# BJT: what are differences between $r_{be}$ and $r_e$; $r_{in}$ and $r_{in(t)}$?

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.

• What textbook? They don't all use the same notations. – Fizz Oct 11 '15 at 18:37
• Transistor Circuit Techniques Discrete and Integrated 3rd Edition, G.J Ritchie. – aukxn Oct 11 '15 at 18:54

$r_{be}$ 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.

• Hi, my textbook say opposite, it is $r_{be}=(1+\beta) r_e$ and in small signal analysis, only $r_{be}$ is used. – aukxn Oct 12 '15 at 5:08

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