Diode

A diode is an active electronic component that limits the direction of current. It doesn't conduct beneath a certain voltage, and variants of the component can be used for purposes such as emitting light and voltage regulation.

1. Characteristics 🔗

Current can normally only pass through from the the anode, to the the cathode, meaning that the voltage from anode to cathode must be positive to conduct. When this is the case, the diode is forward biased.

Anode              Cathode
  o-------|>|---------o

A conducting diode will drop voltage from anode to cathode. This is known as a forward voltage, or a voltage drop. In order for the diode to conduct, the voltage must exceed the forward voltage. The amount of voltage drop is inherent to the type of diode used. Most conventional diodes have a voltage drop of about 0.7V.

Not conducting

+0.3V            0V
  o-------|>|----o

Conducting

+3V          +2.3V
 o-----|>|-----o

If the voltage at the cathode exceeds the voltage at the anode, the diode is reverse biased and won't conduct unless it's either a zener diode, or if it's pushed so hard it breaks. If a diode breaks, it can result in the diode not conducting under any circumstances, or it can short entirely. Diodes breaking is generally considered bad for circuit health.

The humble 1N4148 silicon diode 🔗 🇺🇸 will for example stand up to 100V of voltage from cathode to anode, but will only handle a few hundred mA of current even in forward voltage.

2. Types 🔗

2.1. Silicon 🔗

Silicon diodes are the most commonly used signal diodes because they are cheap and have a fast switching time. Their largest drawbacks are their relatively high forward voltage $(W_F \approx 0.7V)$ .

2.2. Germanium 🔗

Germanium diodes were historically favoured for their low forward voltage $(W_F \approx 0.3V)$ , but are mostly replaced by Schottky diodes which achieve lower forward voltages and are far cheaper. Germanium diodes still see use in audio, in particular in distortion effects as their characteristics ^{(which characteristics?)} contribute to a vintage sound.

2.3. Schottky 🔗

Schottky diodes have a very low forward voltage, making them ideal for power management processes, e.g. rectification. They are available with current ratings up to several Amperes. A consequence of their low forward voltage is that their reverse breakdown voltage is relatively low.

2.4. Light-emitting 🔗

LEDs emit light when current passes through them. They're easily fried when provided with too much current and are usually rated for a low reverse voltage.

2.4.1. Forward voltage 🔗

LEDs' forward voltage increases with the frequency of the light.

Color	Forward voltage
Infrared (IR)	$W_F \approx 1.2V$
Red	$W_F \approx 1.7V$
Yellow	$W_F \approx 2.1V$
Green	$W_F \approx 3.5V$
Blue	$W_F \approx 3.6V$
Ultraviolet (UV)	$W_F \approx 4.0V$

Values will between specific types a lot. Green and white LEDs vary a lot in particular as their colours can be a compound of many different things.

2.4.2. Form factor 🔗

Since LEDs are used in user interfaces, their physical form factor is often defined by the diameter of their bulb. Common sizes include 8mm, 5mm, 3mm, and 1.8mm (flat shape).

8mm and 5mm are popular in guitar pedals. 3mm can be used in them as well. 3mm has historically been used in keyboards sometimes, but would often collides with keycaps and so 1.8mm LEDs were favoured, although both are mostly replaced by RGB lights placed directly on the PCB such as WS2812Bs nowadays.

2.5. Zener 🔗

Zener diodes are unique in that they can conduct in reverse bias (up to a certain voltage) without breaking. This voltage varies but is typically something like 9V or 12V. This allows you to use them for voltage regulation.

2.6. Comparison table 🔗

Type	Forward voltage	Reverse voltage	Current rating	Applications
Silicon	$W_F \approx 0.7V$	$V_R \gtrapprox 30V$	Medium	Signal processing
Germanium	$W_F \approx 0.3V$	$V_R \gtrapprox 30V$	Low	Audio effects, power handling
Schottky	$W_F \approx 0.2V$	$20V \lessapprox V_R \lessapprox 40V$	Low to high	Power handling
Light-emitting	$1.2V \lessapprox W_F \lessapprox 4.0V$ ¹	$V_R \approx 5V$	Low	User interfacing
Zener	$W_F \approx 0.7V$	Variable; does not break	Low to medium	Voltage regulation

3. Applications 🔗

Applications for diodes include rectifying alternating current to direct current, distorting sound, limiting voltage, and emitting light.

3.1. AC rectification 🔗

3.2. Reverse voltage protection 🔗

Connecting a diode between the positive pole of your power input and the rest of the circuit ensures that current does not flow if a power supply of wrong polarity is connected. A schottky diode is ideal for this task as its low forward voltage maximises the voltage available to the circuit.

3.3. Voltage regulation 🔗

A zener diode connected in reverse bias between positive and negative DC power rails ensures that any voltage exceeding the diode's rating is drained to ground, protecting the circuit from over-voltage.

3.4. In audio 🔗

N.B.: This section needs some peer review and concrete, verifiable examples.

Diodes interact wonderfully with audio signals and are mostly used to create various distorting effects. Although diodes do not achieve the same "authentic" overdrive that is caused by literally pushing an amplifying circuit to its limit, the right selection and application of diodes will very frequently achieve nearly indistinguishable results.

3.4.1. Analogue octave up 🔗

The simplest, but probably least common usage of diodes in audio processing, is to simply run the signal across a diode. This will make a half-wave rectifier and cut all negative parts of the signal. While the fundamental frequency for a stable wave will remain the same, the harmonic emphasis will be drastically altered.

Using either four diodes or a transformer/op-amp and two diodes, we can instead create a full wave rectifier. Though a full-wave rectifier is normally used to turn always positive relative to neutral in order to create DC, we can opt to not add AC filtering diodes to instead just get the doubled frequency of the rectified wave.

A four-diode rectifier is arguably the simpler design, but using a centre-tap transformer or an op-amp allows reducing the amount of diodes to two, which makes for a less distorted effect as there is less forward voltage to overcome. By creating an inverted signal and then running both the original and the inverted signals through their own half-wave rectifiers before summing them, a full-wave rectification is achieved, of which the fundamental frequency is twice what the input's was.

For pure octave effects, diodes with low forward voltages are strongly preferred. If more distortion is desired, other diodes may be of greater interest.

3.4.2. Gate 🔗

Running a signal through two diodes in parallel, one forward-biased and one reverse-biased, will cause the signal to go through so long as the forward voltage is met, both on the positive and negative duty cycles. Since this will cut the signal during the transition from positive to negative and vice versa, it will be a distorted gate sound, sharing some similarities to old transistor-based fuzzes.

3.4.3. Distortion 🔗

By running the signal to ground through a two-way diode gate, any voltage above the forward voltage will run to ground and the wave will effectively have its top and bottom cut off. This creates a classic pedal distortion.

By instead doing this with the negative feedback loop of an op-amp, the op-amp will achieve a similar but less harsh effect. This is more akin to a pedal overdrive.

In these circuits, the switching speed of the diode influences the harshness of the distortion. Although germanium diodes have a very low forward voltage and cut the most off a signal, they will produce fewer harmonics.

4. Footnotes 🔗

Scales with wave length, see LED forward voltage ↩