incorrectly calculating hfe/input impedance

[akis]

Hello

Multisim calculates wrongly the input impedance of a common emitter stage - judging from the voltage drop on the base resistor network it appears it assumes a hfe of about 10, when the real value is over 500. I used the 2N2222A as a model, and the error is there on multiple transistors with hfes many multiples of 10.

Could you please let me know what I am witnessing here? 

Thanks

Akis

[OStep]

please post your circuit

Thanks

[Andrew McL]

Hi All,

Please find attached the user's code.

Many thanks,

[Andrew McL]

Sorry all, not sure what happened but the above code doesn't work.

please try the attached.

Thanks

[OStep]

The active region beta for this transistor is around 220.Your circuit is actively biased and a simple current measurements shows that beta=Ic/Ib~= 220. So I am not sure the reason for your assertion that beta is 10 because you need to actually measure it.

Generally I can say with a high degree of confidence that the models for the 2N2222s and their simulation within Multisim are quite accurate.  

Thanks,

Message Edited by MaxNI on 10-14-2009 09:54 AM

[Andrew McL]

Hi All,

I also ran this test and found that the HFE value was around the 220 mark. I wonder if you are looking at a different flavour of 2n2222A transistor? A quick look on google revealed the following components

Microsemi - HFE between 50 and 100

Philips Semi conductors - HFE 75

SemiLab - HFE between 100 and 300.

I think the thing to note is that these transistors can have a quite varying value for HFE. If you navigate to the component browser, find the 2N2222A component and click on the button marked "Detail report". You can see that the hFE value will vary between 35 and 300.

I hope this helps,

Many thanks, 

[akis]

Hi

Thanks for the replies. The circuit I posted does not correctly show what I am seeing, I took my original circuit, did a small analysis to try to find out why Multisim thought that one side of the waveform would be severely clipped, and mistakenly I traced it to the simplified circuit I posted. I will go back to the original circuit and either post it fully or trace it better to find the problem area.

Thanks

Akis