why base is lightly doped

 why base is less lightly doped?

we know that Ic (collector collector) output current depend on Ie due to collector junction is reverse bias and emitter junction is forward bias(for working of transistor as an amplifier) so 99% of collector current(Ic) depend on electrons coming from emitter

                                        

                    Ie = Ib + Ic  ( for npn transistor)

                    Ic = Ie - Ib

As Ib is due of recombination of electrons from emitter with holes(present in base) in base region when electrons pass throw base region due to this recombination the electrons entering into collector will decrease.so to have less recombination we dope the base lightly so that less recombination occur and we can get high collector current(Ic)

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Difference between BJT AND FET

Difference between FET(field effect transistor) and BJT(biploar junction transistor)    

BJT	FET
It requires both holes and electrons for working of transistor	It requires either holes or electrons depending on their majority
It is current controlled device	It is voltage controlled device
It is more noisy than FET	It is less nosiy due to absence of junction
Works well even at high frequency	Performance vary with the frequency
For thermal stability we need to bias the BJT	It has good thermal stability due to absence of minority charge carriers
Terminals are Emitter, Base, Collector	Terminals are Gate ,Source ,Drain

  BJT

    FET

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why we call bjt as bipolar junction transistor

 BJT(bipolar junction transistor)

Bipolar means two type of charge carriers which are electrons and holes .For working of  transistor we require both holes and electrons as Ib(base current) is due to recombination of holes and electrons in base region and this Ib is required for transistor to be in active region .So this Ib comes from recombination of electrons and holes means we require both holes and electrons so we call it as bipolar junction transistor

Graph between voltage gain and frequency response of amplifier

 

     voltage gain vs frequency response of amplifier

from figure shown

• From origin to FL is called as lower cutoff frequency
• FL to FH is called as midband  region
• FH to infinity is called as upper cutoff frequency 

lower cutoff frequency due to coupling capacitors (coupling capacitor info electronics152.blogspot.com)

upper cutoff frequency due to junction capacitors(discussed below)

junction capacitors are the capacitors fixed between two terminal of transistor as shown above

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.

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