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I know that the base-collector region is reverse biased and because of that it offers high resistance but it is the resistance due to biasing.

In a video, the person was explaining the hybrid-pi model and he said that there is also the resistance of the material that comes into play, calculated from base to collector terminal and that resistance is the output resistance of the common-emitter configuration and is high.

If we don't consider the resistance offered by the reverse-biased voltage of the base-collector region as the output resistance then how does the material resistance of from base-collector offer higher resistance? The resistance is also there in base-emitter junction, so should we also call that input resistance high as well?

JRE
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Ahsan
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  • The common emitter circuit acts like a current source at the collector. A (constant) current source has, by definition, a high output impedance. The output voltage depends on the load resistor. A short at the output willl reduce the output to 0 V, increasing the load resistance will also increase the voltage. – Bart May 05 '22 at 08:03
  • Look up this question to get an understanding of a difference between voltage and current sources: https://electronics.stackexchange.com/questions/75017/difference-between-current-source-and-voltage-source – Bart May 05 '22 at 08:13
  • @Bart I am not considering any load resistor at the end and I also know that common-emitter output acts as a dependent current source and has high resistance but when we draw the model of BJT in the small-signal analysis we consider the output resistance if the resistance is not due to reverse biasing then the only resistance left is due to material and the area of collector is high so the resistance of output (collector) should be low how can the output has high resistance? – Ahsan May 06 '22 at 14:02

2 Answers2

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My recommendation: In general, one should not blindly trust "a person" who tries to explain something in a video.

Of course, it is correct that the "material" (ohmic effects?) inside the transistor will contribute to the overall behaviour of the BJT - however, to a very small and negligible amount ony.

I am afraid, the "person" did not know about the Early effect which is responsible to the large output resistance at the collector node (a non-ideal current source). This is not a static but a dynamic resistance - it can be visualized by the small slope of the Ic=f(Vce) characteristic (for constant Ib or constant Vbe).

LvW
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  • The resistance at the output of small signal model of bjt with controlled current source is due to early effect when considering practical. But not anywhere I have seen or read we are including a resistance due to current source in parallel with the early effect resistance can you just tell me this that why we don't consider resistance of controlled current source in both ideal and non ideal conditions – Ahsan Jul 17 '22 at 00:33
  • Why do you think that "we don`t consider" this resistance caused by the Early effect? Of course we do in some cases where it can play a considerable role. For example, this resistance acts as the only load resistance in a differential amplifier with a current mirror in the collector path (active load). – LvW Jul 17 '22 at 08:16
  • no no I am asking why we don't consider resistance in parallel due to controlled current source in the small signal analysis. The early effect is an important parameter and in practical scenarios, we consider the early effect resistance but I haven't seen anywhere anyone including the resistance of controlled current source in doing analysis. We add source resistance in the model sometime but we never include controlled current source resistance in parallel? why is that – Ahsan Jul 17 '22 at 22:37
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Ic is a voltage-controlled(Vbe) current sink (Ic) with some Early Effect leakage resistance but otherwise high impedance output by design.

An Ideal current sink is infinite but over the linear range Rce has the leakage effect of the "Early voltage" resistance.

However, when saturated (Vbc for PN jcn. >0) hFE declines rapidly to a low % of linear hFE then ΔVce/ΔIc=Rce acts as a switch resistor instead of a current sink.

should we also call that input resistance high as well?

Well it's not a current sink but rather an hFE amplifier impedance.

Tony Stewart EE75
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    I rather think than @Ahsan's problem is in understanding the difference between voltage and current sources. And that a high output impedance does not have to mean a high resistance in series with an ideal voltage source. – Bart May 05 '22 at 08:18