BJT Circuit - Determine region of operation

[Blehs]

Homework Statement

See the jpeg image for the circuit. Question is simply determine the region of operation.

The Attempt at a Solution

The answer is cut off, but i don't really get how it is.

Looking at the circuit, emitter voltage Ve should be higher than base voltage Vb (simply because the base current has to go through a resistor Rb). Through this reasoning i say that Veb = Ve-Vb>0 . This means emitter-base junction is forward biased. Voltage at the collector Vc must be higher than Vb because there is a voltage drop between the base and collector terminal. Hence Vbc = Vb-Vc<0 and collector-base junction is reverse biased

That information suggests the circuit is in forward active operation. However i noticed one strange thing - the direction of the base current is going into the base (due to Ve > Vb). However from my notes it says a PNP transistor in forward active operation should have conventional current going out of the base.

So i must be analyzing the circuit wrong...help please?

 

[berkeman]

Homework Statement

See the jpeg image for the circuit. Question is simply determine the region of operation.

View attachment 52778

The Attempt at a Solution

The answer is cut off, but i don't really get how it is.

Looking at the circuit, emitter voltage Ve should be higher than base voltage Vb (simply because the base current has to go through a resistor Rb). Through this reasoning i say that Veb = Ve-Vb>0 . This means emitter-base junction is forward biased. Voltage at the collector Vc must be higher than Vb because there is a voltage drop between the base and collector terminal. Hence Vbc = Vb-Vc<0 and collector-base junction is reverse biased

That information suggests the circuit is in forward active operation. However i noticed one strange thing - the direction of the base current is going into the base (due to Ve > Vb). However from my notes it says a PNP transistor in forward active operation should have conventional current going out of the base.

So i must be analyzing the circuit wrong...help please?

I believe the part of your quote that I bolded is the key.

 

[Blehs]

Ah ok so when i assumed that conventional current was going OUT OF the base, the results seem to make sense and point towards the BJT being in cut off.

However my real concern is - how am i meant to determine that current is in fact going into the base? Was it really just as simple as

1) assume forward region of operation
2) in forward active region current goes INTO base, so its incorrect
3) assume cut off
4) everything adds up so therefore its cut off

Got an exam coming up soon and id like to prepare a good methodical way to approach questions like this

 

[rude man]

The point berkeman was alluding to is that the circuit does not PROVIDE for (conventional) current flowing out of the base. No base current, no emitter or collector current (ignoring reverse-biased b-c junction leakage current).

 

[rezeew]

Based on the information provided, it seems like the circuit is indeed in forward active operation. This is because the emitter-base junction is forward biased and the collector-base junction is reverse biased, which is characteristic of this region of operation.

As for the direction of the base current, it is important to note that the direction of conventional current is opposite to the direction of electron flow. In this case, the base current is flowing into the base, but the electrons are actually flowing out of the base. This is because the base terminal is connected to a positive voltage source, and electrons flow from negative to positive.

Therefore, it is possible for a PNP transistor in forward active operation to have conventional current flowing into the base. This is because the base current is actually carrying electrons out of the base, but in the opposite direction of the conventional current.

In conclusion, the circuit is indeed in forward active operation and the direction of base current is not an issue. It is important to remember that conventional current is opposite to the direction of electron flow.