What is Signal Diode : Working & Its Applications

Diodes are frequently used as simple rectifiers, switches to open or close a circuit, and mixers in the signal compounding. When the diodes are used in the mixers to detect the signals are known as signal diodes. At present, these diodes are used in electrical & electronic circuits to form a type of small elements which are designed from semiconductor crystals. So this article provides information on signal diode and its working with applications.


What is Signal Diode?

The signal diode definition is: It is a non-linear semiconductor device that is used where high frequencies or small currents are involved like in TVs, radio & digital logic circuits. The term ‘signal diode’ is frequently used to reference “small-signal diode”. At present, these diodes are used as small elements in electrical & electronic circuits which are made from semiconductor crystals.

Signal Diode with Symbol
Signal Diode with Symbol

The signal diode function is to process electrical signals within circuits, so they are simply necessary to pass up to 100mA of small currents. The signal diode symbol is shown above. Generally, these diodes include two terminals an anode and a cathode where the cathode terminal of this diode is marked with black color to identify and place the diode in the circuit properly.

Signal Diode Construction and Working

The construction of a signal diode is similar to a Mesa diode because, in a Mesa diode, an element of the PN block can be etched away so that it gives a high topped plateau structure. This diode includes two layers P-type and N-type. The N-type layer is separated into two regions like N- and N+ where one region (N-) is near the p-type material that is lightly doped & another layer is near the cathode contact which is heavily doped.

Construction
Construction

The whole structure is protected through a silicon oxide insulating layer or glass layer to guard the diode against oxidation. So, the construction of the Mess diode will make the diode more consistent which results in better predictability of its parameter.

Once the positive supply is applied to the anode and the negative supply is applied to the cathode of the diode then the diode conducts current. If the voltage is given in the reverse direction then the diode blocks any current. Ideally, this diode must block a negative signal completely.

Types of Signal Diodes

There are two types of signal diode which include the following.

Germanium Signal Diodes

Germanium signal diodes have very less amount of reverse bias resistance values which results in a low forward voltage drop across the PN junction ranging from 0.2V to 0.3V, but it has a high forward bias resistance value to the small PN junction area.

Silicon Signal Diodes

Silicon signal diodes have high reverse bias resistance values which lead to a forward voltage drop of about 0.6 to 0.7V across the PN junction. These diodes have fairly low forward bias resistance values which result in higher forward current & reverse bias voltage values.

Some of the signal diode examples with parameters are listed below.

Diode Name Forward Current Max in mA Max Forward Voltage Drop in Volts Peak Inverse Voltage (V)

OA70

50 1.5 22.5

OA72

10 45
OA73 8

20

OA79 10 2.2

40

OA81

50 1 115

OA85

30 1

110

IN34

400

2 40

IN4148

200 1 75
IN4151 300 1

50

Specifications

The signal diode specifications include the following.

Maximum Forward Current

The maximum forward current is, once the PN junction diode is connected in forward bias, the flow of current throughout its increases exponentially, so the diode will dissipate energy in the form of heat. When more forward current supplies throughout the diode, then a huge amount of heat can be generated across its junction. So the diode can get damaged because an ideal PN junction diode can withstand currents up to 10mA. This is why these diodes have a maximum forward current rating of upto 100 milliamps.

Maximum Reverse Voltage

The maximum reverse operating voltage is applied to this diode with no reverse breakdown & damage. As compared to avalanche breakdown voltage, this voltage is less which ranges from a few volts to 1000s of volts. The PIV rating is an essential factor to consider whenever the diode has to be applied in AC applications.

Total Power Dissipation

The maximum power that signals diode can dissipate within forwarding bias is known as total power dissipation. We have already discussed that these diodes dissipate power in the heat form across the junction.

The flow of current increases exponentially throughout the diode for a little change within the forward voltage. The power dissipation can be derived through the multiplication of the current & forward voltage throughout the diode. So, the whole power dissipation of this diode is in the 500mW range at 25oC.

Diode Capacitance

The junction capacitance of a diode is known as diode capacitance and it is provided for a specific frequency normally in the pF range.

Reverse Recovery Time

The reverse recovery time is an essential factor used for the diode selection within switching applications. The reverse recovery time of the signal diode is 150–200 ns.

Forward Recovery Voltage

Forward recovery voltage is the required voltage of the signal diode to switch back to the forward current level. So this parameter is extremely useful for high-speed switching applications. Generally, it is higher compared to the forward voltage of the diode. The forward voltage typically ranges from 0.6V to 1V for Si diodes whereas for Ge diodes it ranges from 0.2 to 0.5V.

Maximum Operating Temperature

The maximum operating temperature is the junction temperature and it is associated with the power dissipation of the diode. The junction temperature mainly depends on both the ambient temperature & forward current. These two factors will enhance the diode’s operating temperature & junction temperature of the diode. The maximum operating temperature of the signal diode is 25oC or 70oC.

Peak Inverse Voltage (PIV)

This parameter can be defined as the highest amount of voltage that can be given to the signal diode in the reverse direction. So this PIV should not be increased as compared to the voltage because the voltage greater than this PIV may cause the device breakdown. The typical PIV of this diode mainly ranges from a few volts to 1000s of volts.

Signal Diode Characteristics

The typical VI characteristics of signal diode with biasing conditions are explained below.

Generally, the signal can handle high voltage & current. At the Germanium diode’s PN junction, the barrier voltage typical values range from 0.2 V to 0.3 V. For the Silicon diode, it ranges from 0.6 V to 0.7 V. The maximum current allowed by these diodes is up to 100 milliamps.

