CE Amplifier - Manually Computed AC Analysis Versus LT Spice

Discussion in 'Analog & Mixed-Signal Design' started by elec_eng_55, Dec 6, 2018.

  1. elec_eng_55

    Thread Starter Member

    May 13, 2018
    129
    0
    Hello:

    I have attached a schematic for a Common-emitter voltage amplifier. I have also
    attached an ac analysis that I computed manually. My results and LTSpice are not identical. Does anyone know which results are correct and why the difference?


    Assume hfe = 100, Rs = 50Ω

    re’ = 25 mV / 1.01 mA = 24.75Ω

    rc = RC║RL = 4KΩ║10KΩ = 2,857Ω

    Av = rc / re’ = 2,857Ω / 24.75Ω = 115.43

    Rin = R1║R2║hfe * re’ = 829.93Ω║24.75Ω = 621.52Ω

    Av-Divider = Rin / Rs + Rin = 621.53Ω / 50Ω + 621.53Ω = 0.9255


    If Vin = 1 mA

    Vin’ = Vin * Av-Divider = 1 mV * 0.9255 = 925.54 uV = 0.92554 mV

    Therefore Vout = Vin’ * Av = 0.92554 mV * 115.43 = 0.106840 mV


    Ic = Vout / RL = 106.84 mV / 10KΩ = 10.68 uA

    Ib = Ic / hfe = 10.68 uA / 100 = 106.00 nA

    Ie = Ib + Ic = 106.00 nA + 10.68 uA = 10.79 uA


    Ai = hfe = 100

    Av = 115.43

    Ap = Ai * Av = 100 * 115.43 = 11,543

    In particular, I am looking at Vout of 100.642 mV in Spice versus my Vout of 106.840 mV.

    Any help would be appreciated.

    David
     
    Last edited: Dec 6, 2018
  2. danadak

    Well-Known Member

    Mar 10, 2018
    1,898
    395
    The model in LTSpice may be different than the one you use on paper.

    Did you look at the Spice model parameter set to see what it is using ?

    Actually. w/o doing the above, the correlation looks pretty good, not knowing
    what the LTspice model for the transistor is doing. ~ 6%


    Regards, Dana.
     
  3. crutschow

    Expert

    Mar 14, 2008
    20,241
    5,731
    I agree with Dana. Considering the approximations used in the calculations, that seems to be pretty good agreement.
    That being said, I think the LTspice simulation is likely to be closer to results with the actual circuit, since it uses a more elaborate model for the transistor then you used.
     
  4. elec_eng_55

    Thread Starter Member

    May 13, 2018
    129
    0
    Thanks guys. Sounds good to me.
     
  5. LvW

    Well-Known Member

    Jun 13, 2013
    710
    102
    I think, the major reason for the difference between both results is that in your hand-calculation you did not consider the influence of the transistors dynamic output resistance rout which is in parallrl to rc=RC||RL.
     
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    1,983
    598
    re’ = 25 mV / 1.01 mA ==>re’ = 25.8 mV / 1.01 mA at 300K=27C
    Rin = R1║R2║hfe * re’ ==> Rin = R1║R2║((hfe+1) * re’)
     
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