The diode model ladder

Từ vựng

English	Ý nghĩa
pn junction	pn junction: tiếp xúc p-n
constant-drop model	constant-drop model: mô hình điện áp không đổi
piecewise-linear model	piecewise-linear model: mô hình tuyến tính từng đoạn

English

A diode — a pn junction — is a one-way valve with a near-vertical forward curve: ten times the current costs only $\ln 10 \times nV_{T} = 59.9$ mV (n = 1). So the resistor in the loop sets the current — exactly when the constant-drop model (0.7 V) is accurate.
The ladder — ideal (0 V), constant-drop, the piecewise-linear model ($0.7\ \mathrm{V} + r_d$), exponential (Shockley) — is picked by the question, and every answer checks against the next model up.

Example

5 V source, diode, 1 kΩ. Ideal: 5 mA. Constant-drop: $(5 - 0.7)/1000 = 4.3$ mA. Shockley truth (bisection): $v_D = 0.5775$ V, $i = 4.42$ mA — constant-drop is within 2.8% because the resistor dominates. At that point $r_d = 26\,\mathrm{mV}/4.42\,\mathrm{mA} = 5.9\ \Omega$: a ±0.1 V source wiggle drives $0.1/1005.9 = 99.4$ µA, within 0.6% of the resistor-only answer.