Signal Diode VI Characteristics
Signal Diode VI Characteristics

Forward-Biased Region

In this region, the signal diode does not conduct until a fixed range of voltage is applied. Here, the voltage range allows the current flow based on the barrier voltage of that particular PN junction. When the level of voltage is applied crosses the barrier voltage, then the diode’s PN junction will allow the flow of current through it. So the main relationship between voltage and current is non-linear or exponential which is shown in the above diagram. In the forward bias region, the flow of current will increase rapidly when the voltage increases little.

Reverse-Biased Region

If the signal diode is connected in reverse biased, then a small amount of current will exist because of the minority charge carriers crossing the depletion layer & moving away from it which is called leakage current or reverse saturation current.

If the reverse bias voltage which is applied is extremely high, then minority charge carriers will get sufficient energy to collide & split up the covalent bonds to produce electron-hole pairs. So this phenomenon is known as a breakdown.

The highest reverse voltage is given to the signal diode before its breakdown state can be referred to as peak inverse voltage or peak reverse voltage.

Breakdown Region

The signal diode limits the current flow in reverse bias condition until a fixed range of voltage is achieved is known as. breakdown voltage. The flowing current is similar to a forward-biased current except in opposite direction and in different phenomena. In the breakdown region, the signal diode does not get smashed if the diode’s power dissipation rating has not been compromised. So, it is a good practice always to include a series resistor with the diode.

Signal Diode Arrays

The main purpose of signal diode arrays is to regulate the applied voltage to a circuit on the printed circuit board. If the applied voltage increases the rating of maximum voltage, then the surplus energy supplied will be entered like heat which may harm the device.

Signal Diode Arrays
Signal Diode Arrays

In order to protect the PCB board from surplus voltages, signal diode arrays are used by connecting either in different configurations like either series or parallel to provide stable known voltage in the limit.

Once these diodes are connected in series, then the highest current necessary by these diodes within the diode array is similar & the highest voltage drop within the diode array will be the amount of all the forward voltage drops within the diodes array.

In the series configuration of signal diodes, the output voltage will be stable despite changes within the flow of current in the connected load otherwise in the changes of the applied input voltage. Therefore constant voltage supply can be provided through the series arrangement of the signal diode.

In a series arrangement, every diode’s individual forward voltage drop of each diode in a series combination is subtracted from the applied input voltage to leave a fixed sum of the voltage across the load-connected resistor at the circuit’s end. This is mainly because of every diode’s ON resistance in addition to the RL or load resistance.

By including a number of diodes within a series arrangement, a huge amount of degradations within voltage will occur. In addition, the diodes which are connected in a series arrangement, in parallel through the RL perform like a voltage regulator circuit.

Signal Diode Vs Rectifier Diode

The difference between signal diode and rectifier diode includes the following.

Signal Diode

Rectifier Diode

Signal diodes are used in lower voltage or lower current lanes of the circuit. Rectifier diodes are used in power supplies.
These diodes can handle the less forward current. These diodes can handle more forward currents.
These diodes can be designed with semiconductor materials like Ge, Si, and GaAs. These diodes are made from a silicon material.

Signal Diode Vs Power Diode

The difference between signal diode and power diode includes the following.

Signal Diode

Power Diode

In these diodes, the doping level of both P & N sides is similar, so we can get a PN junction.

 

In these diodes, the junction can be formed in between a lightly doped N– layer & a heavily doped P+ layer which is grown epitaxially on a heavily doped N+ layer.
These diodes operate at a higher switching speed. These diodes function at high speeds.
The drift region is not present in the signal diode. The drift region is present in power diodes.
The current, voltage & power ratings are lower. The current, voltage & power ratings are higher.
These are low-power diodes. These are high-power diodes.
Power rating ranges from MW to watts. Power rating ranges from watts to kilowatts.
The current rating ranges from uA to mA. The current rating ranges from A to kA.
Voltage rating is in volts. The voltage rating is in kV.
These are low-power diodes. These are high-power diodes.

Signal Diode Vs Small Signal Diode

The difference between a signal diode and a small-signal diode includes the following.

Signal Diode

Small Signal Diode

The construction of a signal diode is similar to the Mesa diode. The construction of a small signal diode is similar to a PN junction diode.
It functions on high-voltage signals. It functions on low-voltage signals.
Its junction area is high as compared to the small-signal diode.

 

Its junction area is very small, so it stores less charge and has low capacitance.

 

Signal diodes are used to operate small currents & high-frequency signals. These diodes also operate the same as a signal diode.
These diodes are frequently used in TV, radio, switching circuits, and communication applications. These diodes are applicable in parametric amplifiers, high-speed switching & general purpose diode applications.

Advantages and Disadvantages

The advantages of a signal diode include the following.

  • The small-signal diode performance for high-frequency applications is very effective.
  • Generally, these diodes have less power dissipation and a small current carrying capacity which ranges from 150mA to 500mW.
  • These diodes have a quick recovery time.
  • These diodes are small in size.
  • These are efficient & fast through very high-speed signals.

The disadvantages of signal diode include the following.

  • These diodes cannot withstand extremely high reverse voltage.
  • This diode has a reverse saturation current.
  • At high frequencies, the level of noise is high.
  • Once the current flow increases then heat will be increased which can damage the diode.

Applications

The applications of signal diode include the following.

  • These diodes are used in current limiters, as a switch within rectifiers, in voltage snubbers, or in wave-shaping circuits.
  • These are frequently used in electronic circuits within radios, televisions & digital logic circuits, etc.
  • These diodes are used in a variety of signal processing applications.
  • These are used in digital devices for clocking functions and also avoid the reverse signal from harming the microcontroller.
  • These are used in clipping and switching applications where small duration pulse signals are normally clipped off.

Thus, this is all about an overview of a signal diode and its working with applications. These diodes are normally used in high-frequency & low-power circuits like digital logic, rectification, & freewheeling, etc. Here is a question for you, what is a freewheeling diode?

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