# Input and Output Impedance of the Common Emitter Amplifier

#### cikalekli

Joined Dec 10, 2020
103
Yeah, now I got it.

If we apply KVL:

VBE = 3V - 0.7V - (R1 * IB)

Let me ask that sometimes instead of a diode, they draw an external resistor which is called (pi-resistor) instead of the diode.

At this specific point I got confused. I just aren't we gonna use diode everytime? Therefore, when should I have to put pi-resistor inside the model?
edit:

As an exemplifying here is a representation of pi-resistor.
It seems that they did not use any diode inside it. That is why I just got confused, sorry for that.

Actually, this is just an example image for the pi-resistor that I found randomly.

I just wanted to say that, until now I have been always putting a diode, but I really could not understand that why some people prefer the pi-model and instead of using a diode?

#### cikalekli

Joined Dec 10, 2020
103
edit: Yeah not I got it clear what does it mean with your kind help of Rπ (Rpi) and when do we need it...

#### cikalekli

Joined Dec 10, 2020
103
hi cika.
Checked the first part of the video,, it looks OK to me.

So lets do this step by step....

What do you calculate the Ib value? show your maths.

E
So, Ib is:

Is * exp( 3V-0.7V / 26mV ) = β * Ib

I tried to find Ib with this equation. However, due to the question did not give Is value, I am stuck at this point...

#### ericgibbs

Joined Jan 29, 2010
19,114
hi,
More reading for you, refer page #2.

E
I would suggest you download and keep a copy of technical data.

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#### cikalekli

Joined Dec 10, 2020
103
hi,
More reading for you, refer page #2.

E
I would suggest you download and keep a copy of technical data.
I guess the question is supposed to give the value Is, right?

Because the question what I shared initially, the instructor did not give the value of Is...

#### ericgibbs

Joined Jan 29, 2010
19,114
hi c,
You know the value of the Collector load resistor, what current will have to flow in that resistor to put the transistor into saturation.?
E

#### cikalekli

Joined Dec 10, 2020
103
what current will have to flow in that resistor to put the transistor into saturation.?
The current Ic needs to have flowed in that resistor I guess right?

Last edited by a moderator:

#### ericgibbs

Joined Jan 29, 2010
19,114
hi
This clip is from a 2N3904 datasheet, shows the Vcs sat voltage OK.
You also know the supply voltage, so what will be the maximum current thru the Collector resistor.?
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#### cikalekli

Joined Dec 10, 2020
103
hi
This clip is from a 2N3904 datasheet, shows the Vcs sat voltage OK.
You also know the supply voltage, so what will be the maximum current thru the Collector resistor.?
E
View attachment 240440
Ix = Is ((exp(Vx / VT) -1 ))

If Ix = Ib then,

Ib = Is ((exp(3V-0.7V / 26mV) - 1 ))

1mA = Is ((exp(2300/26) -1 )) ----------> Is = 38.1x10^-40 mA

Then,

Ic = Is * exp(
VBE / VT)

Ic = 38.1x10^-40 * exp(3V-0.7V / 26mV)

Ic = 0.99 mA

#### ericgibbs

Joined Jan 29, 2010
19,114
hi cika,
How can this be correct ? Ic = 0.99 mA ????

You have a 10V supply , and the transistors Vsat is say 0.2V .

The collector resistor is 3K

So using ohms law what is the current thru the 3K.?

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#### cikalekli

Joined Dec 10, 2020
103
hi cika,
How can this be correct ? Ic = 0.99 mA ????

You have a 10V supply , and the transistors Vsat is say 0.2V .

The collector resistor is 3K

So using ohms law what is the current thru the 3K.?

E
Oh alright, (10V - 0,2V) / 3K = Ic = 3,2 mA

Update: then Ib is ---> (3V - 0,7V) / 100K = 0,023mA right sir?

#### cikalekli

Joined Dec 10, 2020
103
Oh alright, (10V - 0,2V) / 3K = Ic = 3,2 mA

Update: then Ib is ---> (3V - 0,7V) / 100K = 0,023mA right sir?
I want to ask that if the circuit was not saturated, then still again did we evaluate the voltage drop of the diode which is 0.7V inside the equation please?

#### LvW

Joined Jun 13, 2013
1,773
I guess the question is supposed to give the value Is, right?
Is is a parameter with very large tolerances.
More than that, it is very temperature sensitive - and this is the main reason for the temperature dependence of Ic.
This uncertainty of Is is the reason for the large tolerance values for the "current gain" B resp. beta.

#### cikalekli

Joined Dec 10, 2020
103
Is is a parameter with very large tolerances.
More than that, it is very temperature sensitive - and this is the main reason for the temperature dependence of Ic.
This uncertainty of Is is the reason for the large tolerance values for the "current gain" B resp. beta.
Thank you sir, now it seems more clear to understand that association

#### cikalekli

Joined Dec 10, 2020
103
Is is a parameter with very large tolerances.
More than that, it is very temperature sensitive - and this is the main reason for the temperature dependence of Ic.
This uncertainty of Is is the reason for the large tolerance values for the "current gain" B resp. beta.
Sorry. can you give me some documents to study input and output impedance, please?
All I wanted was to learn how could I accomplish to fins input and output impedances.
Yet, I morely confused about that.

#### ericgibbs

Joined Jan 29, 2010
19,114
hi cika,
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#### cikalekli

Joined Dec 10, 2020
103
hi cika,
E
Hi eric,
Uhm...
Oh alright, (10V - 0,2V) / 3K = Ic = 3,2 mA

Update: then Ib is ---> (3V - 0,7V) / 100K = 0,023mA right sir?
This is right?

and also do you really have a resource for studying alone about Input & Output Impedances of Common Emitters, please?

#### ericgibbs

Joined Jan 29, 2010
19,114
hi,
You have already correctly calculated the Base current as 23uA.
With a beta of 100 , Ic = 100 * Ib.

The Is ranges from 10^-12 thru 10^-15... I would use 10^-12.

What is the problem.?
E

For self study there are number of video's and tutorials on the web