Why the common emmiter

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hello every body, i have a quistion. Why do we use the common emmiter ?

 

[Bob Myers]

hello every body, i have a quistion. Why do we use the common emmiter ?

As opposed to what?

Describe what the CE, CB, and CC configurations
do in terms of voltage and current gain, and input and
output impedance, and you should have your answer.

Bob M.

 

[Eeyore]

hello every body, i have a quistion. Why do we use the common emmiter ?

It's popular because it has both current and voltage gain.

We do also use the common base and the common collector (emitter follower) too
you know.

Graham

 

[Charles]

hello every body, i have a quistion. Why do we use the common emmiter ?

Has the best power gain and moderate input/output impedances.

 

[John Larkin]

It's popular because it has both current and voltage gain.

We do also use the common base and the common collector (emitter follower) too
you know.

Graham

Just look at the transistor symbol. Obviously the emitter is heavier,
so it settles down to ground.

But I'm still confused about PNP's. I figure the emitters must be
helium implanted, since they tend to go up.


John

 

[Eeyore]

But I'm still confused about PNP's. I figure the emitters must be
helium implanted, since they tend to go up.

Except in the 'old days' of course.

Graham

 

[Jon Slaughter]

hello every body, i have a quistion. Why do we use the common emmiter ?

Its the only amplifier configuration.

CB is a switch. It is normally to unstable for use as an amplifier(or you
can think of it as an amplifier with positive feedback that saturates or
cuts off the transistor very easily).

CC is a unity gain buffer. It has a high impedence input so that you do not
load the output of whatever your taking(sorta like how a volt meter works...
volt meters would be useless if when you used them they did not give you the
right voltage). Its output is low impedence which means that it can drive a
load.

CE is a relatively stable amplifier. It will take a voltage signal and
amplify it. Here though there is a loading effect. By adding a CC at the
end then you can buffer the amplifier so that you get the best of both
worlds. You get gain from the CE + low loading effects.

CB and CC do not amplify(in the sense of taking a small single and making it
larger) but they have other important properties.

 

[Eeyore]

CB and CC do not amplify

Ahem !

Graham

 

[John Popelish]

Its the only amplifier configuration.

Man, do you have a lot of wonderful things to learn about
transistors.

Any configuration with more power out than power in has gain.

CE, CB and CC can all have power gain.

CB is a switch. It is normally to unstable for use as an amplifier(or you
can think of it as an amplifier with positive feedback that saturates or
cuts off the transistor very easily).

I don't know where to start. CB puts the input impedance in
the output circuit as negative feedback. This makes the
output more linear (less like a switch).

CC is a unity gain buffer. It has a high impedence input so that you do not
load the output of whatever your taking(sorta like how a volt meter works...
volt meters would be useless if when you used them they did not give you the
right voltage). Its output is low impedence which means that it can drive a
load.

Good. And that current gain at unity voltage gain
represents real power gain.

CE is a relatively stable amplifier. It will take a voltage signal and
amplify it. Here though there is a loading effect. By adding a CC at the
end then you can buffer the amplifier so that you get the best of both
worlds. You get gain from the CE + low loading effects.

CB and CC do not amplify(in the sense of taking a small single and making it
larger) but they have other important properties.

Of course, they do. CB amplifiers can produce voltage gain
and CC can produce current gain. That is getting a bigger
signal (in a power sense) out than you put in.

 

[Michael Black]

Its the only amplifier configuration.

CB is a switch. It is normally to unstable for use as an amplifier(or you
can think of it as an amplifier with positive feedback that saturates or
cuts off the transistor very easily).

Now this is a roundabout way to force us to do the guy's homework.

Common base are used when one wants to do impedance transformation. So
when you want to use that 8 ohm speaker as a microphone, you use a common
base amplifier because the low impedance input will better match the
microphone, and you will get some gain as well as a higher output impedance.

One should point out that common base amplifiers are not uncommon in
radio work, because they do that impedance transformation and because
their relatively low gain is a good thing. If it had more gain, it
would require more effort to ensure that it wasn't unstable.

Michael

 

[John Larkin]

Its the only amplifier configuration.

CB is a switch. It is normally to unstable for use as an amplifier(or you
can think of it as an amplifier with positive feedback that saturates or
cuts off the transistor very easily).

Common-base has the same voltage gain as common-emitter, and it's just
as linear and sometimes more stable.

John

 

[Bob Myers]

Its the only amplifier configuration.

It most certainly is not. Take a look at the output stage
of most audio amps, and get back to us, OK?

(Hint: "amplifier" does not necessary equate to "voltage gain.")

Bob M.

 

[Bob Myers]

Now this is a roundabout way to force us to do the guy's homework.

Well, I tried in my original response to avoid doing that, but it
looks like some folks just have that overwhelming urge to explain....

;-)

Bob M.

 

[Jon Slaughter]

Anyone else want to point out my ignorance and that I must have an IQ of -3?
By amplification I ment voltage amplification... I thought it would be clear
because usually that is how they are configured. A unity gain buffer is
unity in voltage but you don't say a unity gain voltage buffer? When you
talk about amplification it is almost always in the context of voltage
amplification? If you talk about current "amplification" it is almost always
talked about as current "gain"?

Also, aren't all transistors configured for power gain? wouldn't it be
worthless if both voltage and current decreased?

I was wrong about the CC and what I ment was a base biased configuration. I
was trying to give the 3 common modes that transistors are used as:
Switching, Amplification, and buffering.

