Explain how transistor works as a switch with i/p and o/p waveform

 Explain how transistor works as a switch with i/p and o/p waveform

                                               

                                                

Ans. First of all we have to start the transistor or we can say that activate the transistor . So for activating transistor , voltage ( Vbe ) should always in a forward bias and voltage ( Vce ) should always in reverse bias .

If we apply the voltage less than the cut off voltage or we can say that knee voltage , then transistor acts or behave as a open switch .

And if we apply the voltage more than the cut off or we can say that knee voltage , then transistor acts or behave as a close switch .

So , the derivation of these circuit is !

                         Input section of the circuit 

                   Vbb - Vb . Rb = 0

        i.e.     Vbe = Vbb - Ib . Rb    ....equation (1)

            Output section of the circuit 

                   Vcc - Ic . Rc - Vce = 0

                   Vce = Vcc - Ic . Rl

            i.e.   Vo = Vcc - Ic . Rl      ....equation (2)

     

             Therefore case (1)

        i.e.    Vin <  cut in voltage

                  Ib = 0

                  Ic = 0

               From equation (2)

                Vo = Vcc - Ic . Rl

                Vo = Vcc - 0 . Rl

       i.e.    Vo = Vcc       [ Off State ]

             

                      Case (2)

                 Vin  > cut in voltage 

                 Base current ( Ib ) is greater 

                 Collector current ( Ic ) is greater

                 Vo = Vcc - Ic . Rl

              But  ( Rl = Vcc )

        i.e.    Vo = Vcc - Vcc

                  Vo = 0              [ ON State ]

                                                                    

                                                              

                                                                  

 Draw circuit of full wave bridge rectifier and explain its operation with waveform

Ans. 

                                         FULL WAVE BRIDGE RECTIFIER CIRCUIT

DIRECTION OF CURRENT FLOW IN A DIODE

                                       

WAVEFORM OF FULL WAVE BRIDGE RECTIFIER CIRCUIT

So here in this circuit , the full wave bridge rectifier is shown in the above diagram . So first of all , let's see the construction of this full wave bridge rectifier circuit . In this full wave bridge rectifier circuit , there are mainly four diode is required to built it . 

The four diode is given as diode D1 , diode D2 , diode D3 and diode D4 . In this full wave bridge rectifier circuit there is one load resistor is connected . 

The four diode D1 , D2 , D3 and D4 are connected in a closed loop configuration , which it is very efficiently converted an alternate current ( A.C ) into a direct current ( D.C ) . 

So whatever converted alternate current   ( A.C ) into direct current ( D.C ) , So than whatever coming direct current ( D.C ) passes from through along load resistor ( RL ) .

Operation in positive half cycle 

So here in this positive half wave cycle , the upper side of alternate current ( A.C ) sine wave is positive potential and lower side is negative potential , it is the polarity sign of this secondary transformer .

As we know that , the conventional current is always flow from a high potential level to a low potential level , the direction of flowing of current is always opposite in direction of flow of electrons in the conductor .

So when positive half wave cycle is given to this circuit . The current start to flow from point ( A ) . Now from point ( A ) current pass through diode D1 , so why current passes through diode ( D1 ) ? , Because of diode D1 is in forward biased mode , therefore it allow the current to pass .

Now here after current coming from diode ( D1 ) , then you think where it is go now ? So as you see the above given diagram . 

As diode ( D1 ) is in forward biased mode that's why current reach at point (B) . 

So when current reaches at point (B) , Now where it go now from at point (B) ? , Now here this diode ( D2 ) is in the reversed biased mode or we can say that in other words that , the diode ( D2 ) is off in condition . So it means that , the diode ( D2 ) is not permit to flow of current . So than current flow from along load resistor ( RL ) .

So after current flowing from load resistor ( RL ) , it reaches at point (D) . So when current at point (D) , it has two option whether it go to ( D3 ) or whether it go to ( D4 ) ? . The current chooses to flow from diode ( D3 ) , because diode ( D4 ) is in reversed biased mode . 

Here you may got surprised that , the current direction and diode ( D4 ) direction are same , it's mean that , it is in forward biased mode , or in other words that , it is ON in condition , that's why diode ( D4 ) has to be make permit to flow of current ? . 

But you have forgetting one thing is that the current is always flow from a high potential to a low potential . 

So when current starts flowing from point (A) then some voltage drop happen at diode ( D1 ) and the some voltage drop happen at the side of load resistor ( Rl ) . So when current reaches at point (D) , it will chooses to flow from diode (D3) .

Therefore current flow from diode ( D3) , or we can say that diode ( D3 ) is in active state or ON in condition . So after current flow from diode ( D3 ) , than it reaches at point (C) and than current go the transformer .

Operation in Negative half cycle

So here , this is a negative half wave cycle , so in this negative half wave cycle , the upper part be negative sign and the lower part be positive sign .

As we already known that the conventional current is always flow from the high potential to the low potential . So that the current starts flowing from point (C) . After current start flowing from point (C) than it passes through diode (D2) . 

So here , the current has two option or we can say that current has two choice to flow . So one option is from diode (D2) and the second option is from diode (D3) .

So you have question that , which option chosen by the current to flow ?...

Here in this , the diode (D2) is in [ON] state condition , it means this diode allow to current pass or we can say that to give permit to flow .

And this diode (D3) is in [OFF] state condition . It means that it does not allow to current to pass or we can say that it does not give permit to flow of current .

Here this diode (D2) allow the current to pass , because this diode is in forward biased in condition . So in this condition the diode is in [ON] state . So therefore current passes through the diode (D2) .

And after current passes through the diode (D2) it reaches at point (B) . And after point (B) it goes to the resistor (R) . After current flow from the resistor (R) than it goes to the point (D) .

At point (D) the current has two option or we can say that current has two choices to flow .

Here in this diode  (D3) , the diode (D3) is in reversed biased mode . It means that the diode is in [OFF] state in condition .

Now here the diode (D4)  , the diode (D4) is in forward biased mode . It means that the diode is in [ON] state in condition .

So the current choose to flow from diode (D4) , so the question arises here that why current chooses to flow from diode (D4) .

To understanding this concept , we take an one example .

As we see that in the negative half wave cycle , so in this the upper side is negative voltage and the lower side is positive voltage . As we know that the conventional current is always flow from the high potential to the low potential . 

It means that the current start flow from point (C) , so at point (C) the potential is 20 volt and than current goes to point (B) so at point (B) the potential is 19.3 volt and after it goes to the resistor (R) now after crossing resistor it's potential get 0.7 volt and then it goes to point (D) and come back to the zero volt (0) point (A) , or we can say that comes to the transformer . 

So because of conventional current or we can say that simply current always flow from the high potential to the low potential . Therefore current chooses to flow from diode (D4) . And after point (D4) it come back to the point (A) or we can say that comes to the transformer . 

So these is the procedure of full wave bridge rectifier to obtain the direct current from alternate current .