[SOLVED] [No title]

BJT characteristic ,collector current in active and saturation regipon

Ic is not a function of resistance Rc , changing Rc to any level will not affect the level of Ib or Ic as long as we remain in ACTIVE REGION.
(Ic is related to Ib is understood)

but in SATURATION REGION Ic = Vcc/Rc , means Ic is dependent on Rc in saturation region.

first i would like to get it clear how Ic is not dependent on Rc in active region.
and how Ic get dependent in saturation region over Rc.

Re: BJT characteristic ,collector current in active and saturation regipon

Because Ic = Ib * β
So Rc simply isn't a part of the equation.

Re: BJT characteristic ,collector current in active and saturation regipon

Prototyp_V1.0 said:

Because Ic = Ib * β
So Rc simply isn't a part of the equation.

Click to expand...

I have already mentioned in my post that the relation of ic =beta*ib is understood. I want a little depth elaboration.

Re: BJT characteristic ,collector current in active and saturation regipon

If you increase Rc too much, BJT will enter into saturation region.

Re: BJT characteristic ,collector current in active and saturation regipon

RAHUL_KUMAR said:

Ic is not a function of resistance Rc , changing Rc to any level will not affect the level of Ib or Ic as long as we remain in ACTIVE REGION.
(Ic is related to Ib is understood)

but in SATURATION REGION Ic = Vcc/Rc , means Ic is dependent on Rc in saturation region.

first i would like to get it clear how Ic is not dependent on Rc in active region.
and how Ic get dependent in saturation region over Rc.

Click to expand...

That is an easy question to answer. A BJT is a transconductance device. That means a voltage source on the base will control a current source in the collector circuit. As you should know, putting a resistor (Rc) in series with a current source will not change the current value. So to answer your question, Ic is not dependent on Rc because Ic comes from a current source when in the active region. In saturation, the BJT collector no longer has the behavior of a current source.

Ratch

Re: BJT characteristic ,collector current in active and saturation regipon

A saturated transistor is turned on so it is a short piece of wire between its collector and its emitter. Then the current in Rc depends on the voltage across Rc.

Re: BJT characteristic ,collector current in active and saturation regipon

Audioguru said:

A saturated transistor is turned on so it is a short piece of wire between its collector and its emitter. Then the current in Rc depends on the voltage across Rc.

Click to expand...

sir , can i get some depth ,the behaviour inside the transistor in active region .
as u said that in saturation region it become short so ic depend on Rc here. means there is some property of transistor due to which Ic is not dependent on Rc in active region..
whats that property??

- - - Updated - - -

how do BJT act like a current source in active region? kindly elaborate its inside behaviour .

- - - Updated - - -

BigBoss said:

If you increase Rc too much, BJT will enter into saturation region.

Click to expand...

sir, HOW??

Re: BJT characteristic ,collector current in active and saturation regipon

Go to school and learn about a transistor current source and a current sink.
If you keep the collector current constant but you increase the value of Rc then the transistor is turned on as much as it can so it is saturated.

Re: BJT characteristic ,collector current in active and saturation regipon

RAHUL_KUMAR said:

sir , can i get some depth ,the behaviour inside the transistor in active region .
as u said that in saturation region it become short so ic depend on Rc here. means there is some property of transistor due to which Ic is not dependent on Rc in active region..
whats that property??

- - - Updated - - -

how do BJT act like a current source in active region? kindly elaborate its inside behaviour .

- - - Updated - - -



sir, HOW??

Click to expand...

There are many properties and behaviors that BJTs have. We cannot give you a comprehensive course in semiconductor electronics within a simple question and answer forum thread like this session is. You need to teach yourself basic electrical science first, and then study some good material on BJTs. Then, we can answer specific questions you might have.

Ratch

Re: BJT characteristic ,collector current in active and saturation regipon

Do not restart the bickering over whether the BJT is a
nonlinear transconductor (base voltage, collector current)
or a linear-ish cccs (base current, collector current).

I'll relate what I was taught many decades ago, when
BJTs were mainstream and MOSFETs a laughable curiosity.

Assume charge neutrality, and injection of minority carriers
into the base (base current). Each carrier has a minority
carrier lifetime.

Across the base is assumed a sufficiently large potential
(C-E) that it can be neglected (forward active) with regard
to saturation. Now the base wants charge balance on all
the base current carriers busily entering and recombining.
At the collector, carriers may enter when the base is not
neutral (junction blocking depends on this). They contribute
a "fractional neutralization" of charge for as long as they
remain in the base region, the base transit time. Then
another will have to pick up the job.

beta equals storage time divided by transit time, this holds
up pretty well in measurement. Also accounts for Early
voltage, as narrowing base w/ Vcb goes to effective base
width goes to transit time.

Re: BJT characteristic ,collector current in active and saturation regipon

dick_freebird said:

Do not restart the bickering over whether the BJT is a
nonlinear transconductor (base voltage, collector current)
or a linear-ish cccs (base current, collector current).

Click to expand...

There should be no bickering over whether a BJT is a transconductance device or not. The following link proves that it is. Base voltage controls collector current. End of story. https://coefs.uncc.edu/dlsharer/files/2012/04/C4.pdf

I'll relate what I was taught many decades ago, when
BJTs were mainstream and MOSFETs a laughable curiosity.

Assume charge neutrality, and injection of minority carriers
into the base (base current). Each carrier has a minority
carrier lifetime.

Across the base is assumed a sufficiently large potential
(C-E) that it can be neglected (forward active) with regard
to saturation. Now the base wants charge balance on all
the base current carriers busily entering and recombining.
At the collector, carriers may enter when the base is not
neutral (junction blocking depends on this). They contribute
a "fractional neutralization" of charge for as long as they
remain in the base region, the base transit time. Then
another will have to pick up the job.

beta equals storage time divided by transit time, this holds
up pretty well in measurement. Also accounts for Early
voltage, as narrowing base w/ Vcb goes to effective base
width goes to transit time.

Click to expand...

The above explanation is one of the most confusing, disjointed, and disorganized I have ever come across. It mentions particular aspects that have nothing to do with the BJT being a transconductance device, and fails to explain how its transconductance comes about.

Above all, the BJT operates by diffusion. Putting a PN junction together will cause the excess N-slab electrons and excess P-slab holes to diffuse into the the opposite slabs. This diffusion also gives the BJT its non-linearity. The diffusion stops when the barrier voltage caused by the uncovered charges halt the diffusion. Applying forward Vbe lowers the barrier voltage and allows more diffusion current to take place. In a BJT, the collector voltage Vbc causes the current to be attracted to the collector. But no matter, what the collector voltage, the collector cannot attract more charge carriers than the diffusion into the base from the emitter allows. So the BJT allows only a finite amount of current no matter what the collector voltage is. That is the definition of a current source, steady current regardless of the voltage. Vbe controls the diffusion and the diffusion controls the collector current. Although there are secondary effects (like Early), that is the primary way a BJT works as a transconductance device.

Ratch