I have attached a .asc file with teo schematics one of which is DC only giving the Collector current with no signal applied, the other is identical but has a small AC signal imput. From the DC circuit the transconductance works out as 2.055mA/ 25mV giving 80mA/V. From the small signal circuit the voltage between base and emitter is 0.8mV. Multiplying the 80mA/V by 0,8mV should give the small signal collector current ( over and above the DC value.)
My question is, why is this value not equal to that in the small signal schematic ( this is giving 0.3mA).

 

hi nt,
This is what I see, very close correlation?
E

 

2.055mA/ 25mV giving 80mA/V.

Did you forget that Vt is temperature-dependent? In LTspcie by default T is equal to T = 27°C. Therefore Vt = 25.864923072742mV.
And for this circuit gm will be around GM = 1/ (100Ω + 12.46Ω) = 8.892mS.
ΔIc = 10mV * GM = 88.92μA (177.8μA peak to peak).

Also, you should not forget that small-signal is just a approximation and it will never give you an exact solution.
Especially in comparison with numerical simulation which takes into account many more parameters than we do in our simplified hand-made calculations.

 

GM = 1/ (100Ω + 12.46Ω) = 8.892mS

Thanks, I dont know this way of calculating transconductance, can you elucidate?