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### Network # CE EMITTER-BIAS CONFIGURATION

#### Omar.HM

Jul 28, 2012
3
hello
please i need help
could any one tell me how i can get the equations from the circuit? this is pdf
http://www.mediafire.com/?i52skp9hgttii66

page 345-348

8.4 CE EMITTER-BIAS CONFIGURATION
Unbypassed

the equations with Effect of ro

equ (8.30) Zb , (8.32) Zo, (8.34) Av

#### john monks

Mar 9, 2012
693
From the circuit we must conclude that the author is referring to AC.
We know that B is beta and equals collector current divided by base current.
Therefore base current pluse the collector current must equal the emitter current.
Ie = Ib + Ic
We know that the transistor has a certain internal resistance in the emitter re.
So the voltage at the emitter resistor must be the collector current plus the base current times the emitter resistor.
Ib(B+1)Re
The voltage of the internal resistance is base current times the internal resistance. I assume that this has been determined experimentally so that's why the extra 1 has not been added to beta.
Then again it could be that when you add 1 to beta the change is so small it's not worth considering.
So there you have it.
Vi = IbBre + (B + 1)IbRe

Last edited:

#### Omar.HM

Jul 28, 2012
3
From the circuit we must conclude that the author is referring to AC.
We know that B is beta and equals collector current divided by base current.
Therefore base current pluse the collector current must equal the emitter current.
Ie = Ib + Ic
We know that the transistor has a certain internal resistance in the emitter re.
So the voltage at the emitter resistor must be the collector current plus the base current times the emitter resistor.
Ib(B+1)Re
The voltage of the internal resistance is base current times the internal resistance. I assume that this has been determined experimentally so that's why the extra 1 has not been added to beta.
So there you have it.
Vi = IbBre + (B + 1)IbRe

thank you for reply
i know that how we get vi coz ib=0 and take a loop

but what i need is the Zb with effect of ro
how the equation (8.30) came ?

#### john monks

Mar 9, 2012
693
I certainly see sources of confusion.
First of all Rb was not even considered.
What I've done to find Zin is multiplying (Re + re) and (beta + 1) and paralleling that impedance with Zin.
Zi = (Re + re)(B + 1)Rb/((Re + re)(B + 1) + Rb)
Ro or output impedance is simply equal to Rc.
I understand the confusion.
If you want I will explain this in complete detail.

Last edited:

#### Omar.HM

Jul 28, 2012
3
thank you so much Mr

#### Estherfe Rafaela

Oct 15, 2012
1
how is equation 8.30 derived?.

#### john monks

Mar 9, 2012
693
Ro is the impedance of the collector relative to ground. I suppose this information would have to be figured from the datasheet or the characteristic curves or experimentally. Usually this value is several times the value of Rc. In this case I don't know where you would get Ro. But whatever it is it is in parallel with Rc.

#### saba iqbal

Feb 23, 2014
1
CE emitter follower configuration unbypassed including r0

Plz... Tell me how we find output impedence in CE emtiter follower configuration including r0 #### duke37

Jan 9, 2011
5,364
An emitter follower cannot be common emitter, it will need to be common collector.

I have understood the the output impedance in CC is the input impedace, including base resistance divided by beta.

#### john monks

Mar 9, 2012
693
I am unable to call up the original circuit. But what I can say is that generally Ro approximates Rin/(b+1) + 25mv/Ie. That is the impedance at the emitter equals input impedance ( divided by (beta plus 1) ) plus 25 millivolts dicided by your emitter current.
If Rin is low and the current through the transistor is high then you can assume the that your output impedance is very low to zero ohms.
To get emitter current accurately you will have to measure it because every transistor is different.

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