# Computing resistance in given circuit with BJT

• Engineering
• Peter Alexander
In summary, the conversation discusses a task involving finding the Q-point of a bipolar junction transistor between the saturation region and the active region. The task can be solved using Kirchoff's laws and the Ebers-Moll model is not necessary. The main issue is determining the value of VCB, which must be 0 for the solution to work. The conversation ends with the person being grateful for the help and apologizing for not having enough knowledge on BJTs.
Peter Alexander
Hi,
before you proceed with reading this question, I would like you to know what I do not expect anyone to solve this task for me. I have a problem with a single step in the solution and I'm only asking you to help me with this one step.

1. Homework Statement

Compute ##R_B## so that Q-point of given bipolar junction transistor is between the saturation region and the active region.
Other data: ##U_{BE} = 0.7 \text{V}##, ##U_{CC} = 5 \text{V}##, ##R_C = 1 \text{k}\Omega## and ##\beta = 100##.
Attached file includes a circuit in question.

## Homework Equations

This task should be solved with Kirchoff's laws easily. The reason why I'm saying this is because I've had a similar task before and it didn't require equations usually associated with bipolar junction transistors (e.g. Ebers-Moll model)

## The Attempt at a Solution

To keep it short and simple: I can only solve this task correctly if ##U_{CB} = 0## and I don't know why.
I started this task by looking at$$U_{CC} = I_C R_C + U_{CE}$$and$$U_{CC} = I_B R_B + U_{BE}$$which leads to a question: what about ##U_{CB}##?
From the first equation I can derive ##I_C## and use ##I_C = \beta I_B## to compute ##R_B## from the second equation.
My computations yield the correct result, but it is obvious that ##U_{CB} = 0## for this to work. I don't know why is that the case, and I'm not satisfied with "it has to be done in order to find the solution".

If someone would be kind enough to help me out, I'd very much appreciate it.

#### Attachments

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"Q-point of given bipolar junction transistor is between the saturation region and the active region."

What do you think is the definition for this specification?
In which region will the BJT operate for VCB<0 (negative) and for VCB>0 ?

anorlunda and Peter Alexander
LvW said:
"Q-point of given bipolar junction transistor is between the saturation region and the active region."

What do you think is the definition for this specification?
In which region will the BJT operate for VCB<0 (negative) and for VCB>0 ?
Well, that's embarrassing, I should have read a bit more theory on BJTs. Obviously that was my flaw here.

Thank you so much for helping me out and pointing me in the right direction. Solved!

No problem - don`t mention it.
Best wishes to you and good success.

Peter Alexander

## 1. How do I calculate the resistance in a circuit with a BJT?

In order to calculate the resistance in a circuit with a BJT, you will need to use Ohm's Law. This law states that resistance (R) is equal to the voltage (V) divided by the current (I). Therefore, to calculate resistance in a BJT circuit, you will need to measure the voltage across the BJT and the current running through it.

## 2. What is a BJT and how does it affect resistance in a circuit?

BJT stands for Bipolar Junction Transistor, which is a type of semiconductor device commonly used in electronic circuits. A BJT can act as either an amplifier or a switch in a circuit, and its presence can affect the overall resistance in the circuit depending on how it is configured.

## 3. Can the resistance in a BJT circuit change over time?

Yes, the resistance in a BJT circuit can change over time due to factors such as temperature and aging of the components. This is why it is important to regularly check and calibrate circuits using BJTs to ensure accuracy.

## 4. How does the configuration of a BJT affect the resistance in a circuit?

The configuration of a BJT can affect the resistance in a circuit in several ways. For example, a BJT configured in common-emitter mode will have a higher resistance compared to the same BJT configured in common-base mode. Additionally, the resistance can also be affected by the biasing of the BJT.

## 5. Are there any limitations to using BJTs for computing resistance in a circuit?

While BJTs are commonly used in electronic circuits, there are some limitations to consider when using them to compute resistance. For instance, BJTs have a limited voltage and current handling capability, and they may introduce noise into the circuit. It is important to consider these limitations when choosing to use BJTs for computing resistance in a circuit.

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