Now amplification means voltage amplification because buffering is current
"amplification". If not then why call it a unity gain buffer and not a
current amplifier? Ok, maybe some people do but some of you have acted like
I commited a cardinal sin and it must mean I have no clue about what I was
talking about. In fact I was atleast 2/3 right and if you stop grading me
like a test then I might not be as dumb as some of you want to make me out
to be. Maybe I got some termiology mixed up but it is enough to point that
out instead of trying to ridicule me.

Obviously if I said a switch it must be the configuration for a switch. i.e.
base biasing. So just changed CC to Base biased and that fixes that
problem. Just prefix all amplifications with voltage amplification and then
that fixes the other issue.

I guess 10 more people are going to reply to my original and say the same
stuff the last 10 said and call me a retard? Now I really feel like a
retard. Thanks guys.

You know, its one thing to point out the mistake like John Larkin did and
its another to try and make the person completely feel like a retard even
when the mistakes are more about terminology than anything else. Maybe I
should have been more clear or re-read my post but its not like I'm writing
a thesis. Sometimes I jsut get confused on the names cause I don't mess
with transistors much. Actually all I have ever did was a simple *voltage*
amplifier. I have read about them and actually just got finished reading
again... but the details don't stay in my brain when I don't use them. The
book I was reading only discussed CE and CB and base bias so I assumed CC
was that base bias because I recalled that there were only 3 configurations.
I'm going to have to see exactly how CC works now to see the difference.

 

[John Popelish]

Anyone else want to point out my ignorance and that I must have an IQ of -3?

(snip)

Don't take it so hard or so personally. I get focused on an
idea (often a completely mistaken tangent) and say something
stupid, almost daily. I have tried (and sometimes it is
hard) to just smile and say thank you when someone points
out how silly my contribution was.

You gave us all an opportunity to wax poetic on transistor
configurations. Besides, its a lot more fun to criticize
someone than just to dive in and and write a tutorial.
Stick around. You'll get your chance to correct me. ;-)

 

[Jon Slaughter]

(snip)

Don't take it so hard or so personally. I get focused on an idea (often a
completely mistaken tangent) and say something stupid, almost daily. I
have tried (and sometimes it is hard) to just smile and say thank you when
someone points out how silly my contribution was.

You gave us all an opportunity to wax poetic on transistor configurations.
Besides, its a lot more fun to criticize someone than just to dive in and
and write a tutorial. Stick around. You'll get your chance to correct me.
;-)

I know... sometimes its hurts though!! We all want to know everything and
when someone points out that you don't it stings... and when they make you
feel stupid then its even worse..

What sucks is that I just got finished reviewing the damn shit and I should
have known but I completely forgot all the names of the different
configurations ;/ I do understand how the work but unfortunately the book
I'm reading didn't conver CC(or if it did it used a different name or I just
haven't got to it yet). I really need to get in some practice with some
transistor circuits soon... hopefully I'll be able to play around more with
them later.

I guess I'm more mad at myself than anyone. I hate when I try to be helpful
but end up making it worse. I should atleast double check what I'm saying to
make sure but I tend to be lazy and just type what ever pops in my mind...

Oh well... no hard feelings

 

[Michael Black]

Well, I tried in my original response to avoid doing that, but it
looks like some folks just have that overwhelming urge to explain....

I seriously thought that was a good answer of yours. Because it
pointed the way, without actually providing the answer.

Michael

 

[John Larkin]

I know... sometimes its hurts though!! We all want to know everything and
when someone points out that you don't it stings... and when they make you
feel stupid then its even worse..

Everybody makes a mistake now and then, especially so when the
consequences are low, as in no planes will crash or anything like
that. Just say "oops" and get on with life... it's no big deal. I see
my mistakes as reminders that I should brush up on something, which
then keeps me from making the same mistake later when it really
matters, as when serious things might be involved.

John

 

[John Popelish]

I know... sometimes its hurts though!! We all want to know everything and
when someone points out that you don't it stings... and when they make you
feel stupid then its even worse..

What sucks is that I just got finished reviewing the damn shit and I should
have known but I completely forgot all the names of the different
configurations ;/ I do understand how the work but unfortunately the book
I'm reading didn't conver CC(or if it did it used a different name or I just
haven't got to it yet).

They are often referred to as emitter followers, since the
output voltage from the emitter "follows or parallels the
input voltage on the base.

I am dyslexic, and regularly get NPN and PNP substituted for
each other, when describing a circuit. I don't have the
problem with schematics, for some reason.

I really need to get in some practice with some
transistor circuits soon... hopefully I'll be able to play around more with
them later.

I guess I'm more mad at myself than anyone. I hate when I try to be helpful
but end up making it worse.

You made it quite a bit better for the O.P. Lots of people
who were not willing to help with what looked like homework,
were more than happy to correct someone who was willing.

I should atleast double check what I'm saying to
make sure but I tend to be lazy and just type what ever pops in my mind...

You can always offer a double your money back, guarantee.

Oh well... no hard feelings

I don't believe you, but hope you'll heal. ;-)

 

[John Larkin]

I am dyslexic, and regularly get NPN and PNP substituted for
each other, when describing a circuit. I don't have the
problem with schematics, for some reason.

I always draw the emitters up on PNPs and the emitters down on NPN's.
Voltages are more positive at the top of the sheet, and current flows
down.

I draw mosfsts sort of like IGBTs...


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because the "proper" symbol is a mess.

You write awfully well for being dyslexic.

John