add Koukab's formulas

08_bjt.tex

@@ -116,3 +116,12 @@ We evaluate the transistor at its operating point ($OP$ or $Q=(V_{BE},V_{CE})$)
     g_m = \left. \frac{\partial i_C}{\partial V_{BE}} \right|_{OP} = \frac{qI_C}{kT}
 \end{equation}
 
+\begin{equation}
+    \label{label:eq:bjt_ro}
+    g_{ce} = r_o^{-1} = \left. \frac{\partial i_C}{\partial V_{CE}} \right|_{OP} \approx \frac{I_{C}}{V_A}
+\end{equation}
+
+\begin{equation}
+    \label{label:eq:bjt_ro}
+    g_{be} = r_{be}^{-1} = \left. \frac{\partial i_B}{\partial V_{BE}} \right|_{OP} = \frac{g_m}{\beta}
+\end{equation}