Does BJT behave as current source in active mode ; voltage source in saturation?

[matrixofdynamism]

Behaving like a current source means that change in load does not impact the current. This seems true for a BJT since the current depends on the base current in the active mode.

 Behaving like voltage source means that the voltage applied on the load does not change but the current does. This seems true for the BJT in saturation since if we change the load e.g add another equal value resistor in parallel and the transistor gain parameter does not change since it is in saturation and its power dissipation limit is not violated, then in this example the collector current shall double.

 Is this correct way of understanding how BJT behaves?

[Zero999]

In active mode, the BJT behaves as a current source, as the collector current varies very little with any change in load resistance. The collector current depends more strongly on the base-emitter voltage, than the base current, which is unpredictable as Hfe is depends on the collector current.

When the BJT is saturated, it behaves like a closed switch, with a very low voltage drop. The characteristics of the voltage source used to power the circuit dominate the characteristics of the transistor which, for analytical purposes, can be ignored altogether i.e. replaced with a short between the emitter and collector.

A BJT becomes saturated when the base current is too high for it to remain in the active mode.

[danadak]

https://www.google.com/search?q=bjt+saturation+plot&tbm=isch&imgil=HoeT9wQmK4Oj1M%253A%253BoR7MNt5BlzUlXM%253Bhttp%25253A%25252F%25252Felectronics.stackexchange.com%25252Fquestions%25252F207935%25252Fconfusion-in-interpreting-npn-transistors-saturation-region&source=iu&pf=m&fir=HoeT9wQmK4Oj1M%253A%252CoR7MNt5BlzUlXM%252C_&usg=__CfYl9D3ApZuOVlkccXtuZ-8yD3M%3D&biw=1517&bih=720&ved=0ahUKEwiA6ODi_5bSAhVJMSYKHf9IBsoQyjcILQ&ei=jdqmWMDtGMnimAH_kZnQDA#imgrc=loII3EPyn4VDKM:

[TimFox]

To first order, when a BJT switch is "ON", it looks like a constant voltage.  When a JFET or MOSFET switch is "ON", it looks like a resistor.

[rfeecs]

Yes, in the active region the base-collector is reverse biased.  Almost all the emitter current is swept out of the collector.  So the collector current is relatively insensitive to the collector voltage and acts like a current source (or sink).

In saturation, the base-collector is forward biased.  So the transistor looks like two back to back diodes, both forward biased.  The collector voltage is Vbe - Vbc.  Since the diode voltages are relatively constant with current, the transistor looks like a voltage source.

[T3sl4co1l]

To first order, when a BJT switch is "ON", it looks like a constant voltage.  When a JFET or MOSFET switch is "ON", it looks like a resistor.

They both look like resistors; some BJTs are even rated in terms of resistance.  The mechanism is physically different, but it's handy that it looks similar enough!

IGBTs and Darlington transistors don't pull all the way to zero, though (and for an essentially similar reason -- there's an extra Vbe in series with the output, preventing it from pulling down all the way).

Tim

[TimFox]

To first order, when a BJT switch is "ON", it looks like a constant voltage.  When a JFET or MOSFET switch is "ON", it looks like a resistor.

They both look like resistors; some BJTs are even rated in terms of resistance.  The mechanism is physically different, but it's handy that it looks similar enough!

IGBTs and Darlington transistors don't pull all the way to zero, though (and for an essentially similar reason -- there's an extra Vbe in series with the output, preventing it from pulling down all the way). 

Tim

Obviously, both devices have some resistance when ON, but for the BJT the resistance is in series with a (roughly constant, but small) voltage V_CE(sat), while for the FETs the resistance has no series voltage source.  The ON resistance of the JFET is a function of the gate-source voltage, and that can be exploited as a voltage-controlled resistor (at low drain-source voltages) in AGC circuits, etc.

[danadak]

Some transistors with collector and emitter swapped in switch circuit
will result in significantly lower Vcesat. But betas significantly affected,
hence base drive levels.


Regards, Dana.

[T3sl4co1l]

Yeah, hence the nod to physical mechanisms, and the wiggle room of "similar enough".

BJTs are usually made with asymmetrical junctions, so there is a built-in potential, which manifests as elevated Vce(sat) under typical saturation conditions.  And, curiously low Vce(sat) when inverted!

Low-Vce(sat) transistors are curiously symmetrical, having high inverted hFE, and often having resistance ratings.  They're quite nice, especially for low voltage circuits -- if you're doing a switching task, they look more like super-high-gain MOSFETs with leaky gates!

Tim