Short note: Graphical analysis of diode circuit

Graphical analysis (load line method) finds the operating point of a diode circuit when the diode's I-V curve is non-linear and an exact equation is hard to solve.

Consider a series circuit: supply VS, resistor R and diode D.

KVL gives: VS = ID R + VD, so ID = (VS − VD)/R. This is a straight line called the load line.

Procedure

1. Draw the diode characteristic (ID versus VD) from the data sheet or from measurement.
2. Plot the load line:
   • VD = 0 → ID = VS/R (point on the current axis)
   • ID = 0 → VD = VS (point on the voltage axis)
3. Join the two points. The intersection with the diode curve is the operating (quiescent) point Q, giving the actual ID and VD.

 ID
 VS/R |\
      | \       diode curve
      |  \    /
      |   \  /
      |    Q*
      |   / \
      +--/----\------ VD
       0.7      VS

Example: VS = 5 V, R = 1 kΩ. Load line: 5 mA on the ID axis and 5 V on the VD axis. If the curve crosses at VD ≈ 0.7 V, then ID = (5 − 0.7)/1 k = 4.3 mA.

The method also shows how Q shifts when VS or R changes, and it can be extended to circuits with ac signals to find the output waveform.