# Finding base resistor of BJT question

Discussion in 'Homework Help' started by torx11, Feb 23, 2014.

1. ### torx11 Thread Starter New Member

Oct 6, 2011
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I need help finding the value of R so that the transistor operates in the saturation region.

The transistor has a max VBE of 0.85V, VCE,SAT = 0.2V, and BDC of 300.

Please could somebody point me in the right direction as to how to start this problem? Is it to do a KVL? To find Ic?

Last edited: Feb 23, 2014
2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,595
393
hi,
If the Gain is 300 when saturated and the collector load is 1K.

What do you calculate the Ic [ collector current] that is required to drop 10V across 1K.?

When you have that Ic value and the Gain of the transistor is 300, what would do you calculate the Ib [ Base current] that is required to produce the known Ic.??

When you know the Ib then you can calculate R.

Do you follow that OK.?
E

3. ### torx11 Thread Starter New Member

Oct 6, 2011
13
0
Thank you for your help. I have a little problem finding the formula for R. What would we do with the 5V and also would I need to find VB?

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,595
393
hi,
You are given Vbe is 0.85V and the Source is +5V, so 5-0.85 = vvv

So R = vvv/ Ib.... What do you make Ib , knowing Ic and the Beta..?

E

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5. ### torx11 Thread Starter New Member

Oct 6, 2011
13
0
Ah thank you so much for your persistent help. I thought the formula at first was R = (5V - VB)/IB, but I wasn't sure how to find VB or if it was even right in this case.

6. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You are given VBE=0.85V (please don't drop the units).

Q1) If the transistor is ON and the emitter is at 0V, what is the voltage at the base?

Q2) What is the voltage at the left of the base resistor?

Q3) What is the voltage at the right of the base resistor?

Q4) What is the voltage across the base resistor?

Q5) What is the voltage at the collector of the transistor?

Q6) What is the voltage across the collector resistor?

Q7) What is the current in the collector resistor?

Q8) What is the current in the base of the transistor?

Q9) What is the value of the base resistor?

Compare the answers to the above with the comparable parts of what ericgibbs was asking. You should get comparable answers.

As a practical note, the beta of the transistor will fall as it starts to saturate. In fact, many small signal transistor data sheets define saturation as the point at which the beta has dropped to a value of 10.

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Jan 29, 2010
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8. ### torx11 Thread Starter New Member

Oct 6, 2011
13
0
I want to say thank you guys for helping me with this problem. I really appreciate the support.

Regarding the same circuit, I am stuck at this question; find the value of R so that the transistor operates in the cut-off region where Ic = IB = 0.

If Ic = IB = 0, then how would we calculate the value for R? Could it be R is infinity or there some exact value for this?

Jan 29, 2010
2,595
393
10. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You've basically got it. We need to make R large enough so that essentially no base current flows. In theory, it needs to be infinite resistance. In practice, it depends on how small the collector current needs to be before we consider it to be essentially zero. For some applications that might be a few milliamps and for other applications it might be a few nanoamps or even smaller.

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