Basic BJT amplifiers

Discussion in 'General Electronics Chat' started by full, Oct 3, 2014.

  1. full

    Thread Starter Member

    May 3, 2014
    225
    2
    Hello everybody,

    I study basic BJT amplifiers , and I can change the DC Bipolar Junction Transistor to AC Bipolar Junction Transistor......

    but there is problem I don't know how I can do loop in AC Bipolar Junction Transistor....

    please can you help me, how I can do loop in this circuit AC?

    thanks,
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    there is no ac bjt, BJT refers to bipolar junction transistor, not bipolar. have you read your thread on bjt posted on monday? the bipoolar refers to n type and p type materials, not an ac operated transistor. transistors operate off dc, not ac.
     
  3. full

    Thread Starter Member

    May 3, 2014
    225
    2
    thanks sir,
    if I change the (npn common emitter) Bipolar Junction Transistor figure 6.28 to figure 6.29 , what is mean?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the first one is the schematic reperresentation, how the circuit is built, the second is the mathematical representation.
     
  5. full

    Thread Starter Member

    May 3, 2014
    225
    2
    nice ,
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    In figure 6.29 you have a small signal mode . A DC circuit that represent/simulates the behavior of the transistor for AC signals.
    So simply use a Dc network analysis techniques. For example a KVL ; nodal or mesh and solve it.
     
  7. full

    Thread Starter Member

    May 3, 2014
    225
    2
    thanks sir,
    please can you do KVL for figure 6.29...
    because I when understand how I can do in another time ......
    thanks ,
     
  8. full

    Thread Starter Member

    May 3, 2014
    225
    2
    if I change the (npn common emitter) Bipolar Junction Transistor figure 6.28 to figure 6.29 , what is mean?
    what happen if I change from figure 6.28 to figure 6.29?
    why I change the circuit from figure 6.28 to figure 6.29?

    thanks,
     
  9. full

    Thread Starter Member

    May 3, 2014
    225
    2
    why in circuit figure 6.33 (gm v-bi) and in figure 6.28 (B vb)?

    thanks,
     
  10. full

    Thread Starter Member

    May 3, 2014
    225
    2
    now I understand :)
    thank you
     
  11. full

    Thread Starter Member

    May 3, 2014
    225
    2
    the AC analysis circuit in figure 6.33 very difficult :confused:

    please can you help me how I can see the circuit and analysis?

    ScreenHunter_m.jpg
     
  12. full

    Thread Starter Member

    May 3, 2014
    225
    2
    I have small question, what different between (gm v-bi) &(B ib )?
    what meant by transconductance ?
    when I use (gm v-bi) and (B ib ) in circuit?
    is mean the (gm v-bi) = (B ib ) same in calculation?
    thanks:)
     
  13. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Hi, do you know anything about dependent sources ? Because we can model/represented the BJT in two equivalent ways.
    Ordinary we treat BJT as a current controlled current source. The collector current is beta times the base current Ic = β * Ib.
    And the current gain is β = Iout/Iin = Ic/Ib
    http://forum.allaboutcircuits.com/threads/common-emitter-amp-doubts-and-questions.91084/#post-663195
    But sometimes we use alternative equivalent model but this time transistor is represented as a voltage controlled (Vbe) current source (Ic).
    So the gain is equal to Iout/Vin = Ic/Vbe = transconductance = gm in unit, Siemens (S)
    gm = Ic/Vbe and also notice that Vbe = Ib * rpi.
    gm = Ic/Vbe = Ic/(Ib * rpi) = β/rpi
    http://www.ittc.ku.edu/~jstiles/412/handouts/5.6 Small Signal Operation and Models/BJT Small Signal Parameters lecture.pdf
    http://www.ittc.ku.edu/~jstiles/412/handouts/5.6 Small Signal Operation and Models/Steps for Small Signal Analysis lecture.pdf
     
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