diff --git a/docs/2a/ece240.md b/docs/2a/ece240.md
index 545f065..9b8c325 100644
--- a/docs/2a/ece240.md
+++ b/docs/2a/ece240.md
@@ -73,3 +73,18 @@ $$r_d=\left(\frac{\partial i_D}{\partial v_D}\right)^{-1} = \frac{V_T}{I_D}$$
     
     $$i_D(t)=I_D+\frac{1}{r_d}V_d(t)$$
 
+### Signal analysis
+
+1. Analyse DC signals
+  - assume blocking capacitors are open circuits
+  - turn off AC sources
+2. Analyse AC signals
+  - assume blocking capacitors are shorts
+  - turn off DC sources
+  - replace diode with effective resistor (the differential resistor)
+
+!!! tip
+    Most $R$s in the circuit can be assumed to be significantly greater than $r_d$, so $r_d$ can be removed in series or $R$ can be removed in parallel.
+
+!!! warning
+    Oftentimes, turning off a DC source to nowhere is actually a short to ground.