Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AC Emitter resistance.

  1. Nov 19, 2011 #1
    Hello experts!!

    AC emitter resistance is given as,
    rE'=25mV/iE

    My question is that what does this 25mV tells us? and where does it come from?

    Thanks in advance.
     
  2. jcsd
  3. Nov 19, 2011 #2
    Vt = 25 mV is a thermal voltage at room temperature, about 20 degrees Celsius I believe. This value changes with temperature.

    It comes from semiconductor physics where you try to describe a PN junctions diode, you arrive at Shockley's equation which describes its IV characteristics. It turns that the Shockley's equation is highly non-linear.

    BJT transistors are composed of two such diodes which also behave non-linearly. So ultimately, when you try to describe the behavior of BJT transistor amplifier, it will be non-linear. But because it's difficult to deal with non-linear equations, the diode equations have been linearized by making some assumptions into what is called the small-signal model. As a results a couple of new parameters come into play, such as r_e, r_pi, g_m.

    When you work this out, Vt makes its way into the linearized model and that's why see it.

    For more info check out: Sedra/Smith.
     
    Last edited: Nov 19, 2011
  4. Nov 20, 2011 #3
    Thanks for reply.

    You mean it is thermal voltage at room temperature and it varies if temperature varies. Am I right?
    It is the voltage assumed on 20°C? Am I right?
     
  5. Nov 20, 2011 #4
    Thermal voltage temperature is at 300 K, so that is about 27°C (room temperature)
     
    Last edited: Nov 20, 2011
  6. Nov 20, 2011 #5
    OK. Thanks a lot. :approve:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook