Understanding Transistor Concepts: Confusion with R1, R2, and IB

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In summary, the conversation discusses the concept of transistors and the confusion surrounding the relationship between current flow and potential difference. The use of a base bias resistor and its absence in a specific circuit is also mentioned. The conversation concludes with a discussion on determining the values of I1, I2, and IB. Ultimately, the summary highlights the need for further clarification and understanding of these concepts.
  • #1
bottlesheep
http://home.netvigator.com/~cyberforce/img/phy.jpg
I have learn transistor just few lessons..

For a)
what i learn is that R1 and R2 would share Vin
that's mean there is p.d. when current flows via R1, R2
thus, Vin = VR1 + VR2
and i am confused if the current flows via them is the same
that's mean I1 and I2 are the same...
but if they are the same...
how come the question ask me both of the seperately??
..Should they be different??

and for IB, I am confused too.
Because when i learned Transistor,
there is a Base bias resistor,say RB, in the circuit
thus, VR2 = (IB)(RB) + VBE
but now there is no RB, so VR2 = VBE..
then I think that there is no potential difference, so no IB
but it seems that I am wrong...
coz part b) assume the current gain is 100..
that means (IC)/(IB) =100, which implies there is IB...

so I don't know how to do the question at all..
I am not good at English and presentation...
I don't know if you understand what I don't understand...

my physics is poor...
please help...
are my concepts totally wrong..? > <..
thank you for your help...
 
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  • #2
When you studied the transistor did you learn something about VBE?
 
  • #3
Originally posted by Guybrush Threepwood
When you studied the transistor did you learn something about VBE?

yes, I know..
then..??
 
  • #4
Originally posted by bottlesheep
yes, I know..

what exactly do you know?
 
  • #5
Originally posted by Guybrush Threepwood
what exactly do you know?

current IB starts to flow when VBE is greater than 0.5V...
in my graph...VBE is the potential difference between any points on the wire which between resistors R1and R2 and any points on the earthing wire...
 
  • #6
ok, when I was doing that kind of problems in the college VBE was always considered to be 0.6V. That's a reasonable approximation for the real world.
I don't know if this is acceptable for your problem (you have to check similar problems you've made before in class).
 
  • #7
Originally posted by Guybrush Threepwood
ok, when I was doing that kind of problems in the college VBE was always considered to be 0.6V. That's a reasonable approximation for the real world.
I don't know if this is acceptable for your problem (you have to check similar problems you've made before in class).

ok..
but this circuit is quite different from my previous exercises..
as I mention, there is no Base bias resistor in this circuit which always appears in the exercises I have done before...

and also, I really want to know if I1 and I2 are the same or not?? can you tell?
 
  • #8
Originally posted by bottlesheep
and also, I really want to know if I1 and I2 are the same or not?? can you tell?

no they're not equal, I1 = I2 + IB
start by writing how VBE depends on I2
 
  • #9
Originally posted by Guybrush Threepwood
no they're not equal, I1 = I2 + IB
start by writing how VBE depends on I2

OH..IC..
that means here, I2=IB..
since VR2=VBE...
Is it correct?
 
  • #10
nope
VBE = VR2 = R2*I2 and you get I2. I2 would be equal to IB if you'd had a series circuit, but it's not the case...

then you find out VR1, I1 and after that IB
 

1. What is the purpose of R1 and R2 in a transistor circuit?

R1 and R2 are resistors that are used to bias the transistor in the active region, ensuring that the transistor operates within its linear range. R1 and R2 also help to stabilize the operating point of the transistor.

2. How do R1 and R2 affect the collector current and base current in a transistor circuit?

R1 and R2 determine the amount of current that flows through the base of the transistor, which in turn controls the amount of current that flows through the collector. If R1 and R2 are too high, the base current may be too low and the transistor will not be fully turned on. If they are too low, the transistor may be damaged due to excessive current.

3. What is the purpose of the base current (IB) in a transistor circuit?

The base current (IB) is used to control the flow of current through the transistor. By varying the amount of current flowing through the base, the amount of current flowing through the collector can be adjusted, allowing the transistor to act as a switch or an amplifier.

4. How do I calculate the values of R1 and R2 in a transistor circuit?

The values of R1 and R2 can be calculated using Ohm's law. R1 is typically chosen to be 10 times larger than R2 to ensure that the transistor is properly biased. The exact values will depend on the specific transistor and circuit design.

5. What happens if R1 and R2 are not properly chosen in a transistor circuit?

If R1 and R2 are not properly chosen, the transistor may not function correctly. If the values are too high, the transistor may not turn on fully and may not amplify the signal properly. If the values are too low, the transistor may be damaged due to excessive current. It is important to choose the appropriate values for R1 and R2 to ensure proper operation of the circuit.

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