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Transistor regions of operation

  1. Mar 11, 2007 #1
    here's some newb questions for ya:

    if you apply an AC signal to the base of a transistor (DC sources and coupling capacitors all included in the circuit to treat it like an amplifier), and on your output (Vce in this case) you get clipping on the negative side of your sinusoidal waveform, what two regions would this be operating in?

    I want to say Vce saturation and Ic saturation, but once again I'm a newb.

    Now what about if there is clipping on the positive side of the sinusoidal waveform? Cutoff?
     
  2. jcsd
  3. Mar 12, 2007 #2

    berkeman

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    Thread moved to homework forums.

    The three regions are traditionally called cutoff, linear and saturation. You are correct that when the bottom of the output sine wave is clipping, Vce is as low as it can go, and this is called saturation. But as the waveform comes back up out of saturation, what is that region called?
     
  4. Mar 12, 2007 #3
    When it comes back into the region of operation, it would be in the active region.

    Is that just another name for "linear"??

    so when only the bottom is clipping:

    it is partially in the saturation region and partially in the linear region.

    when only the top is clipping:

    it is partially in the cutoff region and parially in the linear region.
     
  5. Mar 12, 2007 #4

    berkeman

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    Yeah, sorry. I misremembered the term for the middle region. "Linear" is not the best description -- "Active" is the regular term. My bad.

    BTW, when I pulled out my old transistor book just now, I saw that there is a 4th forward bias region that we haven't talked about yet. Quiz question -- what is that 4th forward bias region called, and what is happening there?
     
  6. Mar 12, 2007 #5
    well I couldn't think of it off the top of my head, so I did some research and found out it's called the "Reverse region". I couldn't find a straight answer for what's going on, but it has something to do with the fact that transistors are not semmetrical (the doping levels are different for the collector and emitter), and it has something to do with reversing the roles of the collector and emitter...

    oh yeah and I found plenty of sites that claim this region is not very useful...hmm...maybe it's similar to reversing a bias of a diode
     
  7. Mar 13, 2007 #6

    berkeman

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    Yeah, there is a reverse region if you run the transistor upside-down. But I was referring to the forward breakdown region, where the fan of Ice/Vce lines goes to the right on the plot, and then turn straight up due to Vce breakdown. :smile:
     
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