Diodes are two-terminal and nonlinear circuit elements which only allow current to flow in one direction, implemented using p-n junctions.

Current/Voltage Models

Ideal Model

Open circuit when $V_{D} < 0$ , and shorted when $V_{D} > 0$ .

Constant Voltage Model

Only passes current when $V_{D} > V_{D, ON}$ . Typically $0.7 V$ for silicon diodes.

Non-Ideal Exponential Model

Shockley Diode Equation
$I_{D} = I_{S} e^{\frac{V _{D}}{V _{T}}} - 1$
$I_{S} =$ reverse-bias saturation current
$V_{T} =$ thermal voltage $= \frac{k T}{q}$ ( $26 mV$ at $25 \textdegree C$ )
Can also be modelled with linear piecewise function.

Solving for current and voltage
$I_{D} ≅ I_{s} e^{V_{D} / V_{T}} for i ≫ I_{S}$
$I_{D} = \frac{V _{DD} - V _{D}}{R}$
Solve iteratively

Small Signal Model

$r_{d} = \frac{V _{T}}{I _{D}}$

Uses

Rectifier

Rectifiers convert AC to DC by using a diode to only allow half of an AC source’s voltages. A capacitor may be used for filtering out AC frequencies and to act as a reservoir.

Half Wave Without Capacitor

Half Wave With Capacitor

$V_{r i ppl e} = \frac{V _{ma x} - V _{D, ON}}{R _{L} C f}$

Full Wave Without Capacitor

Full Wave With Capacitor

$V_{r i ppl e} = \frac{V _{ma x} - 2 V _{D, ON}}{2 R _{L} C f}$

Voltage Limiter

Clips voltages which exceed certain thresholds.
$V_{o} = ⎩ ⎨ ⎧ - V_{b} - V_{D, ON} V_{i} V_{a} + V_{D, ON} if V_{i} < - V_{b} - V_{D, ON} if V_{b} \leq V_{I} \leq V_{a} if V_{i} > V_{a} + V_{D, ON}$

Voltage Regulator

A voltage regulator generates constant DC voltage. A simple voltage regulator uses many diodes in series, while a more common implementation involves using a zener diode in breakdown.

Power-supply ripple: The unwanted sinusoidal residue in a regulator.

DC Power Supply
Power transformer reduces 120V voltage to 8-12V
Diode rectifier converts bipolar voltage to unipolar
Filter reduces voltage variations
Voltage regulator further reduces voltage and ripple

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Diode