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.

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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 1N 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 1N. 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?

Key Takeaways

• Nonlinear components sometimes need a linear approximation to quickly learn about a circuit&#;s electrical behavior.

• The quickest way to approximate a nonlinear component&#;s electrical behavior is to use a small signal model, which uses a Maclaurin series expansion around a specific operating point.

• Using a small-signal model helps you understand more about your circuits, but it fails when the input signal gets too large. Too large as compared to? This is discussed later, so a couple of words is all that is needed here.

A small-signal model for a diode gives you a quick way to analyze nonlinear circuits.

When you&#;re a physicist or mathematician, you spend plenty of time taking series expansions of different functions. Engineers sometimes need to do the same, but they often don&#;t know it. One instance where you really need to take a look at series expansions is when you are examining the behavior of nonlinear electrical components and systems. Doing this makes your job of analyzing circuit behavior much easier.

The classic nonlinear component most engineers see is a diode. A small-signal model for a diode is extremely easy to understand, and every other small-signal model can be derived using the same mathematical process. To understand what the small-signal diode model means for circuit analysis, we must first understand how it works. 

The Small-Signal Diode Model

Saying a model for an electronic component is a &#;small-signal&#; model means something very specific. In particular, we mean that the voltage drop across the component is only a small fraction above or below some desired operating voltage. Developing a small-signal model is all about approximating the voltage drop across the diode and the diode current using a derivative. The goal is to describe how the output (the diode current) changes when there is a small change in the input (the voltage drop).

To start, let&#;s look at the equation for the current in a diode as a function of voltage drop across the diode:

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Current in a diode as a function of voltage drop across the diode. Please define n, k, and T

Here we need to approximate the current as a function of voltage near some operating voltage. First, define V0 as the operating voltage across our diode. The goal of a small-signal model is to get a value for the admittance (or impedance) of the component. The admittance is simply the derivative of the diode current, with respect to the voltage drop evaluated at the operating point:

Admittance of the diode at its operating point.

It&#;s important to note that a change in the operating point V0 also changes the admittance. This is to be expected, as the current in a diode is a nonlinear function of the voltage drop. With this in mind, we can approximate the diode current as a linear function of voltage drop across the diode, i.e., as I = VY:

Small signal model for the current in a diode as a function of its admittance and voltage drop V near the operating point V0.

This equation basically defines Ohm&#;s law for a diode within a small range of voltages. If, for example, you send an AC signal with small amplitude across the diode, the above equation tells you the relationship between the voltage and current. Simply plug in the function for the AC voltage, and it will give you the current. This enables the use of Kirchoff&#;s laws to analyze the current in a circuit with a diode near the operating voltage, including circuits with reactive components.

Why Use a Small Model?

There are two reasons to use a small signal model for circuit analysis in nonlinear circuits:

1. DC current and voltage in a nonlinear component often requires solving a transcendental equation, which often does not have closed-form analytical solutions.

2. When AC voltage is dropped across the nonlinear component, higher-order harmonics are generated due to frequency mixing. Using a small signal model simply ignores the potential for harmonic generation.

To see how this is helpful, consider the following circuit:

Example circuit with a diode.

If you try to calculate the total current in the circuit I1 using series and parallel rules, you&#;ll find that this current is a function of the voltage drop across the diode. The voltage drop across the diode is then equal to Vd = V - V40 - V20. This gives a complicated transcendental equation for I1 as a function of the current in the remaining components. Using a small-signal model allows the standard Gauss-Jordan matrix technique in SPICE simulations for linear circuits to be used to determine the voltage and current in every component (both for DC and AC inputs).

The other reason to use a small-signal model is to avoid the need to consider harmonics generated for AC signals. Mathematically, the current/voltage relationship in a nonlinear component can be approximated using a Taylor or Maclaurin series, which gives a high degree polynomial. For an AC signal, taking powers of the input voltage will produce higher-order harmonics of the AC input.

When the AC input is small enough, the generated harmonics will also be small and can be ignored. Otherwise, at large AC input, the current will contain extra harmonics that will be seen in the time domain and frequency domain. More complex methods like harmonic balance analysis are needed to consider the full AC behavior in reactive nonlinear circuits in the frequency domain.

Beyond Diodes

The methodology used to describe diodes at a specific operating point can also apply to other components. The same series expansion and operating point technique can be used to approximate linear behavior in:

• Back-to-back diodes

• Transistors (note that we&#;re not referring to load lines)

• Photodiodes and other electro-optical components

• Iron core inductors and transformers

• Electrolytic solutions

• Varactors or barium strontium titanate capacitors

• Power amplifiers operating near saturation

• Analog mixers, limiters, and multipliers

Small-signal models aren&#;t limited to individual components. Any circuit or N-port network that contains at least one nonlinear component is a nonlinear circuit. Therefore, the overall relationship between inputs and outputs can be described with a small-signal model. In terms of modeling with your circuit design software, you can construct a phenomenological model for a component and include it in a different schematic or circuit. You only need to examine the relationship between each input and output in your nonlinear circuit; what happens within each component is less important.

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