Large signal model of pn diode

The large signal model of a pn junction diode is an approximate equivalent circuit used when the voltage and current swings are large (as in rectifiers, clippers and switches), so the diode is treated as switching between ON and OFF states rather than working at a small region around a Q-point.

The real diode follows the exponential law I = Is (e^(V/ηV_T) − 1). This is replaced by straight-line (piecewise-linear) pieces. Three common levels are used:

1. Ideal diode model

• Forward bias: short circuit (V_D = 0, any current).
• Reverse bias: open circuit (I = 0).
• Used when supply voltages are much larger than 0.7 V.

2. Constant voltage drop (simplified) model

• ON when V_D ≥ V_γ: a battery V_γ (0.7 V for Si, 0.3 V for Ge) in series with an ideal diode.
• OFF when V_D < V_γ: open circuit.
• Most often used in hand analysis.

3. Piecewise-linear model

• ON: battery V_γ in series with the forward resistance r_f (slope of the straight line, typically a few ohms to tens of ohms) and an ideal diode.
• OFF: open circuit (or a very large reverse resistance).
• I_D = (V_D − V_γ)/r_f for V_D > V_γ.

Model         Equivalent (ON)        I-V shape
Ideal         --o--                  vertical at 0
Const. drop   --|+ -|--  (0.7 V)     vertical at 0.7 V
Piecewise     --|+ -|--/\/\--        slope 1/r_f
                 V_γ    r_f          from 0.7 V

  I |      /   piecewise linear
    |     /
    |    /  slope = 1/r_f
    |___/____________ V
    0  V_γ

The diode is first assumed ON or OFF, the circuit is solved, and the assumption is checked (I_D > 0 for ON, V_D < V_γ for OFF). The large signal model is different from the small signal model, which uses only the dynamic resistance r_d = ηV_T/I_D around a Q-point.