Application of phototransistor

Definition: Phototransistors resemble normal transistor except the fact that the base terminal is not present in case of the phototransistor. Phototransistors convert the incident light into photocurrent. Instead of providing the base current for triggering the transistor, the light rays are used to illuminate the base region.

The base terminal is made up of the material which shows sensitivity towards the light. The circuit symbol of the phototransistor is similar to that of the conventional transistor but the base terminal can be omitted.

The two arrows point towards phototransistor indicates that the phototransistor is triggered by the light incident on it. The Phototransistors are manufactured in the similar way by which normal transistor is manufactured, the only difference is the area of the base and collector region in case of phototransistors is quite large as compared to the normal transistor. This is because the more the light falls on the phototransistor the more current it will generate.

The collector and base region are formed by the techniques of ion-implantation and diffusion. The transistor which were used earlier was made of semiconductor material such as Germanium and Silicon and the resulting structure becomes a homogeneous material consist of either Silicon or Germanium.

On the contrary, contemporarily, phototransistors are made up of Group-III and Group-V materials such as GaAs Gallium Arsenide in such a way that gallium and arsenide, each of these are used on either side of the transistor. The resulting structure becomes heterogeneous in nature. This type of structure is used widely because the conversion efficiency increases several times as compared to the conversion efficiency of the homogenous transistor.

The output of the phototransistor is taken from the emitter terminal and the light rays are allowed to enter the base region. The magnitude of the photocurrent generated by the phototransistor depends on the light intensity of the light falling on the transistor. It can be of three terminals or two terminals we can omit base as per our requirement.

The phototransistor can be operated in three regions that are the cut-off region, active region, and the saturation region.

application of phototransistor

The cut-off region and saturation region can be used to operate the transistor as the switch. The active region is used for generating current. The current generated from phototransistor depends on several factors apart from luminous intensity such as. The output characteristics of phototransistor can be understood with the help of the diagram below. It shows the variation of collector current with respect to the variation in the emitter-collector voltage.

Phototransistors are the crucial optoelectronics device, it is also used in optical fibres. Due to its several advantages over photodiodes, it is more preferred over photodiodes.

Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Leave a Reply Cancel reply Your email address will not be published.It is a 3-layer semiconductor device that consists of a light sensitive base region. It is basically a transistor whose action depends on the application of light.

Hence named phototransistor. The phototransistor is basically an enhancement of Photodiode. As phototransistor has the ability to give larger gain than that of the photodiode.

It is similar to a normal BJT but the only variation is that in phototransistor, virtually the base region exists but it is kept unconnected with the external supply. This means that as in normal BJT, base current is used to drive the circuit, however, in phototransistor light energy falling on the base region acts as the overall input of the device. A phototransistor is said to exhibit the combined operation of the photodiode as well as a normal transistor.

It operates on the principle of Photoelectric effect. As it changes light signal incidenting on its surface into its electrical equivalent form.

And the transistor action permits it to perform amplification of the current flowing through it. As we have already discussed that a phototransistor is nothing but a normal transistor whose action depends on the incident radiation falling at its base. At the time of constructing the phototransistor, the base and collector region is provided with a larger area in comparison to a normal BJT. The figure below represents the constructional structure of an NPN Phototransistor :.

Here, as we can see that the light is majorly allowed to incident at the base collector junction. Initially, phototransistors were fabricated from silicon or germanium as their basic material that resultantly provides homojunction structure. However, in recent times, these are constructed using materials likes gallium or arsenide. Thereby, providing a heterojunction structure. This is so because these structures exhibit large conversion efficiency. This implies they are more capable of changing light energy into electrical energy as compared to homojunction transistors.

Phototransistors are mainly enclosed in a metallic case that consists of the lens at the top in order to gather the incident radiation. The figure below represents the symbolic representation of a Phototransistor :.

Here, the symbolic representation is almost similar to a normal BJT but the only variation is the presence of two inward arrows at the base region that shows the incident of light radiation. The operation of a phototransistor depends on the intensity of radiation falling at its base region. Its working is almost similar to a normal transistor, however; the variation lies in the input current that drives the circuit.

And in the case of a phototransistor, the incident light generates driving current. In the circuit arrangement, we can clearly see that the base region is kept unconnected with the external supply voltage and is used as the region for radiation incidence.

application of phototransistor

Only the collector region is connected to the positive side of the supply provided along with emitter which is connected to the negative side. However, the output is taken at the emitter terminal of the transistor.Photodiodes and phototransistors are semiconductor devices which have their p-n semiconductor junction exposed to light through a transparent cover, so that external light can react and force an electrical conduction through the junction. A photodiode is just like a regular semiconductor diode example 1N consisting of a p-n junction, but it has this junction exposed to light through a transparent body.

