BJT ( bipolar junction transistor)

[encoder]

BJT is a current control device why we calling it as current control device.

 

[alexan_e]

Small variations in the base current will have big variation in the collector current.

Check transistor explanation in the following link Tutorials, Electronics made EASY, Williamson Labs

Alex

 

[syeda amna]

this is because collector current depends on base current. Ic=β*Ib
see the similar threads.

https://www.edaboard.com/threads/81771/

https://www.edaboard.com/threads/51884/

 

[encoder]

Small variations in the base current will have big variation in the collector current.

Check transistor explanation in the following link Tutorials, Electronics made EASY, Williamson Labs

Alex

Thanks for the link it is beneficial to understand the transistor.

 

[fg]

Bipolar junction transistors are voltage controlled. The fact that the base current and collector current have a fairly constant relationship does not mean that variations in the latter are caused by variations in the former, or vice versa. It would be grossly unscientific to assume so.

This issue has been recently addressed in this thread: https://www.edaboard.com/threads/196092/

 

[alexan_e]

Yes you must have a voltage to create the current flow in the base but since the hfe factor is based on the base current you need to know the actual current you are feeding.
Voltage is the cause and current is the result so it couldn't be any other way (current exists only where voltage exists),
how would you calculate the collector current from the base voltage, you would have to calculate the current of the base and i guess this is why we call it a current controlled device.

On the other hand in a mosfet you can calculate the output voltage/current only knowing the Vgs voltage (assuming it is an ideal low impedance source that can charge the gate capacitance immediately).

Do you consider these two cases the same?

 

[fg]

Yes you must have a voltage to create the current flow in the base but since the hfe factor is based on the base current you need to know the actual current you are feeding.
Voltage is the cause and current is the result so it couldn't be any other way (current exists only where voltage exists),
how would you calculate the collector current from the base voltage, you would have to calculate the current of the base and i guess this is why we call it a current controlled device.

On the other hand in a mosfet you can calculate the output voltage/current only knowing the Vgs voltage (assuming it is an ideal low impedance source that can charge the gate capacitance immediately).

Do you consider these two cases the same?

In my view, both BJT's and FET's are voltage controlled current sources more than anything else, but they have major differences in their voltage-to-current transfer characteristics. The drain current of an FET depends quite linearly on its gate-source voltage. Likewise, the collector current of a BJT depends on its base-emitter voltage, but this relationship is logarithmic. Both relationships are equally real and both should be treated as cause-effect relationships.

For a BJT, the key to understanding its operation is this: When one varies the base-emitter voltage, the charge in the transistor's base region changes proportionally. The amount of movable charge carriers in the base region is increased if you increase the forward voltage bias on the base-emitter junction. If, on the other hand, you decrease the forward bias voltage on the BE junction, the number of movable charge carriers in the base region becomes less. The more movable charge carriers in the base region, the more charges can flow between the collector and emitter. This flow of charges is what we call the collector current.

I can't explain this stuff very clearly. In case any of you wants to learn more about this viewpoint -- that BJT's are controlled by the charge in the base region, which, in turn, is controlled by the base-emitter voltage -- the best thing you can do is try and look for better explanations.

Some may find this useful: SCIENCE HOBBYIST: how transistor works, an alternate viewpoint

 

[alexan_e]

I would like to make my point using the following circuit:
I use a NPN transistor and a N mosfet, both have a 100 ohm resistor as load connected to a 10v source,
also both have 10v fed in the base and gate through a 100M ohm resistor.
The simulation graph shows the collector current to the left (green trace)
and the drain current to the right (red trace).

As you can see the transistor output current is 28uA (micro amperes) while the mosfet has an output current of 98.4ma (the max current for the load of 100 ohm would be 10v/100=100ma).
I think the difference between the two devices is quite clear,and it is more than just a logarithmic voltage controlled device and a linear one,
the voltage controlled mosfet has no problem with the current limiting resistor in the gate and can give the full current,
on the other hand the transistor can barely output any current because we have limited the current in its base to 10/10000000=1uA.
So unless you feed the base of the transistor constantly with current you will not get any current in the output.
This is why i believe it would be fair to say that the transistor is current controlled, if you prevent the current flow you don't get an output current.
Yes i know that the voltage of the base drops over the resistor and you may say that since we have a very low base voltage we don't have an output
but this is because the base is a low impedance load that needs current and we are limiting it with a resistor.
Also when you want to calculate the input voltage to the base of the transistor so that you have a specific current in the collector, how will you do that?
There is no info or way to do that in a transistor datasheet unless you calculate the base current.

Alex

 

[encoder]

In my view, both BJT's and FET's are voltage controlled current sources more than anything else, but they have major differences in their voltage-to-current transfer characteristics. The drain current of an FET depends quite linearly on its gate-source voltage. Likewise, the collector current of a BJT depends on its base-emitter voltage, but this relationship is logarithmic. Both relationships are equally real and both should be treated as cause-effect relationships.

For a BJT, the key to understanding its operation is this: When one varies the base-emitter voltage, the charge in the transistor's base region changes proportionally. The amount of movable charge carriers in the base region is increased if you increase the forward voltage bias on the base-emitter junction. If, on the other hand, you decrease the forward bias voltage on the BE junction, the number of movable charge carriers in the base region becomes less. The more movable charge carriers in the base region, the more charges can flow between the collector and emitter. This flow of charges is what we call the collector current.

I can't explain this stuff very clearly. In case any of you wants to learn more about this viewpoint -- that BJT's are controlled by the charge in the base region, which, in turn, is controlled by the base-emitter voltage -- the best thing you can do is try and look for better explanations.

Some may find this useful: SCIENCE HOBBYIST: how transistor works, an alternate viewpoint

this is the nice information thanks for sharing the information.

 

[boob_fet]

This is why i believe it would be fair to say that the transistor is current controlled, if you prevent the current flow you don't get an output current.

My man... you got it wrong....buddy.

take example nPn transisteat.

the EB section function as electron injector, and the BC section is the electron capture element.
you inject electron man...
and how would you control how much current you would inject..go figure it out