What is a Diode and What Is Its Purpose?

Many people’s concepts of electronic components still revolve around passive components such as resistors and capacitors, which are primarily used for the consumption or storage of electrical energy without generating it. However, once we go beyond this concept, we enter a broader field known as the semiconductor domain, where the diode is a widely used component. So, what is a diode?

Whether you are an expert or a scholar, at the mention of this term, the first reaction may be a sense of confusion. What is a diode? Is it an LED? Just an LED?

When we suddenly bring up the term ‘diode,’ our initial response might be this:

Several different colors of LEDs

Yes, LED is one of the most common diodes we encounter, also known as a light-emitting diode. For example, common daily items like flashlights, LED bulbs, LED screens, and more.

However, LED is just one of them. In fact, it has many siblings. In this article, TechSparks will delve into the concept and uses of diodes, helping you better understand the microelectronics world.

Table of Contents

What is a Diode

A diode is a semiconductor device that has a history of over a century in the electronics world. It has evolved from early forms like the ‘cat’s whisker crystal’ and vacuum tubes to the widespread use of materials such as germanium or silicon.

In a circuit, a diode is represented by a straight line and an arrowhead. Typically, the arrowhead points in the direction of the current flow, resembling a funnel where water can only enter from the larger end. This illustrates the rectifying function of the diode, allowing current to flow in only one direction.

As a component with two polarities (anode and cathode), when the positive supply is connected to the anode of the diode, it is termed ‘forward bias.’ In this state, current flows through when the forward bias voltage exceeds the diode’s forward voltage drop determined by its manufacturing material. When the negative supply is connected to the anode, it is termed ‘reverse bias,’ blocking the flow of current. This is the working principle of a diode, controlling the flow of current by judiciously managing the voltage.

Forward and Reverse Bias of Diodes

Apart from rectification, diodes can serve various functions such as detection, regulation, isolation, and more. Different types of diodes play distinct roles in electronic circuits.

What Are Diodes Used For

Reverse Connection

In most cases, diodes are connected in the forward direction in a circuit, as mentioned earlier, known as forward biasing. However, sometimes, to protect other components, a diode is connected in the reverse direction at the power source. This is referred to as a reverse polarity protection diode or reverse power supply protection diode.

When using it, you can connect it in series or parallel in the circuit, considering a few factors:

Low Forward Voltage Drop: When powered by the power supply, the forward voltage drop can cause the overall circuit’s operating voltage to decrease. If the forward voltage drop is too high, it may prevent the entire circuit from reaching an acceptable level, ultimately leading to abnormal circuit operation or failure to start properly.

High Current Capacity: The diode must be able to handle the current required by the entire circuit during normal operation. Considering the overall current capacity of the system, the diode’s current capacity should be sufficient without becoming a bottleneck for the entire system.

Power Dissipation: Due to the reverse connection in the circuit, the diode will inevitably generate resistance when operating in the forward direction, leading to additional power dissipation in the circuit. If the power dissipation is too high, it can cause the diode to overheat, affecting its performance and lifespan. Therefore, in the design, these additional losses must be acceptable.

Transient Protection

According to Faraday’s law of electromagnetic induction, a changing magnetic field can induce an electromotive force (EMF). In an inductor, when the current undergoes a sudden change, the magnetic field also changes abruptly, resulting in a potentially large induced EMF. To prevent such high-voltage pulses from causing damage to the circuit, it is common to use a parallel diode, known as a “reverse-blocking diode” or “flyback diode.” In this scenario, the diode provides a low-impedance path, allowing the current in the inductor to flow through, thus protecting the adjacent circuit.

Diode instantaneous protection circuit

Clamping

You can use a reverse-biased diode to clamp the input signal, ensuring that it does not exceed the normal operating range of operational amplifiers (OP-AMPs) or other circuit components. This design is commonly referred to as a voltage clamping circuit.

Diode clamp function

Once the amplitude of the input signal exceeds VCC by 0.2V or 0.6V, it will directly flow to the positive power supply terminal. Similarly, if it goes below VEE by 0.2V or 0.6V, it will directly flow to the negative power supply terminal. This protects the input terminals of the operational amplifier.

Doubling

Alternatively, you can combine a capacitor with a diode to form a voltage-doubling rectifier circuit, which can amplify the peak-to-peak voltage of the input AC signal to twice its original value.

Diode voltage doubler rectifier circuit

This circuit relies on the periodic variation of the signal, making it suitable for cases where the input signal is an AC signal:

Positive half-cycle: D1 conducts, charging the capacitor C1 to the peak value of the input signal.

Negative half-cycle: D1 cuts off, and D2 conducts. At this point, capacitor C2 starts charging, while capacitor C1 maintains the previously charged voltage.

Positive half-cycle: D2 cuts off, and C2 discharges through the load. The final output voltage is the voltage across capacitor C2, which is twice the peak value of the input signal.

Periodic changes in circuit signals

Types of Diodes

Diodes can be classified based on different criteria. Here, TechSparks categorizes diodes based on functionality and introduces three typical types of diodes:

Detector Diodes

In wireless radio communication, signals are often transmitted in a modulated form, such as Amplitude Modulation (AM) or Frequency Modulation (FM). In such cases, the task of a detector is to restore these modulated signals to their original audio or data signals. The essence of detection is rectification, meaning the conversion of alternating current (AC) signals into direct current (DC) signals. Common types of detector diodes include regular diodes, Schottky diodes, and PIN diodes. These diodes have a point-contact structure, resulting in smaller junction capacitance, making them suitable for high-frequency operations.

Clamping Diode

When a diode is forward-biased, its forward voltage drop remains relatively constant. For silicon diodes, this voltage drop is approximately 0.7V, while for germanium diodes, it is around 0.3V. This characteristic provides interesting opportunities for designing circuits with nonlinear voltage-current characteristics.

A clamping diode, also known as a voltage-stabilizing diode, serves as a protective component, ensuring that the input signal does not exceed a specific voltage range, preventing overvoltage damage to other components. Its operation is as follows:

When the input voltage is below the set lower limit, the clamping diode is in a high-impedance state, preventing current flow.

When the input voltage is within the specified range, it exhibits relatively low impedance, allowing current to pass.

If the input voltage exceeds the set upper limit, it returns to a high-impedance state, absorbing the excess voltage and limiting the amplitude of the voltage.

Mixer Diode

A mixer diode is a type of diode used in mixer circuits, designed to combine two or more signals, resulting in a new signal that includes the sum, difference, or other harmonic effects of the original signals. When subjected to different voltage and current conditions, the mixer diode exhibits varying nonlinear responses. Therefore, achieving the desired frequency-mixing effects requires careful design.

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