why we use coupling and bypass capacitors?

     why we use coupling and bypass capacitor?                                                                                   

     

    Here c1,c2 are coupling capacitors and

            c3 is bypass capacitor

     what is the use coupling capacitor?

    1.It blocks the dc voltage(explain clearly below).

    2.It also use for coupling(joining) the AC signal of one circuit with another circuit  using capacitors.

    

    How it blocks the dc voltage?

    As we are using capacitor we know that capacitor act as open circuit for DC signal and short circuit for AC signal  and proof is shown  below

                                            let Xc is reactance of capacitor

                    we know that  Xc =1 / j(w*c)  (formula for reactance of capacitor)

                            where w=angular frequency

                                        c=capacitance of capacitor

                                As we know that DC signal has w=0

                                then  Xc=1/0 (one divided by zero) so Xc tends to infinity

     As Xc is infinity so large value of reactance(resistance) .As open circuit have infinite resistance so capacitor will act as open circuit  for DC signal  and AC signal have some frequency so it have some w value . so Xc  won't be so high and allows AC signal

    

so it allows only AC signal  and doesn't allow DC signal .As it doesn't allow DC signal, Q point will not shifted and transistor will remain in Active region .This blocking of DC and allowing of AC signal done from both sides of transistor as we use capacitors from both side(C1,C2 shown in figure) 

why we use bypass capacitor?

we use bypass capacitor due to it bypass the AC signal to the ground so next question comes in our mind is why we need to bypass the AC signal to ground, as we are input AC signal so there will be some AC current in the flows in emitter so this alternating current(AC) is changing it's magnitude continuously due to this the voltage drop across emitter will change and this change  Ie(emitter current) and change in Ie changes Ic(collector current) also changed .Due to change in Ic the Q point will be shifted and this may make transistor to fall into cutoff or saturation region which is not desired  if the transistor works as amplifier .To avoid this we use a capacitor in parallel to emitter so that capacitor draw all AC signal and pass only DC signal to  emitter (capacitor act as open circuit for AC and blocks DC signal .So that DC signal will pass throw Re and AC will pass throw capacitor  ) in this way dc current only pass. Gain of voltage can be increased by using bypass capacitor

   Hope this will clarify your doubt.

   Thanks for reading