Why is the internal resistance of BJTs considered in small signal AC analysis but not in DC analysis of BJT amplifiers?
For ideal resistors, the voltage drop across a resistor is equal to the current through the resistor times the resistor's resistance. This is Ohm's law.
$$V = IR$$
This law, rearranged is
$$R = \frac{V}{I}$$
For a non-linear device, there is no single value of R that relates V and I in a linear way. We can still refer to a &quot;resistance&quot; \$R=\frac{V}{I}\$ as long as we understand that it is merely the ratio of V and I for particular values of V and I, and not a constant.
Small signal &quot;resistance&quot; is something different.
$$r = \frac{dV}{dI}$$
Small signal &quot;resistance&quot; tells us the change in voltage for a small change in current.
For a non-linear device,
$$\frac{V}{I} \ne \frac{dV}{dI}$$
(Except perhaps for some very specific values of V and I).
So, we cannot use r to determine either V or I. That is why r is not used in DC analysis.