Its working can be understood by imagining a standard silicon diode connected in reverse biased fashion across a supply source as shown below. However, suppose we have the same diode with its outer opaque cover scraped of or removed and connected with a reverse bias supply.

application of phototransistor

This will expose the PN junction of the diode to light, and there will be an instant flow of current through it, in response to the incident light. This may result in a current as much as 1 mA through the diode, causing a rising voltage to develop across R1. The photodiode in the above figure can be also connected on the ground side as shown below.

This will produce a opposite response, resulting in a decreasing voltage across R1, when the photodiode is illuminated with external light. The working of all P-N junction based devices is similar and will exhibit photo-conductivity when exposed to light.

Compared to cadmium-sulphide or cadmium-selenide photocells like LDRsphotodiodes are generally less sensitive to light, but their response to light changes is much faster. Due to this reason, photocells like LDRs are generally used in applications that involve visible light, and where the response time does not need to be quick.

On the other hand, photodiodes are specifically selected in applications that require fast detection of lights mostly in the infrared region. You will find photodiodes in systems such as infrared remote control circuitsbeam interruption relays and intruder alarm circuits. There's another variant of photodiode which uses lead-sulfide PbS and there working characteristic is quite similar to LDRs but are designed to respond only to the infrared range lights.

The phototransistor is generally in the form of a bipolar NPN silicon transistor encapsulated in a cover with a transparent opening. It works by allowing light to reach the PN junction of the device through the transparent opening. The light reacts with the exposed PN junction of the device, initiating the photoconductivity action.

A phototransistor is mostly configured with its base pin unconnected as shown in the following two circuits. In the left side figure the connection effectively causes the phototransistor to be in the reverse bias situation, such that it now works like a photodiode. Here, the current generated due to light across the base collector terminals of the device is directly fed back to the base of the device, resulting in the normal current amplification and the current flowing out as the output from the collector terminal of the device.

This amplified current causes a proportionate amount of voltage to develop across the resistor R1. Phototransistors may show identical amounts of current at their collector and emitter pins, due to an open base connection, and this prevents the device from a negative feedback.

PhotoTransistors - Transducers - Light Energy to Electrical Energy - Working EXPLAINED in EASY steps

Due to this feature, if the phototransistor is connected as shown at the right side of the above figure with R1 across emitter and ground, the outcome is exactly identical as it had been for the left side configuration. Meaning for both the configurations, the voltage developed across R1 due to phototransistor conduction is similar. Although the working principle is similar for the two counterparts, there are a few noticeable differences between them. A photodiode may be rated to work with much higher frequencies in the range of tens of megahertz, as opposed to a phototransistor which is restricted to only a few hundred kilohertz.

The presence of the base terminal in a phototransistor makes it more advantageous compared to a photodiode. A phototransistor can be converted to work like a photodiode by connecting its base with ground as shown below, but a photodiode may not have the ability to work like a phototransistor. Another advantage of the base terminal is that the sensitivity of a phototransistor can be made variable by introducing a potentiometer across base emitter of the device as shown in the following figure.

In the above arrangement the device works like a variable sensitivity phototransistor, but if the pot R2 connections are removed, the device acts like a normal phototransistor, and if R2 is shorted to ground, then the device turns into a photodiode.

Phototransistor

In all the circuit diagrams shown above the selection of R1 value is usually a balance between voltage gain and bandwidth response of the device.Phototransistor circuits are normally relatively straightforward, especially of the detector is only required to detect the presence of absence of a particular light source.

As a result of their ease of use and their applications, phototransistors are used in many applications. The phototransistor can be used in a variety of circuits and in a number of ways dependent upon the application.

Being a low cost device the phototransistor is widely used in electronic circuits and it is also easy to incorporate. The phototransistor can be used in a variety of different circuit configurations. Like more conventional transistors, the phototransistor can be used in common emitter and common collector circuits. Common base circuits are not normally used because the base connection is often left floating internally and may not be available. If the base connection is required, then it is necessary to buy a phototransistor with a base connection available.

The choice of common emitter or common collector phototransistor circuit configuration depends upon the requirements for the circuit. The two phototransistor circuit configurations have slightly different operating characteristics and these may determine the circuit used.

The common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected. The circuit actually acts as an amplifier.

