bjt as an amplifier and as a switch pdf

Bjt As An Amplifier And As A Switch Pdf

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4.2: The Bipolar Junction Transistor (BJT) as a Switch

In this Transistor tutorial, we will learn about the working of a Transistor as a Switch. Switching and Amplification are the two areas of applications of Transistors and Transistor as a Switch is the basis for many digital circuits. As one of the significant semiconductor devices, transistor has found use in enormous electronic applications such as embedded systems, digital circuits and control systems.

In both digital and analog domains transistors are extensively used for different application usage like amplification, logic operations, switching and so on. This article mainly concentrates and gives a brief explanation of transistor application as a switch.

Almost in many of the applications these transistors are used for two basic functions such as switching and amplification. The name bipolar indicates that two types of charge carriers are involved in the working of a BJT. These two charge carriers are holes and electrons where holes are positive charge carriers and electrons are negative charge carriers. The transistor has three regions, namely base, emitter and collector.

The emitter is a heavily doped terminal and emits electrons into the base. Base terminal is lightly doped and passes the emitter-injected electrons on to the collector. The collector terminal is intermediately doped and collects electrons from base. This collector is large as compared with other two regions so it dissipates more heat. These two transistors can be configured into different types like common emitter, common collector and common base configurations.

Depends on the biasing conditions like forward or reverse, transistors have three major modes of operation namely cutoff, active and saturation regions. In this mode transistor is generally used as a current amplifier. In active mode, two junctions are differently biased that means emitter-base junction is forward biased whereas collector-base junction is reverse biased. In this mode current flows between emitter and collector and amount of current flow is proportional to the base current.

Cutoff Mode In this mode, both collector base junction and emitter base junction are reverse biased. This in turn not allows the current to flow from collector to emitter when the base-emitter voltage is low.

In this mode device is completely switched off as the result the current flowing through the device is zero. Current flows freely from collector to emitter when the base-emitter voltage is high.

In this mode device is fully switched ON. The below figure shows the output characteristics of a BJT Transistor. In the below figure cutoff region has the operating conditions as zero collector output current, zero base input current and maximum collector voltage.

Therefore, the transistor is completely in OFF condition. Similarly, in the saturation region, a transistor is biased in such a way that maximum base current is applied that results maximum collector current and minimum collector-emitter voltage.

This causes the depletion layer to become small and to allow maximum current flow through the transistor. Therefore, the transistor is fully in ON condition. This type of switching application is used for controlling motors, lamp loads, solenoids, etc.

A transistor is used for switching operation for opening or closing of a circuit. This type solid state switching offers significant reliability and lower cost as compared with conventional relays. Some of the applications use a power transistor as switching device, at that time it may necessary to use another signal level transistor to drive the high power transistor. Based on the voltage applied at the base terminal of a transistor switching operation is performed. Therefore, the transistor acts as a short circuit.

Similarly, when no voltage or zero voltage is applied at the input, transistor operates in cutoff region and acts as an open circuit. In this type of switching connection, load here LED lamp is connected to the switching output with a reference point. At the base input a signal varying between 0 and 5V is given so we are going to see the output at the collector by varying the V i at two states that is 0 and 5V as shown in figure.

From the above calculations, the maximum or peak value of the collector current in the circuit is 7. And the correspond base current to which collector current flows is So, it is clear that when the base current is increased beyond the Consider the case when zero volt is applied at the input. This causes the base current zero and as the emitter is grounded, emitter base junction is not forward biased.

Therefore, the transistor is in OFF condition and the collector output voltage is equal to 5V. Therefore the base current is greater than Thus the output at the collector becomes approximately zero. This type of switching is used for negative ground configurations. For the PNP transistor, the base terminal is always negatively biased with respect to the emitter.

In this switching, base current flows when the base voltage is more negative. Simply a low voltage or more negative voltage makes transistor to short circuit otherwise it will be open circuited or high impedance state. In this connection, load is connected to the transistor switching output with a reference point. When the transistor is turned ON, current flows from the source through transistor to the load and finally to the ground.

Similar to the NPN transistor switch circuit, PNP circuit input is also base, but the emitter is connected to constant voltage and the collector is connected to ground through the load as shown in figure. In this configuration base is always biased negatively with respect to the emitter by connecting the base at negative side and the emitter at the positive side of the input supply.

Therefore, for the conduction of transistor emitter must be more positive with respect to both collector and base. In other words base must be more negative with respect to the emitter. For calculating the base and collector currents following expressions are used. Consider the above example, that the load requires milli ampere current and the transistor has the beta value of Then the current required for the saturation of the transistor is.

Therefore, when the base current is 1 mA, the transistor will be fully ON. But practically 30 percent of more current is required for guaranteed saturation of transistor. So, in this example the base current required is 1. As discussed earlier that the transistor can be used as a switch. It is also possible to control the relay operation using a transistor. With a small circuit arrangement of a transistor able to energise the coil of the relay so that the external load connected to it is controlled.

Have you got the clear idea of how a transistor can be used as a switch? We acknowledge that the furnished information clarifies the entire switching concept with relevant images and examples. Further any doubts, suggestions and comments on this post you can write below.

Loved a lot…. More n more information given in this I have not understood when my teacher explained but I understood when I red it once tx for this info…… Too helpfull Very good explanation……???

O was really stuck on transistors for almost 2 months, studying class notes but got nothing. Today I made it. Your email address will not be published. Comments Great tutorial. Thanks a lot. Very nice explaination. It can be better if you use animated ckt. Sir very Useful article and very good explanation thanks sir. Leave a Reply Cancel reply Your email address will not be published.

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The Bipolar Junction Transistor (BJT) as a Switch

The Bipolar Junction Transistor can be used as a switch. This is required when the typical digital output max current output 20ma does not provide sufficent current drive for high current devices like lamps, solenoids or motors. The transistor current gain provides the order of magnitude increase in collector current. Notes Observe the node voltages when you change VIN by adjusting the knob. To calculate the node voltages Use equation 4 to calculate I B. See diode circuit analysis for explanation.


Circuit using Transistors. Input1 of differential amplifier is connected to the base of transistor Q1 and input2 of the differential is connected to the base of another transistor. DC load line and operating point of Q2, transistor with T model as shown in Fig.

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A bipolar junction transistor BJT can be used in many circuit configurations such as an amplifier, oscillator, filter, rectifier or just used as an on-off switch. If the transistor is biased into the linear region, it will operate as an amplifier or other linear circuit, if biased alternately in the saturation and cut-off regions, then it is being used as a switch, allowing current to flow or not to flow in other parts of the circuit. This lab activity describes the BJT when operated as a switch.

A transistor connected in the three modes illustrated in Figs. What is a Transistor? Describe the Transistor Action in Detail. Nei transistor metallici di potenza vedi fig. Hi, Disclaimer. I need to replace a BJT transistor well I'm pretty certain that is what it is on a PCB and am unfamiliar with how to make the right selection.

Amplifier & a Switch, Large Analysis,. BJT at DC Biasing BJT Small Signal. 1. BJT at DC The amplifier is biased at a point Q, and a small voltage signal vi is.


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