# CE Amplifier - Manually Computed AC Analysis Versus LT Spice

#### elec_eng_55

Joined May 13, 2018
214
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

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Last edited:

Joined Mar 10, 2018
4,057
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.

#### crutschow

Joined Mar 14, 2008
26,735
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.

#### elec_eng_55

Joined May 13, 2018
214
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.
Thanks guys. Sounds good to me.

#### LvW

Joined Jun 13, 2013
1,212
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.

#### Bordodynov

Joined May 20, 2015
2,801
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’)