# BJT trans. in Satuation Mode

Discussion in 'Homework Help' started by mo2015mo, Sep 13, 2013.

1. ### mo2015mo Thread Starter Member

May 9, 2013
157
1
Hi guys,

i have been solved the problems as attached photos
problem 1 as attached photo #1
What is the value of EE-voltage to make the transistor in Saturation mode??
btw I solved this problem and found IE = 0.36mA , Ic= 0.342mA and Vcb = 7.8 volts

Problem 2 as attached photo #2
What is the value of Rb-resistor to make the transistor in Saturation mode??
btw I solved this problem and found IB = 0.23mA , Ic= 9.2mA and Vce = 21.2 volts

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2. ### WBahn Moderator

Mar 31, 2012
17,716
4,788
What is the value you are using for Vce when the transistor is in saturation? What value of Vee makes this happen? Note that, in saturation, the beta is not the same as it is in the active region. For practical purposes -- as well as a large fraction of datasheets -- you often assume a beta of 10 in saturation.

What is the voltage across the 10kΩ resistor is the collector current is 342μA? What does this make the voltage at the collector?

Why are you finding Vcb and not Vce?

3. ### mo2015mo Thread Starter Member

May 9, 2013
157
1
Vce = -Vcb - Vb = -8.5 volts

4. ### WBahn Moderator

Mar 31, 2012
17,716
4,788
Vce is Vc - Vb.

How can you calculate the collector-emitter voltage using only the voltage from the collector to the base and the voltage at the base? Doesn't the voltage at the collector have to be in there somewhere?

Did you mean Vbe instead of Vb?

Using subscript arithmetic, you would need:

Vce = Vcb - Vbe

Or you could just find Vc and Ve and take the difference.

Vc = -12V + 342μA*10kΩmV = -8.58V
Ve = Vb + 0.7V = 0.7V
Vce = Vc - Ve = -8.58V - 0.7V = -9.28V

Last edited: Sep 14, 2013
5. ### WBahn Moderator

Mar 31, 2012
17,716
4,788
In the first circuit, what does the collector voltage need to be in order for it to be in saturation?

What does the collector current need to be to produce that collector voltage?

What does the emitter current need to be in order to produce that collector current?

What does VEE need to be in order to produce that emitter current?