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I'm new with circuitlab & trying to get familiar with it while reading some tutorial about transistor NPN tutorial. But I've notice something not quite right when doing some DC Sweep. I've construct the same circuit in http://www./circuit/, and their result was different than what circuitlab does..

Is it a bug or am I missing something?

Hi Kokang There are different math modells to simulate a components behaviour. Every modell simplifies certain aspects which may lead to different results. Without closer inspection falstad assumes a linear current hfe which is not the case in reality. (See http://www.fairchildsemi.com/ds/2N/2N3904.pdf and have a look at the graph current gain.) . Conclusion: If you build a cirquit relaing on certain parameters of a component (here hfe current gain) make sure the simulator model used reflects reality. Moreover, all component have fabrication tolerances and some of them change siginificantly with ambient temperature for example. (Have a look at the datasheet Base Emiter ON voltage vs Collecotr current). If in doupt build it and test......

For info:

SPICE parameters for devices have little to do with datasheet specifications. They are much more to do with semiconductor physics and the particular process under which a given device is manufactured at the foundry. Consequently it is not always a simple task to map datasheet information onto the model parameters.

If you want to try to understand more about what the SPICE - and hence CL - parameters mean in diodes, bipolar transistors and MOSFETs, then you might like to have a look at:

http://www3.imperial.ac.uk/pls/portallive/docs/1/56133736.PDF

with individual slide sets:

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292571.PDF

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292572.PDF

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292573.PDF

For more detailed information about bjt's in particular, have a look at this book:

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01480193

available from:

http://www.lulu.com/spotlight/iangetreu

and

http://www.amazon.com/Modeling-Bipolar-Transistor-Ian-Getreu/dp/B000EYPQLU

Another excellent (and free) book about transistor modelling, go to:

http://www.aeng.com/spice_modeling.htm

and register to get a copy of:

Definitive Handbook of Transistor Modeling