What is a signal diode and how does it work?

Signal Diode is a non-linear semiconductor device which is basically used in electronic circuits and include small currents or high frequencies. Radio, television, and digital logic circuits are some of examples of signal diode. Small two-terminal signal diodes conduct current when forward biased and stop current flow when reverse biased.

When compared to larger Power Diodes, signal diodes such as the Point Contact Diode or the Glass Passivated Diode are physically quite small. A small signal diode's PN junction is normally enclosed in glass to preserve it, and it has a red or black band on one end of its body to help distinguish which end is the cathode terminal. The 1N4148 and its counterpart 1N914 signal diodes are the most commonly utilized of all the glass encased signal diodes.

When compared to rectifier diodes, small signal and switching diodes have much lower power and current ratings, around 150mA and 500mW maximum, but they can perform better in high frequency applications or clipping and switching applications that deal with short-duration pulse wave-forms.

The characteristics of a signal point contact diode varies depending on whether it is made of germanium or silicon, and are as follows:

  1. Germanium Signal Diodes: These have a low reverse resistance, resulting in a reduced forward volt drop across the junction (usually 0.2 to 0.3 volts), but a greater forward resistance due to their tiny junction area.
  2.  Silicon Signal Diodes: These have a high reverse resistance and a forward volt drop of 0.6 to 0.7 volts across the junction. They have relatively low forward resistance, resulting in high forward current and reverse voltage peak values.

The electronic symbol for any type of diode is an arrow with a bar or line at one end, as shown below, along with the Steady State V-I Characteristics Curve.

V-I Characteristic Curve of Silicon Diode

The arrow always points in the direction of the traditional current flow through the diode. That is, the diode conducts only when the positive power supply is connected to the anode terminal (a) and the negative power supply is connected to the cathode terminal (k), so the current flows in only one direction and is an electric one-way valve. It works like (forward).

However, when you connect an external power supply in the opposite direction, the diode blocks the current flowing through it and instead behaves like an open switch (reverse bias state) as shown below.

Forward and Reversed Biased Diode

The ideal small signal diode passes current in one direction (forward conduction) and cut off current in the other direction (reverse blocking). Signal diode performs as closed switch in In forward bias and thus short circuited for driving current solely in one direction (from the positive terminal to the negative terminal). On the other hand, silicon diode performs as open switch in reverse bias and thus open circuited for blocking the current flowing in the diode. Signal diodes are used in a variety of applications. Signal diodes are used as a rectifier, current limiter, voltage limiter, or switch for waveform shaping circuits.

Prasun Barua

Prasun Barua is an Engineer (Electrical & Electronic) and Member of the European Energy Centre (EEC). His first published book Green Planet is all about green technologies and science. His other published books are Solar PV System Design and Technology, Electricity from Renewable Energy, Tech Know Solar PV System, C Coding Practice, AI and Robotics Overview, Robotics and Artificial Intelligence, Know How Solar PV System, Know The Product, Solar PV Technology Overview, Home Appliances Overview, Tech Know Solar PV System, C Programming Practice, etc. These books are available at Google Books, Google Play, Amazon and other platforms.


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