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Effects of dc beta(hfe) in BJT characteristics

  1. Sep 16, 2012 #1
    I need help in answering the post lab questions given to us by our professor.

    1. Is dc beta constant on every point of the characteristics? What is its effect in the linearity of the transistor?

    2. What is the effect of a high dc beta on the characteristic curves?

    I have attached an image of The bjt configuration.

    Our data shows that dc beta isn't constant, but I can't see its effects in the linearity. I don't even know how to define linearity for a three terminal device like the BJT, but I guess that it must be based on Vce and Ic. Even if my guess is right, beta only relates Ib with Ic , and I can't think of any effects on linerarity and the characteristics.Applying KVL to the output side of the configuration produces a linear equation relating Vce and Ic. I can't really find an answer.
    Please reply. Thanks in advance!!
  2. jcsd
  3. Sep 16, 2012 #2
    I can't upload the file, I made it with paint but it exceeds the limit for file size.

    The bjt configuration has a variable dc source and resistor connected to each of the collector and base
  4. Sep 16, 2012 #3


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    Science Advisor

    Reload your file with Paint and save it again as a .PNG file.

    It should then be a lot smaller than the .BMP file you probably had before.

    However, you should be aware that we can't directly answer coursework questions.

    But what effect does the variation in DC current gain have on the collector current for different input voltages? Your input signal will usually be a voltage, not a current.
  5. Sep 17, 2012 #4
    In our experiment the base current was considered as the input, and the collector current as the output.The voltage Vbb being fed across the base and emitter was controlled to establish a fixed base current. We set the base current to be equal to 50 micro amps, then 100 and 150. Then A voltage source Vcc was fed across the collector and emitter. We measured the voltage across the 0.33 kilo-ohm resistor to calculate Ic and then the beta.

    From our data it seems that, beta increases with Vce and Ic, but I really need to know how and why. I reallt do not know what's next.
  6. Sep 17, 2012 #5
    Here's the image

    Attached Files:

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