Large signal / piecewise linear diode models

When the signal applied to a diode is large compared with its knee voltage (about 0.7 V for silicon), the exponential I-V curve is replaced by straight-line approximations. These are the large signal (piece-wise linear) models. In each model the diode is a switch plus extra elements: ON when forward biased beyond the cut-in voltage, OFF otherwise.

1. Ideal diode model

• Forward bias: short circuit (VD = 0, no resistance).
• Reverse bias: open circuit (I = 0).
• Used when the supply voltage is much larger than 0.7 V.

2. Constant voltage drop (battery) model

• Diode ON when VD ≥ Vγ (Vγ = 0.7 V for Si, 0.3 V for Ge). It is replaced by an ideal diode in series with a battery Vγ.
• OFF when VD < Vγ: open circuit.

3. Piece-wise linear model with forward resistance

• The forward curve is approximated by a line of slope 1/rf through Vγ: VD = Vγ + rf·ID. The diode is an ideal diode in series with a battery Vγ and a resistance rf (typically a few ohms to tens of ohms, rf = ΔV/ΔI of the linear portion).
• Reverse: open circuit (or a very large reverse resistance).

 I                       Ideal diode: ON  = wire
 |      /  slope 1/rf    Const. drop: ON  = --|+ -|--
 |     /                 PWL        : ON  = --|+ -|--/\/\--
 |    /                              Vg         rf
 +---+----------- V      OFF (all models) = open circuit
    Vg

Example: Vs = 5 V, R = 1 kΩ, Si diode. Ideal: I = 5 mA. Constant drop: I = (5 − 0.7)/1 k = 4.3 mA. With rf = 20 Ω: I = 4.3/1.02 = 4.22 mA.