# Common Emitter Amplifier (1 Unknown Resistor)

#### Fotic Pap

Joined Aug 24, 2019
11
Hi everyone,
I have been going around in circles on pages and forums for a couple days now, but just cannot seem to make any proggress with my problem.

I have the below recuit (image attached).
I need to find R1.

Vcc = 22 V
R2 = 2 kΩ
Rc = 5 kΩ
RE = 5 kΩ
VBE = 0.7 V
b = 300

If anyone can help me, i would be grateful.

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#### ericgibbs

Joined Jan 29, 2010
9,824
hi FP,
Welcome to AAC.
Could you please post your effort at solving the problem, we can then guide you.
E

#### Fotic Pap

Joined Aug 24, 2019
11
hi FP,
Welcome to AAC.
Could you please post your effort at solving the problem, we can then guide you.
E
Hello, thansk for the effort

(On the same circuit as R2: Unknown)

I use this types to find R2. (I'm not sure if they are all correct)
Vc = RE * Ic
VB = VE + VBE
IB = IC/ b
IR1 = (VCC-VB)/ R1
IR2 = IRC - IB
R2 = VB/IR2

MOD:

Last edited by a moderator:

#### WBahn

Joined Mar 31, 2012
25,566
Hi everyone,
I have been going around in circles on pages and forums for a couple days now, but just cannot seem to make any proggress with my problem.

I have the below recuit (image attached).
I need to find R1.

Vcc = 22 V
R2 = 2 kΩ
Rc = 5 kΩ
RE = 5 kΩ
VBE = 0.7 V
b = 300

If anyone can help me, i would be grateful.
You need to select a value of R1 in order to achieve what goal?

#### WBahn

Joined Mar 31, 2012
25,566
Hello, thansk for the effort

(On the same circuit as R2: Unknown)

I use this types to find R2. (I'm not sure if they are all correct)
Vc = RE * Ic
VB = VE + VBE
IB = IC/ b
IR1 = (VCC-VB)/ R1
IR2 = IRC - IB
R2 = VB/IR2

MOD:
View attachment 184661
What is R2? Your diagram has five resistors, none of which is labeled R2.

#### Audioguru

Joined Dec 20, 2007
11,249
Why did you make Re the same value of Rc? Then you are throwing away a lot of output voltage swing. Make Re about 1/10th of Rc.
Why did you make the value of R2/R3 so low when the b is very high? The value of R1 can be much higher than Rc.

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#### Fotic Pap

Joined Aug 24, 2019
11
What is R2? Your diagram has five resistors, none of which is labeled R2.
Hello, thanks for the effort.
See the attached image.. the 2nd image i didnt upload it.

Why did you make Re the same value of Rc? Then you are throwing away a lot of output voltage swing. Make Re about 1/10th of Rc.
Why did you make the value of R2/R3 so low when the b is very high? The value of R1 can be much higher than Rc.
Hello, also thanks for the effort.
All the values I wrote are from the professor.

#### WBahn

Joined Mar 31, 2012
25,566
Hello, thanks for the effort.
See the attached image.. the 2nd image i didnt upload it.

Hello, also thanks for the effort.
All the values I wrote are from the professor.
By "attached image" are you talking about the image in the original post?

Again, what is the goal you are trying to achieve? You can put a random value of R2 in the circuit and the circuit will behave a certain way. But will it behave the way you want it to? Who knows? We can't even begin to know that until you tell us how you want it to behave?

This is like asking someone what kind of car you should buy without telling them whether you want the car to get around the city by yourself with the best fuel economy, take your six kids on long trips, or haul dirt and rocks because you are a landscaper.

#### Fotic Pap

Joined Aug 24, 2019
11
By "attached image" are you talking about the image in the original post?

Again, what is the goal you are trying to achieve? You can put a random value of R2 in the circuit and the circuit will behave a certain way. But will it behave the way you want it to? Who knows? We can't even begin to know that until you tell us how you want it to behave?

This is like asking someone what kind of car you should buy without telling them whether you want the car to get around the city by yourself with the best fuel economy, take your six kids on long trips, or haul dirt and rocks because you are a landscaper.
Yes the image i post.

Sorry for the missing infos.

Exercise Pronunciation:
Computer the resistanse of R1 on the circuit so that the operating point Q of the transistor is in the middle of the DC direct load.

(i hope to make sense because my english is not very good)

#### WBahn

Joined Mar 31, 2012
25,566
Yes the image i post.

Sorry for the missing infos.

Exercise Pronunciation:
Computer the resistanse of R1 on the circuit so that the operating point Q of the transistor is in the middle of the DC direct load.

(i hope to make sense because my english is not very good)
We'll work through any language issues; I assure you your English if far, far better than my ability to speak your native language.

I'm going to infer that they mean that the collector voltage is midway between the highest and lowest voltages that it can be under DC conditions. If that's not what they mean, then please correct me.

