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I have always been confused between the difference between AC base resistance and AC emitter resistance which is usually denoted by r_e'. I've got the diagram as below:enter image description here

As we can see there is another resistance they are considering denoted by r_b', in floyd's book its written that they ignore this r_b' due to negligible value, my question is: is it correct to assume r_b' as the ac base resistance? If no, please explain what exactly it is.

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  • \$\begingroup\$ Where does the picture come from? Can this be at all answered by anyone not knowing the picture source and not having a copy of your book? Maybe you are restricting the number of people able to answer this inadvertently? \$\endgroup\$
    – Andy aka
    Commented Jan 7, 2021 at 11:25
  • \$\begingroup\$ @andy aka te.kmutnb.ac.th/~msn/225303report56.pdf, please see P.P 276 \$\endgroup\$
    – Sayan
    Commented Jan 7, 2021 at 11:30

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I can understand your confusion. The problem is as follows:

The resistive symbol r´e in the small-signal equivalent diagram is, in reality, no resistor at all. It symbolizes the transconductance gm=d(Ic)/d(Vbe)=1r´e which constitutes the connection between the control input (base-emitter pn junction) and the output (current Ic). The transconductance is the slope of the Ic=f(Vbe) curve - determined at the DC quiescent current.

During calculation we can assume that r´e=1/gm is a small-signal (dynamic) resistance - and that is the reason that there is such an equivalent diagram. But this has no physical meaning - it does not reflect the physical working principle of the transistor.

As far as r´b is concerned, it is a kind of differential "path resistance" between the outer base node and an "inner point" of the base region - and it can be neglected in practice (in comparison with the much larger base-emitter resistance rbe=r_pi)

Personal comment: That is the reason I do not like at all the so-called T-model. It can (as we have seen here) create confusion.

Example: Looking at the most right model, it seems that the input resistance at the base would be r´e=1/gm (in practice some tenth of volts). However, that is completely wrong - normally, some kOhms (because the contribution of the generated current is to be considered).

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