What is Small Signal Diode : Working & Its Applications

A diode is one type of semiconductor device that allows the flow of current only in one direction from anode to cathode. So these are used in different application circuits like AC-DC rectifier, logic, AM detection, level-shift, snubbers & protection circuits. There are different types of diodes available in the market as a small signal diode, LED, Schottky diode, Shockley diode, Zener diode, and many more which are used based on application. This article discusses one of the types of diodes namely; a small signal diode and its working with applications.

What is a Small Signal Diode?

A small-signal diode definition is; a type of PN junction diode which functions on low voltage signals. This is a small non-linear semiconductor diode normally used in electronic circuits. The junction area of this diode is very small so, this junction is capable of less charge storing & low capacitance. This diode has a high switching speed through a very fast recovery time. These diodes are used in audio, video, radio signals, and also digital signals that may be found in automotive, home, industrial equipment, aeronautic & musical systems. Generally, these diodes are available in both surface mount and wire-ended formats.

The small-signal diode symbol is shown below. This diode includes two terminals an anode and a cathode. To specify the cathode terminal, it is marked with red or black color.

Specifications

The specifications of the 1N4148 small signal diode are listed below.

• This diode is available in the DO-35 package.
• A number of pins – 2.
• Single diode configuration.
• Maximum repetitive reverse voltage Vrrm is 100V.
• Forward current If is 150mA.
• The maximum forward voltage (VF) is 1V.
• Maximum reverse recovery time (trr) is 4ns.
• The maximum forward surge current (Ifsm) is 2A.
• The maximum operating temperature is 200°C.
• The case style of the diode is DO-204AH.

Small Signal Diode Construction

Small-signal diodes are constructed as PN junction diodes with a tiny junction area. A smaller junction area gives the diode smaller junction capacitance. This also reduces the reverse recovery time of the diode to a few nanoseconds or less.

This diode can be made with either Ge or Si type semiconductor material, although the diode characteristics change based on the doping material. This diode can be protected from contamination by a glass envelope which is also called a Glass Passivated Diode. The most popular small signal diode is the 1N4148. As compared to power diodes, these diodes are very small in appearance.

The performance of this diode is very effective in high-frequency applications. These diodes have a power dissipation and small current carrying capacity which ranges from 150mA & to 500mW correspondingly.

Small Signal Diode Application Circuit

The typical applications of the small-signal diode are waveform clipping, waveform clamping otherwise DC restoration, and input protection. Here one of the application circuits of small-signal diode-like waveform clipping is discussed below.

Generally, signal processing may also need the clipping process and this process is similar to rectification which means changing the signal shape by eliminating the positive otherwise negative peak of a signal. Here, the clipping action on a sine wave is shown below figure.

The first circuit a includes a resistor as well as a small signal diode. This circuit is very helpful in clipping the positive half cycles of a sine wave. Here assume the diode used is with zero junction potential, then the output signal will be half of the input wave amplitude & totally negative-going.

In the second circuit b, the diode is connected reversely and this diode is assumed as an ideal diode. So now the negative half cycle of the wave is removed and the positive half cycle of the sine wave is left.

In the third circuit c, two diodes are used which are assumed to not be theoretical ideal diodes, but these are small signal silicon types. These diodes will have about 0.7V of a forward junction potential, so do not clip the sine wave at 0V, although at +0.7V & -0.7V leaving a 1.4Vpp of the square wave.

Small Signal Diode Resistance

When the junction diodes are the same, and each diode has an equal current flow through it like ID =2.9 mA, then small-signal resistance for every junction diode is similar like rD=8.6Ω.  This is called small signal diode resistance.

Characteristics

The characteristics of a small signal diode include the following.

• Peak Reverse Voltage: The peak reverse voltage of this diode is denoted with ‘VPR’ and it is the maximum reverse voltage that can be given to the small-signal diode before it breaks down.
• Reverse Current: The reverse current is denoted with ‘IR’ and it is the current that supplies once it is reverse biased.
• Maximum forward voltage (VF) at peak forward current (IF).
• Reverse Recovery Time: The reverse recovery time is denoted with ‘Trr’ and it is the time necessary for reverse current to drop from forward current to reverse current.

Advantages

The advantages of a small-signal diode include the following.

• It is a small non-linear semiconductor diode.
• These are available in small sizes as compared to normal power diodes.
• This diode’s PN junction is generally encapsulated within the glass to defend it.
• As compared to rectifier diodes, these diodes have better power ratings & lower current.
• It can perform better in high-frequency-based applications.

The disadvantages of a small-signal diode include the following.

• As compared to rectifier diodes, these diodes have very low power & current ratings of approximately 150mA, 500mW maximum.
• The junction area of this diode is small so it gives a smaller junction capacitance to the diode.

Applications

The applications of small-signal diode include the following.

• These are used where small currents or high frequencies are necessary like in TV, radio & digital logic circuits.
• These diodes are used in switching, clipping, and high-frequency applications which deal with short-duration pulse waveforms.
• These are used as a switch within snubbers, rectifiers, limiters & wave-shaping circuits.
• These diodes are used in creating a regulated voltage supply by connecting diodes in series to provide a stable DC voltage across the combination of diodes.

Thus, this is an overview of small-signal diode & its working with applications. Here is a question for you, what is a signal diode?