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On the topic of small signal analysis in my textbook Transistor Circuit Techniques: Discrete and Integrated; 3rd Edition, G.J Ritchie, I encounter the two pairs of terms: \$r_{be}\$ and \$r_e\$; \$r_{in}\$ and \$r_{in}(t)\$, but cannot find a definition that clearly distinguishes them. Can someone clarify the difference between the members of these two pairs and perhaps their prose names? In particular, I see \$r_{in}(t)\$ used in some AC equivalent models but not others. In what ways is it different from \$r_{in}\$?

Also in this textbook, it says to determine \$r_{out}\$ by short-circuiting the input voltage source (\$v_{in}\$ I assume) and applying a voltage across the output terminals. I don't understand what the former means.

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    \$\begingroup\$ What textbook? They don't all use the same notations. \$\endgroup\$
    – got trolled too much this week
    Commented Oct 11, 2015 at 18:37
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    \$\begingroup\$ Transistor Circuit Techniques Discrete and Integrated 3rd Edition, G.J Ritchie. \$\endgroup\$
    – aukxn
    Commented Oct 11, 2015 at 18:54

2 Answers 2

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\$r_{be}\$ stands for the resistance located in the path of Base to Emitter (and reverse, it's bidirectional) which is actually placed in Base side and needs to be multiplied to \$\beta\$ if you want to move it to Emitter. It's also known as \$r_{\pi}\$.

\$r_{e}\$ stands for the total resistance seen from Emitter of a transistor, in approximate:

\$r_{e} = \beta r_{\pi} || r_{o} \approx \beta r_{\pi}\$

\$r_{o}\$ is the resistance between Emitter and Collector of a BJT which is infinite as usual in approximations.

In high frequencies, some capactive behaviors will be appeared in BJT Transistors. \$r_{in}\$ stands for the input resistance seen usually from Base and it gets time variant if capacitors appear in transistor model, so you'll have a time-dependent resistance in high frequencies notationed as \$r_{in}(t)\$. It's related to advanced topics of BJT transistors.

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  • \$\begingroup\$ Hi, my textbook say opposite, it is \$r_{be}=(1+\beta) r_e \$ and in small signal analysis, only \$r_{be}\$ is used. \$\endgroup\$
    – aukxn
    Commented Oct 12, 2015 at 5:08
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rbe = resistance between the base and emitter terminals re = the resistance at the emitter terminal I believe rin is the input impedance at the base

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