Describe transistor as a switch with neat sketch.
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From the circuit we can see that the control input Vin is given to base through a current limiting resistor Rb and Rc is the collector resistor which limits the current through the transistor.

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When a sufficient voltage V is given to input, transistor becomes ON & it goes into saturation. During this condition the Collector Emitter voltage Vce will be approximately equal to zero, ie the transistor acts as a short circuit & Vo = 0. When input voltage V=0, transistor becomes OFF & it goes into cutoff. The transistor acts as an open circuit. During this condition the Collector Emitter voltage Vce=Vcc. Therefore Vo = Vcc.

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Transistor is a three terminal semiconductor device that amplify the current and used as a open close switch . Transistors plays a very important role in replacing vacuum tubes. Because transistors are ideal component for using in the digital circuits as switch.

A transistor can be used as a switch because its collector current is directly controlled by the base current . If the base current is greater than 0.7 volts than the transistor acts as a closed switch. To understand this let’s take a look of current characteristics of a transistor .

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The characteristic curve above shows the three regions.

Saturation region Cut off region Active region


When we are using the transistor as switch we deals with only saturation region and cut off region.


Transistor in Cut off region :

In the cut off region the base voltage is less than 0.7 V so the base current is also negligible. In the transistor the collector current is directly proportional to the base current. So the collector current Ic is also negligible. But the collector to emitter voltage VCE is maximum. Which creates a large depletion region and no current flows through the transistor. So this region is called as cut off region.

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The conditions for the transistor in the saturation region are Vb < 0.7 V, Collector Current ( Ic) = minimum, Collector to Emitter Voltage ( VCE ) = maximum.


Transistor in Saturation region : 

In this mode the maximum base current is applied that results with high collector current and lower collector to emitter voltage. And this results a lower depletion region layer. By this the large amount of current can flow through the transistor.

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Fig. Transistor in saturation region


The conditions for the transistor in the saturation region are Vb > 0.7 V, Collector Current ( Ic) = maximum, Collector to Emitter Voltage ( VCE ) = minimum. 

When the base bias voltage is lower than 0.7 V. The transistor operated in the cut off region. But when the base bias voltage increases from 0.7 V the base saturated the collector and the current starts flow. In this state the transistor acts as a closed switch.

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Transistor as Switch: A transistor can be used for two types of applications viz. amplification and switching. For amplification, the transistor is biased in its active region. For switching applications, transistor is biased to operate in the saturation (full on) or cut-off (full off) region.

(i) Transistor in cut-off region (Open switch):

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In the cur-off region, both the junctions of transistor are reverse biased and very small reverse current flows through the transistor. The voltage drop across the transistor (VCE) is high, nearly equal to supply voltage VCC. Thus, in cut-off region the transistor is equivalent to an open switch as shown in fig.(a).


(ii) Transistor in Saturation region (Closed switch): 

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When Vin is positive, a large base current flows and transistor saturates. In the saturation region, both the junctions of transistor are forward biased. The collector current is very large, the voltage drop across the transistor (VCE) is very small, of the order of 0.2V to 1 V, depending on the type of transistor. Thus in saturation region, the transistor is equivalent to a closed switch.

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The transistor can be used for two types of application viz. amplification and switching. For the amplification as a transistor is biased in its active region. 

Whereas for switching applications it is biased to operate in the saturation (full on) or cut off (full off) region. 


a. Transistor in cut- off region (open switch): 

In the cut –off region both the junction of a transistor are reverse biased and very small reverse current flows through the transistors. 

The voltage drop across the transistor (VCE) is high. Thus, in the cut off region the transistor is equivalent to an open switch. 

Therefore , VCE = VCC


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b. Transistor in the saturation region(closed switch): 

When Vin is positive a large base current flows and transistor saturates. 

In the saturation region both the junctions of a transistor are forward biased. The voltage drop across the transistor (VCE) is very small, of the order of 0.2 V to 1V depending on the type of transistor and collector current is very large. In saturation the transistor is equivalent to a closed switch. 

Therefore , VCE = 0 

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In Bipolar Transistor as a Switch the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the “ I-V ” characteristics curves. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below. 


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1. Cut-off Region 

Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”. 


2. Saturation Region 

Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.

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