How to determine Polarity of transistors in a circuit?

[Hamid1]

Hi all,
In following image thare are Positive(+) and Negative(-) signs on above and under V_EB and V_CB
What's meaning of these signs?are they polarity of transistor?
why beside connection to ground is a negative sign?

thanks in advance.

 

[stevenb]

Those signs simply define the polarity of their respective voltages. This is similar to the current arrows which define the current direction.

When analyzing a circuit it is good to clearly label/define all current directions and voltage polarities.

There exists an unofficial notation convention (or notational aid) in which a label Vab is implied to mean Va-Vb, where Va and Vb are the voltages with respect to ground, on each side. This example seems to use that notation.

 

[Hamid1]

Thank you stevenb.
Can you explain how do I determine which one is Positive and which on is Negative?

 

[stevenb]

Thank you stevenb.
Can you explain how do I determine which one is Positive and which on is Negative?

I'm not sure I understand your question. Do you mean, how do you determine which case has VEB or VCB postitive and which negative?

If so, then you simply solve the circuit equations and calculate the values of the voltage drops. If you get a positive answer, then you know that you guessed the correct polarity when you set up the problem. If you get a negative answer, then you know that you guessed the wrong polarity. Note, that I'm using the word "wrong" loosly here, since there is no right and wrong with a convention.

In the case of the NPN transistor, it's clear that VEB is negative and VCB is positive. You can see this just by looking at the battery polarity which is biasing the transistors. You can also see that the current directions are defined correctly (using the positive current convention with positive charge flow representing positive current).

 

[Hamid1]

Thanks.
That's exactly what I asked.