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Transistor bias configuration

Laplace

Apr 4, 2010
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I have no idea who is right and who is wrong in that other thread LOL
If you are thinking in terms of right & wrong, then you have probably missed the essence of that discussion. The issue is whether it is most appropriate to explain transistor circuit operation using concepts from semiconductor physics, or using principles from engineering circuit design. While it is true that Vbe controls Ic, the relationship is highly non-linear and temperature dependent. It is therefore not a useful relationship for performing engineering circuit design. Ask any experienced circuit designer if they have ever used the principle of Vbe controlling Ic for any circuit design; my expectation would be the answer is "No". It is not that the principle is wrong, it is just too difficult to analytically quantify to have any useful purpose.

The advanced courses for integrated electronics that I'm familiar with begin with a review of semiconductor physics, PN junctions, and transistor operation. If it is a course for creating integrated electronics, it then delves into the partial differential equations relating physical structures in the crystal lattice to the electrical characteristics. If it is a course for circuit design, it develops transistor models and how to use those models to design transistor circuits - leaving the semiconductor physics behind. Transistor circuit designers do not design with transistors, they design using transistor models. Attached diagrams show some of these models both for large-signal analysis and small-signal analysis.

Note that in all those BJT models only one even acknowledges the existence of Vbe, and then merely as a constant voltage source equal to one standard diode drop which is not controlling anything at all.

In summary, it is not wrong to explain that Vbe controls the currents in a transistor. But I don't need any hand-waving explanation; what I need is a quantifiable relationship that is useful as the basis for analyzing and designing transistor circuits. So when reading that other forum thread, just keep in mind that it is not wrong to be useless.
 

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M.Sridhar

Sep 16, 2015
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Sep 16, 2015
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I thank u really.Actually I understood how the transistor works in self bias condition.We should design the circuit in such a way that Vbe must be always in fb condition and there are various ways in which we can do this.
Self bias is one such circuit in which voltages will be independent of beta value but requires more components
However thanks a lot
 

Ratch

Mar 10, 2013
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Mar 10, 2013
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I thank u really.Actually I understood how the transistor works in self bias condition.We should design the circuit in such a way that Vbe must be always in fb condition and there are various ways in which we can do this.
Self bias is one such circuit in which voltages will be independent of beta value but requires more components
However thanks a lot

There is a lot more to bias than just getting the voltages correct. There is also stability with respect to variations in temperature, beta, components, and power supply fluctuations, Each one of these has a defined stability factor which can be calculated.

Ratch
 
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