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Homework Help: Transistor Problem

  1. Jun 20, 2012 #1
    The circuit is shown below and i am asked to find out the voltage gain A(v)

    Beta = 100 and Vcc = 13.7 volts Common emitter configuration.

    Voltage Gain is equal to the ratio of output voltage to input voltage .

    3. The attempt at a solution

    Don't know how to proceed as there wasn't any v(i) or v(o) given in the equation, how should i start ?

    Attached Files:

  2. jcsd
  3. Jun 20, 2012 #2

    If you input 1 volt, what would the change in output voltage be?

  4. Jun 21, 2012 #3
    if i would know what will be the currents across 100 kohms, 12 kohms and 10 kohms i can use kvl including v(i), but then i wouldn't know what v(cb) is ? Voltage gain is ratio of output to input right then how am i supposed to know output voltage since beta is multiplied with input current giving i(c) value which will passed through load resistant to give output voltage ! i think thats how single stage amplifier works ?
  5. Jun 21, 2012 #4

    Why are you worrying about the current through the 100k resistor? Most all your signal current will exist the the base-emitter path. If you know the current change in the base circuit, then you know the current change in the collector, right? And when you know the current change in the collector, then you know the voltage change across the 12k resistor.

  6. Jun 21, 2012 #5
    ya but how should i find that base current ? and without considering 100 kohms i have to consider v(ce) for kvl ! isnt that so ?
    or else how would i be able to calculate i(b) ?
  7. Jun 21, 2012 #6

    I told you already. The 100k value is so much higher than the base-emitter resistance that almost no signal will flow through it. So it can be ignored for small signal analysis.

  8. Jun 21, 2012 #7
    i got the solution right now and the person is considering 100 kohm as the part for conduction i am posting the solution over here please help , i am not able to understand !
  9. Jun 21, 2012 #8
    here it is !

    Attached Files:

  10. Jun 21, 2012 #9

    Base current change in the base-emitter junction for 1 volt input will be 1/10k = 0.1ma. Multiply base current by beta gives 10ma collector current change. Multiply collector current by 12k gives 120volts. So small signal gain is 120/1 = 120

  11. Jun 21, 2012 #10
    but A(v) is 10
  12. Jun 21, 2012 #11

    So where did I go wrong? Find my mistake.

  13. Jun 21, 2012 #12
    do we calculate the voltage across Rc or across load resistance that will be emitter resistance i guess ? And isn't the 13.7 volts the biasing voltage ! won't it affect the base current in some manner ?
  14. Jun 21, 2012 #13
    plus we are just assuming it to be having 1 volt input signal and considering it small signal analysis giving linear model ! can't it be large signal in which each gap in Ib will create non linear effects in Ic!
  15. Jun 21, 2012 #14
    according to the solution given to me i wasn't able to understand the equivalent circuit the person has drawn !
  16. Jun 21, 2012 #15

    What is there about my analysis you don't understand?

  17. Jun 21, 2012 #16
    ya i understood it !
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