Assuming it is:

Q1) What is the highest voltage that the collector voltage can be?
Q2) What is the lowest it can be?
Q3) How did you determined them?
Q4) What is the collector voltage that you are trying to achieve as an operating point?

Let's get that far and then move forward once any issues here are resolved.

#### LvW

Joined Jun 13, 2013
975
As our experienced forum member WBahn has asked several times....
"what is the goal you are trying to achieve"?
You really should try to be able for answering this question.
More than that, it is really necessary to UNDERSTAND the working principle of the whole circuit - that means:
You should know the role and the task of each component in the circuit.
Do you know the purpose of the emitter resistor and the parallel capacitor?

One final (but important remark): In general, there exist a (nearly) infinite number of design alternatives for selecting component values. S - it is not asimple calculation. You start with a suitable choice of some key parameters (values) corresponding to your requirements ...and then you can calculate the rest.

#### Fotic Pap

Joined Aug 24, 2019
11
We'll work through any language issues; I assure you your English if far, far better than my ability to speak your native language.

I'm going to infer that they mean that the collector voltage is midway between the highest and lowest voltages that it can be under DC conditions. If that's not what they mean, then please correct me.

Assuming it is:

Q1) What is the highest voltage that the collector voltage can be?
Q2) What is the lowest it can be?
Q3) How did you determined them?
Q4) What is the collector voltage that you are trying to achieve as an operating point?

Let's get that far and then move forward once any issues here are resolved.
Im too nobbie on electronic circuits.
As i said before, i need to find R1 resistanse when Q operating point is in the midway of the DC.
On the 1st post, ipost the values that is given to me for the exercise.

#### WBahn

Joined Mar 31, 2012
25,566
Im too nobbie on electronic circuits.
As i said before, i need to find R1 resistanse when Q operating point is in the midway of the DC.
On the 1st post, ipost the values that is given to me for the exercise.
In solving any problem we need to work from what we know to what we need to find.

We know that we need the operating point of the collector to be at the midway point of it's DC output range (or at least we'll assume that this is the proper interpretation of the problem statement).

We need to find the value of R1 that will make that happen.

So how do we get from what we know to what we need? Baby steps that get us part of the way toward the goal each time.

Does it not make sense that the first baby step might be to figure out what the actual collector voltage is that is the midway point of its DC output range?

Does it not make sense that to find the midpoint of some range, it might be helpful to find the endpoints of that range?

So, once again, please answer the following. There is no point going further until you have done so.

Q1) What is the highest voltage that the collector voltage can be?
Q2) What is the lowest it can be?
Q3) How did you determined them?
Q4) What is the collector voltage that you are trying to achieve as an operating point?

Once again, answer them as best you can. That will give us something to work with as far as understanding what you do and don't comprehend and filling in whatever gaps you might have.

#### Fotic Pap

Joined Aug 24, 2019
11
In solving any problem we need to work from what we know to what we need to find.

We know that we need the operating point of the collector to be at the midway point of it's DC output range (or at least we'll assume that this is the proper interpretation of the problem statement).

We need to find the value of R1 that will make that happen.

So how do we get from what we know to what we need? Baby steps that get us part of the way toward the goal each time.

Does it not make sense that the first baby step might be to figure out what the actual collector voltage is that is the midway point of its DC output range?

Does it not make sense that to find the midpoint of some range, it might be helpful to find the endpoints of that range?

So, once again, please answer the following. There is no point going further until you have done so.

Q1) What is the highest voltage that the collector voltage can be?
Q2) What is the lowest it can be?
Q3) How did you determined them?
Q4) What is the collector voltage that you are trying to achieve as an operating point?

Once again, answer them as best you can. That will give us something to work with as far as understanding what you do and don't comprehend and filling in whatever gaps you might have.
Sorry but the professor didnt gave us any extra info, only that i post it at the first place.
If i use this types below, i will get correct result?

Rdc = Rc + Re
Ic = Vcc / Rdc
Vb = ReIc + Vbe
Ib = Ic/ b
IR2 = Vb / R2
IR1 = Ib + IR2
R1 = (Vcc - Vb) / IR1

#### WBahn

Joined Mar 31, 2012
25,566
Sorry but the professor didnt gave us any extra info, only that i post it at the first place.
If i use this types below, i will get correct result?

Rdc = Rc + Re
Ic = Vcc / Rdc
Vb = ReIc + Vbe
Ib = Ic/ b
IR2 = Vb / R2
IR1 = Ib + IR2
R1 = (Vcc - Vb) / IR1
You have everything you need, provided you paid any kind of attention in your prior electronics courses. If you didn't, then you need to drop this course and go retake them and pay attention the second time around, otherwise you will just be digging yourself a deeper and deeper hole that will just be increasingly difficult to every get out of.

What are the two extreme modes for a BJT transistor to be operated in?