Please follow our facebook page for more info Link : https://www.facebook.com/Electroconcepts357/ As an electronics student, you may have heard many times that transistor is used as a switch. But did you know how transistor is used a switch ? What properties of transistor allows it to be used as a switch ?

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Transistor as you know is basically, a three terminal device consisting of Base, emitter and collector. (If we are talking about BJT)

In digital circuits, we often refer to digital logic “1” and “0” which can be referred as logic “High” or logic “Low”. Almost any electronic device works on these two logic standards. Although there are many logic families like TTL, ECL, CMOS etc (complicated stuff) But, lets talk about the simplest application where a BJT can be used as an INVERTER. INVERTER is a logic gate which inverts the input. If input is “1” output will be “0” similarly if input is “0” output will be “1” But how can a transistor be used as an INVERTER ? Consider an NPN transistor connected as shown in the above figure. There can be two cases when transistor is used as a switch. Case 1 : Transistor is in cut off region 

• When Vb (base voltage) is very low, the base current (Ib) will be negligible.
• Due to this, the collector current (Ic) will also be minimum and transistor will be in cut off region.
• As the collector current (Ic) is very low, there will be almost no voltage drop across resistance (Rc) thus IcRc = 0 volts

By using Kirchoffs voltage law (KVL) we can say that, Vcc = Vce + IcRc But since, IcRc = 0 volts  Vcc = Vce + 0 = Vce Thus Vce = Vcc (supply voltage Vcc is usually 5 volts) For e.g if we give a base voltage (Vb) of 0 volts, we get collector to emitter voltage (Vce) of 5 volts. Thus if we give input as logic “0” we get output as logic “1” Case 2 : Transistor is in saturation region 

• When Vb (base voltage) is very high, the base current (Ib) will be maximum.
• Due to this, the collector current (Ic) will also be maximum and transistor will be in saturation region.
• That is, if we increase Ib, then Ic will also increase but, beyond a certain point Ic cannot increase and it will attain a maximum value.
• As the collector current (Ic) is very high, The voltage drop across Rc which is IcRc = very high

By using Kirchoffs voltage law (KVL) we can say that, Vcc = Vce + IcRc  But since, IcRc = very high, All the voltage supplied by Vcc will be across IcRc and Vce will be almost 0 volts. But practically in saturation region Vce is very low (0.2 volts) which is almost considered as 0 volts. Vcc = 0 + IcRc = IcRc Thus Vce = 0 volts  For e.g if we give a base voltage (Vb) of 5 volts, we get collector to emitter voltage (Vce) of 0 volts. Thus if we give input as logic “1” we get output as logic “0”

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Thus if we summarize the following data we can infer that,

• When Vb = 0 volts (logic 0) , Transistor will be in cut off region. Vce = 5 volts (logic 1) . Transistor will act as an open switch.

• When Vb = 5 volts (logic 1) , Transistor will be in cut off region. Vce = 0 volts (logic 0) . Transistor will act as a closed switch.

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