Diode (sv: Diod)

Tags: electronics, components

A diode is an active electronic component that limits the direction of current.


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.


Applications

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

AC rectification

Voltage protection

DC direction protection

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.

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.

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.

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.


Diode types

Silicon diodes

Germanium diodes

Light-emitting diodes

Schottky diodes

Zener diodes


Further reading