Re: Problem with measure Rin!

From: Francesco (roxfNOSPAM_at_libero.it)
Date: 08/09/04 

Date: Mon, 09 Aug 2004 12:08:39 GMT

Helmut Sennewald wrote:

> "Francesco" <roxfNOSPAM@libero.it> schrieb im Newsbeitrag
> news:ZixRc.33373$1V3.835898@twister2.libero.it...
>
>>Hi,I have a problem with this circuit
>>http://digilander.libero.it/checco23/Schemi/Per_sito1.GIF , I try first
>>to calculate and after to measure in a simulation the value of Rin, but
>>the two results are much different!
>>
>>Where is the mistake?
>>
>>
>>Data:
>>
>>Vi= 100mV;
>>
>>Ii( AC current at the transistor's base ) = 794nA;
>>
>>hfe = 83.6;
>>
>>re = 42 Ohm;
>>
>>Ri ( calculate ) = (hfe+1)*( re + RE ) = 84.6*1042 = 88153.2 Ohm;
>>
>>Ri ( measured ) = Vi/Ii = 100m/794n = 125944.6 Ohm;
>>
>>The difference between the two result is too much!
>>
>>Help me to understand where is my error!
>
>
> Hello Francesco,
> the value of Ib and Ic depend on the SPICE parameter Ise, Ikf and Vaf.
> If you have specified Vaf in your transistor model, then hfe can reach
> roughly about BF*(1+(Vce-Veb)/Vaf) for currents Ic<< Ikf.
> On the other end, the parameter Ise causes a drop in hfe for very
> small values of Ic.
> .MODEL 2N2222 NPN(VAF=... BF=... IKF=... )
>
> If you really want that someone can repeat your result, then you
> have to disclose the model of your 2N2222. Post your model here(or
> send it to me) so that other can try to repeat your results.

My 2N2222 MODEL NPN(IS= 81.2e-15 VAF= 98.60 BF= 195 IKF= 480m)

Are they all the necessary?

> I would check it with LTSPICE of course.
> The model in question should look like ".MODEL 2N2222 NPN(Is=...... )".
>
> Best regards,
> Helmut
>
>
> Hint:
> hfe = beta = dIc/dIb
> B = Ic/Ib
>
> 

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