# Finding the necessary currents for AC analysis for CE NPN BJT amplifier

I have this circuit with a CE NPN-BJT amplifier and I am struggling to find the necessary values for calculating r(pi) for AC analysis, i.e. the hybrid-pi model:

$$r_{pi} = \frac{V_{thermal}}{I_b}$$

I need to find the current going into the base of the CE amplifier. I am assuming that the infinite capacitance to be short circuited in the DC analysis. This means that V(b) = 0 I can then use the active region model of NPN BJT for DC analysis to write down the KVL which gives me something like this: -0.7 - 60(I[e]) = 0

I solve the equation and end up with a negative emitter current. This does not adhere to the model. What am I doing wrong?

[ • Even if your base capacitance is infinite, if you are running any simulation tool, at DC, all caps are considered open. Meaning in your case, that Vb is not 0. – vtolentino Mar 30 '20 at 6:54
• @vtolentino No space before question mark! – winny Mar 30 '20 at 10:18
• @winny I didn't know about it. Is there any guideline written somewhere? – vtolentino Mar 30 '20 at 11:54
• @vtolentino grammarbook.com/punctuation/qMarks.asp – winny Mar 30 '20 at 14:36
• @vtolentino As far as I'm aware, the only language that puts spaces before question marks is French. Every other language that uses question marks has them immediately following the last letter of the sentence. – Hearth Mar 30 '20 at 17:19

## 1 Answer

@vtolentino already got it right in the comments: in DC analysis all capacitors are open-circuited, it should be fairly simple to find what you want after that.

I'd like to add that this BJT is not a Common-Emitter BJT, it's a Common-Base BJT because the base terminal is common to both inputs and outputs, so you're dealing with a CB NPN-BJT.