The current generated by the light affects the base region. This is amplified by the current gain of the transistor in the normal way. The common collector, or emitter follower phototransistor circuit configuration has effectively the same topology as the normal common emitter transistor circuit - the emitter is taken to ground via a load resistor, and the output for the circuit being taken from the emitter connection of the device. The circuit generates an output that moves from the low state to a high state when light is detected.

The phototransistor circuits can be used on one of two basic modes of operation. They are called active or linear mode and a switch mode. Operation in the "linear" or active mode provides a response that is very broadly proportional to the light stimulus. In reality the phototransistor does not give a particularly linear output to the input stimulus and it is for this reason that this mode of operation is more correctly termed the active mode. The operation of the phototransistor circuit in the switch mode is more widely used in view of the non-linear response of the phototransistor to light.

When there is little or no light, virtually no current will flow in the transistor, and it can be said to be in the "off" state.In this tutorial, we will learn about Phototransistors, characteristics of photo transistors, what are the things to consider when selecting a phototransistor and a few example circuits using phototransistor as a Light Sensor.

Before going into the details of Phototransistors, let us refresh our understanding of Sensors and in particular, Optical Sensors. From the perspective of an Electronics Engineers, a Sensor is a device that responds to a physical phenomenon or property with an electrical signal. The input of a sensor can be ay physical quantity like Light, Sound, Temperature etc. The above definition of a Sensor can lead to another interpretation of Sensor i.

An Optical Sensor is a device that measures the intensity of light, usually the Electromagnetic Radiation in wavelength range from Ultraviolet to far Infrared. Since atypical Light Sensor is associated with absorption of photon by the sensing material, almost all the Light Sensors are divided into two types.

They are:. Optical Sensors that fall under the quantum detectors category usually operate in the Ultraviolet to mid Infrared range of the EM Spectrum while that fall under thermal detectors operate in the mid to far Infrared range of the EM Spectrum.

Before understanding Phototransistors, let us first briefly understand what a Photodiode is. In simple words, a Photodiode is an optical sensing PN Junction diode but in reverse bias condition so that the current is very low. When a photon of sufficient energy hence, the dependency of the wavelength of the light hits the photodiode, an electron is freed with an energy to pass the barrier i. Keeping this mind, a Phototransistor is a Photojunction device so, is Photodiode that is similar to a regular transistor except that it has a light sensitive Base terminal or Collector — Base Junction, to be precise.

In other words, a photo transistor can be considered as a Photodiode with current amplifier. A phototransistor converts photons to charge directly, just like a photodiode, and in addition to this, a phototransistor also provides a current gain. Like regular transistors, phototransistors also have large gain but the main difference is the size of the base — collector junction. In phototransistors, the size of base — collector junction is larger as it is a light sensitive region of the sensor.

The larger size of the junction results in a significantly larger junction capacitance and as a result, phototransistors have low frequency response than photodiode in spite of the high gain. The principle of operation of a phototransistor is similar to a photodiode in combination with an amplifying transistor. The light incident on the base of a phototransistor will induce a small current. This current is then amplified by normal transistor action, which results in a significantly large.

Usually, when comparing to a similar photodiode, a phototransistor can provide a current that is 50 to times that of a photodiode.

As phototransistors are basically Bipolar NPN Transistors with large base — collector junction, the characteristics of a Phototransistor are similar to that of a simple BJT. Phototransistors are available as two-leaded or three-leaded devices.

In a two-leaded Phototransistor, the Base terminal electrically unavailable and the device is completely dependent on light. Collector terminal is usually at a higher potential than Emitter to induce reverse bias at base — collector junction.

When there is no light falling on the phototransistor, a small amount of leakage current known as Dark Current flows from collector to emitter. When there is enough light falling on the base terminal, a base current is produced, which is proportional to the intensity of the light.

The base current will then trigger the amplification process and a collector current with high gain flows. The following image shows the collector current characteristics curve. From the above curve, it is clear that as the intensity of the light increases, the collector current also increases.

As mentioned earlier, phototransistors are also available as three-terminal devices. In this case, the usage of Base terminal is optional. When used, it acts as a regular BJT and when not used, it acts as a Phototransistor. When choosing a Phototransistor, there are a few things or properties to be considered so that the phototransistor can be used at its best.

As mentioned earlier, only photon of specific energy can excite the electrons and this means that the wavelength of the light is an important factor. Phototransistor usually have a specific range of wavelengths that they can sense. Another important property of a phototransistor is the linearity of the output. How linearly the output varies according to the intensity of the light is an important consideration. Sensitivity of the phototransistor is the ratio of the output signal to the input intensity of the incident light.It seemed to leap off my bookshelves into the purses of friends.

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Basics of Phototransistor

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application of phototransistor

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