Voltage at the base of a transistor when a current source is connected to it

[Vijay Vinay]

Hi,
I have connected a current source to the base of the transistor, with Iref as 2mA. Now, I would like to know the value of voltage of the base of the transistor. Could anyone help me in finding it. Is there any particular formula for finding it out. In am getting the value of that voltage to be 1.95V. How is it so? Thanks in advance

 

[Audioguru]

The datasheet for most transistors shows a graph of base-emitter voltage with different currents and temperatures.
Your schematic does not show a negative supply or a ground.

 

[RobertFalbo]

About 0.6 volts more than the emitter.

 

[Vijay Vinay]

Hi RobertFalbo,
Could you elaborate a little bit on how its 0.6V more than the emitter voltage. Thanks in advance

 

[Vijay Vinay]

Hi guys,
sorry Iref is 4mA. My mistake

 

[Audioguru]

Could you elaborate a little bit on how its 0.6V more than the emitter voltage.

You can read about the physics that causes it on the internet.
Almost every transistor datasheet has a graph showing the base-emitter voltage at different currents and at different temperatures. For a silicon transistor it is from about 0.6V to about 0.8V, averaging 0.7V.

 

[Vijay Vinay]

You can read about the physics that causes it on the internet.
Almost every transistor datasheet has a graph showing the base-emitter voltage at different currents and at different temperatures. For a silicon transistor it is from about 0.6V to about 0.8V, averaging 0.7V.

I know that Vbe is 0.7V, but RobertFablo has mentioned that the base voltage is 0.6V more than that of the emitter voltage. I need to know how its possible and why is it so?

 

[andre_luis]

As Audioguru said, you did not show the reference for that measurement, therefore we don't know if the 1.95v "base voltage" you mentioned refers to the Vbe of the transistor.

 

[RobertFalbo]

I know that Vbe is 0.7V, but RobertFablo has mentioned that the base voltage is 0.6V more than that of the emitter voltage. I need to know how its possible and why is it so?

I guess, with 2 ma base current you would be closer to 0.7 volts above the emitter voltage.


The typical voltage drop of a PN junction (diode) for silicon is, as stated above, 0.6 to 0.8 Volts.