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

Determining h parmaters of bjt practically

  1. Oct 22, 2014 #1
    so i know that h11 is defined as v1/i1 when v2=0 and so on...but the thing is how do i do it practically...my professor told me that v1,i1,v2,i2 in h parameter equations are instantaneous values of current and voltage of ac in a bjt. he even wrote that to get h11, you have to set v2 of ac=0. but since, the lecture was a recorded one, i cudnt get it cleared.i have the following doubts-
    1) how can v2 of ac be set to zero....if at all, say in ce config, i short the c and e terminals that is replace load resistance by a short circuit, how is it practically possible...
    2) for h12, i will have to set i1=0 ie. in ce config, i will have to set i base as zero. which means i collector=0. which means that i2=0 which means h12 is infinity. not possible!!
    3) he told me to set v2=0, we use a capacitor... and for i1=0 we use an inductor at the base side...didnt get it.
    4)on further reading on the net, i read that ib is kept constant so that i1 becomes zero..

    i am all confused.pls help me asap
  2. jcsd
  3. Oct 22, 2014 #2
    h-parameter are only used for small-signal analysis and designing a amplifier circuits. And don't forget that all h parameters are AC parameter.
    So 0V means 0V ac voltage at given point. v2 = 0 V means that the collector AC voltage is 0V (Collector is short via large capacitor to ground).
    Capacitor is a short for AC signal and coil is a open circuit for AC.
    But don't worry too much, in real world we don't use h-parameters to design an amplifier. Only EE students are forced by their mad professors to use it.
  4. Oct 22, 2014 #3


    User Avatar

    Staff: Mentor

    Jony has explained it well.

    1) shorting collector to ground using an appropriately large capacitor will set vce 0, for determining hie.

    2) an appropriate inductor (RFC) is series with the base will still allow biasing current, but present an AC impedance →∞ . You then couple a signal generator to the collector and measure what fraction of its voltage appears at the base, for determining hre. Yes, you can apply input to the terminal you normally think of as output. Just keep the signal voltage low and Q1 won't be offended. ;)
  5. Oct 24, 2014 #4
    thank you people....nascentoxygen JONY[]....helped me a lot....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook