# Finding the re model, voltage gain(Avmid=Vo2/Vi , Avs =Vo2/Vs)

#### candemir

Joined Apr 8, 2020
5
Hello,

#### ronsimpson

Joined Oct 7, 2019
940
off frequency of each capacitor
C = 5uF output cap, can not give a answer because we do not know the load to the right of the cap.
Ce is simple.
Cs C=5uF. The "R" if the formula is (Rb2//Rb3)//(Re X 200) …………… {// is parallel}
Avmid=Vo2/Vi , Avs =Vo2/Vs
Amplification Voltage from Vi to Vout is Rc/Re. (Wrong because Ce!)
Amplification Voltage from Vs to Vout, ….. You have a voltage divider of 1k and (Rb2//Rb3). I did not include the impedance of Q1 Base. The gain will be slightly lower. You do the math and come back.

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#### Ylli

Joined Nov 13, 2015
978
Since Re is bypasses, Av = Rc/R'e

• ronsimpson

#### The Electrician

Joined Oct 9, 2007
2,791
Since Re is bypasses, Av = Rc/R'e
Only at frequencies where the bypassing is effective. The problem asks for the "cutoff frequency" of each capacitor, so that means to consider the effect of CE.

#### The Electrician

Joined Oct 9, 2007
2,791
Hello,

What network analysis techniques have you learned? Superposition, branch, nodal, mesh?

Can you post an image of a point earlier in the text where it is shown how to solve a problem like this, using a simpler network? This will allow us to see what the text expects of you.

#### MrAl

Joined Jun 17, 2014
7,814
That is an interesting little amplifier.

They are giving the transistor Betas so i would think you have to do a full analysis to find the cutoff frequencies caused by each capacitor. For the output capacitor you will have to consider the equivalent resistance infinite.
I dont think there are any shortcuts because for example the input impedance of the bottom transistor works with the input cap as well as the bias resistors. So you have some work to do here.

Do you have to calculate this or can you use a simulator?

#### candemir

Joined Apr 8, 2020
5
That is an interesting little amplifier.

They are giving the transistor Betas so i would think you have to do a full analysis to find the cutoff frequencies caused by each capacitor. For the output capacitor you will have to consider the equivalent resistance infinite.
I dont think there are any shortcuts because for example the input impedance of the bottom transistor works with the input cap as well as the bias resistors. So you have some work to do here.

Do you have to calculate this or can you use a simulator?
That is an interesting little amplifier.

They are giving the transistor Betas so i would think you have to do a full analysis to find the cutoff frequencies caused by each capacitor. For the output capacitor you will have to consider the equivalent resistance infinite.
I dont think there are any shortcuts because for example the input impedance of the bottom transistor works with the input cap as well as the bias resistors. So you have some work to do here.

Do you have to calculate this or can you use a simulator?
That is an interesting little amplifier.

They are giving the transistor Betas so i would think you have to do a full analysis to find the cutoff frequencies caused by each capacitor. For the output capacitor you will have to consider the equivalent resistance infinite.
I dont think there are any shortcuts because for example the input impedance of the bottom transistor works with the input cap as well as the bias resistors. So you have some work to do here.

Do you have to calculate this or can you use a simulator?
Dear Sir, first of all I have to draw the re model and I really dont know how to draw it. Can you help with it firstly?

#### LvW

Joined Jun 13, 2013
1,086
Are you really required to use the "re" model?
For my opinion, it is the worst model and can create confusion...as an example: In post '4' it is written as R´e.
This is confusing (and wrong) because re is a differential quantity and must be written in small letters.
More than that - and strictly spoken - it is NOT a resistance...
Why not using models with a voltage-controlled current source (transconductance gm) or a current-controlled current source?
These models have more physical relevance than the re model.

#### candemir

Joined Apr 8, 2020
5
Are you really required to use the "re" model?
For my opinion, it is the worst model and can create confusion...as an example: In post '4' it is written as R´e.
This is confusing (and wrong) because re is a differential quantity and must be written in small letters.
More than that - and strictly spoken - it is NOT a resistance...
Why not using models with a voltage-controlled current source (transconductance gm) or a current-controlled current source?
These models have more physical relevance than the re model.
Dear Sir, thanks for your interest. I must use the re model, because of our professor’s requirement

#### MrAl

Joined Jun 17, 2014
7,814
The re model is fine you just have to know full well how to use it and calculate the value of re itself.
Furthermore, the re model does not preclude the use of a current controlled current source for the transistor in fact it works very well with it.
Also in fact, once you calculate re you can calculate the gain quite easily and it will MATCH the simulation exactly.

So first, do you know how to calculate the value of re?

#### candemir

Joined Apr 8, 2020
5
The re model is fine you just have to know full well how to use it and calculate the value of re itself.
Furthermore, the re model does not preclude the use of a current controlled current source for the transistor in fact it works very well with it.
Also in fact, once you calculate re you can calculate the gain quite easily and it will MATCH the simulation exactly.

So first, do you know how to calculate the value of re?
re is 26mV/IE, so I think first we have to find Ie. Will be waiting for your help

#### The Electrician

Joined Oct 9, 2007
2,791
re is 26mV/IE, so I think first we have to find Ie. Will be waiting for your help
Since the β of Q1 and Q2 is a rather high 200, for a first approximation you could ignore the base currents and calculate the current in the 3 bias resistors, and thence the voltage at the bases of Q1 and Q2. Then the voltage at the emitter of Q1 will follow, which gives the emitter current. Can you do all this? Show your work.

#### LvW

Joined Jun 13, 2013
1,086
The re model is fine you just have to know full well how to use it and calculate the value of re itself.
Furthermore, the re model does not preclude the use of a current controlled current source for the transistor in fact it works very well with it.
Also in fact, once you calculate re you can calculate the gain quite easily and it will MATCH the simulation exactly.

So first, do you know how to calculate the value of re?
OK - I know what you mean...and I suppose you are refering to the second re-model as shown in the following link
http://www.zen22142.zen.co.uk/Theory/re_model.htm

I think, this one (te second one) is not really an "re model" (in fact, it is the Pi-model) because the input resistance rbe=h11=hie is only expressed with two ther parameters gm=1/re and beta.

My personal objections are primarily against the first "re-model" in the linked document. For example - having a quick look at this model a beginner could think that the resistance between B and E would be "re" - but this is wrong!

#### MrAl

Joined Jun 17, 2014
7,814
Oh yes the first one is very poorly defined.
The second one is defined for what looks like small signals only? It does not allow calculating the DC bias point because the base does not connect directly to re as they seem to show.

I posted an all in one formula for calculating the bias point and at the same time the value of re. It's around this forum somewhere. This is homework help though so i should not post it again here.

#### MrAl

Joined Jun 17, 2014
7,814
Dear Sir, first of all I have to draw the re model and I really dont know how to draw it. Can you help with it firstly?
Oh well the way i draw it and the way you might find it on the web are two different things. The way i draw it accounts for the Vbe voltage drop while most of the models i see on the web only have re.

But some preliminaries...
Do you know how to use a current controlled current source?