BJT

Why we are prefering common emitter configuration in transistors?

Are we? I sounds like your professor wants you to explain what a bipolar transistor in common emitter configuration does for you and why you would do it that way in contrast to other configurations.

I am not aware of any conspiracy to prefer one configuration over the other. You don't start out saying you're going to use common emitter and then structure the circuit around that. You start with a problem, and if a BJT in common emitter configuration looks like a good fit you use it.

Common emitter is probably the most common configuration for a bipolar transistor because it happens to be a good fit to solve common problems.

I'm willing to discuss some of the tradeoffs, but don't want to just answer your homework question for you. You need to show you've at least tried to look into this subject and then ask specific questions about things that confuse you.

We also need to know your current level of understanding? What BJT configurations do you know about? Can you show at least one application where each would be useful and explain why?

I have no such preference to be honest, it's pretty much application dependent. What do you want to do/understand how it works, that you would prefer a common emitter configuration?

I'll ignore for a moment that this might be a school project and explain to you (in the hope that you will better understand and figure it out yourself) what is the difference between the BJT transistor configurations.

There are basically 3 basic (simple) configurations for a BJT:
- common emitter
- common collector
- common base

Common emitter is basically a voltage amplifier configuration, since most electronics work using voltage as information carrier or reference values you can imagine it can be used for a great deal of applications (I'll let you figure them out).

Common collector is basically a current amplifier configuration that keeps the output voltage at the same value with the input voltage, why it's also called an emitter follower. Again I'll let you figure out the applications.

Common base (or grounded base) is also a voltage amplifier configuration, but unlike the common emitter the output current is the same as the input current - also called current buffer. Same thing with the applications.

Since the incertitude of whether this is or isn't homework I will not go further and assume you know how the configurations look like and what are the locations and polarities of the inputs and outputs.

Waiting for a little more details from you so the answer can be better suited to your needs.

I've seen from your profile you do not wish to be contacted, so I will say this here: you would make a great engineering college professor... I've had my fair share of morons with teaching rights so I know when I see someone who should teach. Your resume also confirms this, congratulations to you and your sons team!

Thanks, Dorin. I have the direct contact shut off else I'd get several emails a week asking for help with homework problems. It's easy to find me if someone really wants to, but the current setup deters the homework weenies well enough.

I guess you saw the reference to my son's Lego League team. That was when he was in middle school, which is grades 6-8 here. He's now a senior in college and should get is BS in applied math next spring.

What part of the world are you from? It doesn't say anything in your profile, but the name suggests the Balkans maybe?

The shut off is understandable, I also imagined the Lego League thing was sometimes in the past... but not that far away. I always wanted a Lego Technics kit when I was at that age, but it wasn't available and affordable in Romania, which by the way it's where I'm from (didn't get the time to finish my profile with having to finish my Master's thesis around the time I created my account on EEWeb). The Balkans was only a Danube's width away, but it's understandable why my name might have lead there, my last name is of Serbian origin.

I noticed most homework weenies are from India and most questions look extremely vague and not thought about in advance... copy-paste in short. Haven't you?

Best of luck to your son with his Bachelor degree. And I welcome him to the world of engineering (applied mathematics is close enough to be called engineering in my opinion).