Though my question looks trivial, I am compelled to ask this on my research experience where I a find large amount of constant behavioural deviation from results prescribed by model of frequency. Will this issue be resolved if we consider every internal capacitance for every frequency a.c analysis?
For my opinion - in fact, we do consider internal parasitics in case we perform a simulator- based analysis.
However, if we try to analyze such a circuit by hand we are using pre-formulated equations and formulas which do NOT contain parasitic influences.
Of course, it would be possible - theoretically - to extend all the formulas and to include parasitic influences.
But in this case, one should ask: Does the manufacturer provide us with all the necessary information about parasitics (with sufficient accuracy)?
SIR @lutz i am attatching data sheet of bc547 will you kindly tell me which parameters i must refer to extend my question to theoratical level
I am not sure if I fully understand your question.
The data sheet contains two capacitances - however, I don`t know what the associated tolerances are.
Do you have formulas which contain these capacitances?
Sir i have not come across any formula with parasitic capacitance being considered at mid-frequency analysis. I am trying to include parasitic capacitance in the mid freq analysis to improve theoratical calculation near to practical behaviour reg which i seeking help here
sir in data sheet there is CE and CC given at 1Mhz..but i wishes to know that how true is our thinking that these CE and CC will not play any role influencing the output behaviour at freq less than 1MHz
I think, there is no doubt that both parasitic capacitances indeed play a role.
However, another question is: Is it necessary to calculate the influence of these parasitics for a special application.
For my opinion, this is a typical engineering task: namely, to estimate if a certain parameter has to be considered during circuit analysis or not.
And this question can be answered only if
* the values of ALL influencing quantities are known, and
* if the corresponding tolerances (uncertainties) are also known.
Remember: In the field of electronic circuit design no formula is (and cannot be) 100% correct. And it is one of the most important engineering tasks to decide for a particular application which parameters are important - depending on the required accuracy. Example: It makes no sense to consider pico-Farad parasitics if lumped nano-Farad capacitors with 5% tolerance are used.
@lutz this is the first time on two weeks that i have got the answer up to my satisfaction.....
Dear Colleagues,
I agree with you that the equivalent circuit parameters of the transistor as an amplifier varies appreciably from sample to sample but with the advancement of the semiconductor technologies the tolerances in the parameters decreases. In principle the transistor parameters extracted from Data Sheets or picked up directly from them are only average values rather than specific for certain specific transistor.
The high frequency model recommended for the bipolar transistor amplifier circuit is the hybrid pi model. Not all the parameters of these model are directly measurable but partly measured and partly calculated from the measured transistor characteristics. The parameters of the hybrid pi model depends appreciably on the DC operating point of the transistor but they are fairly independent of the frequency.
Then before calculating the amplifier characteristics one has to set the dc operating conditions and determine the equivalent circuit parameters at this operating point.
In case of the R-C coupled amplifier. one can then calculate the midband gain and the
-3dB cut off frequencies as depicted in any standard electronic circuits text book.
At midband, the effects of the reactive elements are negligible as a consequence of the flat midband gain normally for several decades. Expressed in an other way the gain in the midband frequencies is more or less independent of frequency which justifies the neglection of the parasitic capacitances of the transistor for midband analysis.
As an example the midband vpltage gain is given by Av= -gm*RLe provided that the amplifier is voltage fed. The transconductance gm=IC/Vt where IC is the DC collector current and Vt the thermal voltage= 25.6 mV at 300K. RLe is the equivalent load resistance which is equivalent to the parallel combination of the collector load resistance in parallel with the ad load resistance RL and the ac collector to emitter resistance.
As a final comment, the capacitors Ce and Cc in the data sheet are the junction capacitances of the emitter and collector junctions at the described test conditions in the data sheet.
I hope that i shed some light on the midband gain and its parameters for the bipolar transistor amplifier.
For more details please refer to the link:
https://www.researchgate.net/publication/256346392_The_bipolar_transistors_theory_and_basic_applications?fulltextDialog=true
Thank you
.
Book The bipolar transistors, theory and basic applications
Thank you Shaikh for your question it is not a trivial one.It is interesting. I will add a course on the bipolar transistor which treats this subject matter in much greater detail. I will inform you as soon as i download it in my page on the researchgate.The link is given in my post above.
I hope it may be useful.
@abdelhalim really great answer i will wait for that link
Dear Shaikh,
Thank you very much. The link is:
https://www.researchgate.net/publication/256346392_The_bipolar_transistors_theory_and_basic_applications?fulltextDialog=true
Best regards
Book The bipolar transistors, theory and basic applications
To A. Zekry:
The above article on the BJT is one of the best contributions in transistor theory I ever have seen. Thank you.
Thank you Lutz for the good evaluation of my article on transistor. You always encourage me.
Thank you Shaikh for adopting my text to your students. I am pleased that you find it useful. It is your question that motivated me to make this available on the researchgate. Again you colleagues who gave the text this value.
Best regards
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