanism. For a common collector config, there is a maximum voltage excursion (transistor not conducting), set by the Vcc and a minimum excursion, (transistor hard on) set by the Vss. With many amplifiers, the transfer characteristic, between those limits is near-linear so that will give clipping, top and/or bottom, depending upon where the input bias is set.
    This link, along with many others you can find, could help you.
     
  6. Sep 21, 2014 #5
    "Downward" clipping can also occur if the transistors are saturated. It depends on the circuit topology. In most modern autio amplifiers, the output stages are emitter followers, so clipping is most likely to occur when you "run out of" supply voltage. :)

    ERic
     
  7. Sep 21, 2014 #6

    jim hardy

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    Here's how to wrap your mind around what the guys are telling you.

    Take the circuit in your link to the extreme:
    Imagine that transistor as a switch instead of a linear device.

    When the switch is open, that is transistor driven to OFF state, what's voltage at the collector?
    Will driving the transistor harder OFF change that? Of course not, off is off.
    That's positive clipping.
    If not intentional, it's caused by applying too much input signal and driving the transistor OFF..

    When the switch is closed, that is transistor driven into saturation, what's voltage at the collector ?
    Will driving the transistor harder into saturation change that? Of course not, full on is full on.
    That's negative clipping.
    If not intentional, it's caused by applying too much input signal and saturating the transistor..

    A prudent circuit designer arranges his bias circuit to handle as much signal as he can. He'll clip top and bottom of the waveform at about same amount of input signal.

    Your link sorta explained that the other direction.
    Halfway down it in paragraph "Amplitude Distortion due to Clipping" is a nice diagram. That'd be a good job of biasing , it clips both top and bottom about same signal amplitude.
    As author noted though, musicians like distortion...

    hope i didn't belabor the obvious. Sometimes exaggeration is a useful thinking tool.

    old jim
     
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