Please help me, I have some homework to do. I need to know everything possible about bootstrap biasing, and emitter feedback biasing of an NPN transistor. Please help. Thank you
Derek Belore

 

hi,

i will try to be as simple as i can get, Bootstrapping is used to increase the input impedance of a given circuit stage. however it is effectively only if the emitter resistor #1 is >> than the transistor junction resistance dictating not less than the maximum voltage gain.

it must also be beared in mind that bias stability is also degraded since β (Re1 + Re2) must now be much greater than Rb1 // Rb2 + Rb3 and Rb3 is usually large.

check this formula Zin = Rb3 + (rj + Re1/ rj) // β (rj + Re1)

provided that Rb3 >> Rb1 // Rb2
and Rb1 // Rb2 >> Re1

in the emitter follower it also gives a high input impedance to the source and a low output impedance to the load. bootsrapping parts like a resistor and capacitor connected in series between the transistor emitter and base are used to eliminate Rb1 & Rb2 from the input impedance equation. the voltage gain is non-inverting and slighly less than 1.

 

You may find the following links here on All About Circuits useful:

Biasing Techniques

and

Feedback

 

Thank you very much guys, I am finishing my project now as we speak.