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What happens before and after the saturation of bipolar transistor?

N

nabi

Jan 1, 1970
0
If base current keeps increasing, then the collector current increases
upto the saturation point. Books say.
What happens if the base current does not decrease at the saturation
point?
The transistor blows up or there will be no collect current flow?

Secondly, when the saturation happens at the collector side, the
volate of the collector point becomes zero volt (ground), so it can be
used to be a temporary ground for another path? A flash light circuit
- which has two LEDs and two bipolar transistor and capacitors on each
collector side, runs this way.

Thanks in advance.
 
J

John Popelish

Jan 1, 1970
0
nabi said:
If base current keeps increasing, then the collector current increases
upto the saturation point. Books say.
What happens if the base current does not decrease at the saturation
point?
The transistor blows up or there will be no collect current flow?

Secondly, when the saturation happens at the collector side, the
volate of the collector point becomes zero volt (ground), so it can be
used to be a temporary ground for another path? A flash light circuit
- which has two LEDs and two bipolar transistor and capacitors on each
collector side, runs this way.

The transistor saturates as the collector voltage swings to
the emitter side of the base voltage. For example, an NPN
with grounded emitter and a collector resistor to positive
supply voltage...

As the base current rises, the collector current rises many
times faster, because of the current gain of the transistor.
The base to emitter junction acts much like any other
forward biased diode, so that the base voltage is about .6
volts more positive than the grounded emitter.

But as the collector current rises, the collector voltage
falls toward the ground rail, as more and more current is
passed through the collector resistor. Once the collector
voltage falls to below the base voltage, the collector to
base junction is no longer reverse biased, and the
transistor is said to be in saturation.

If you keep raising the base current, the collector voltage
falls so much that the collector to base junction becomes
forward biased enough that a significant fraction of the
base current passed directly to the collector, making it
difficult for the current gain of the transistor to pull the
collector any lower. In effect, a significant part of the
collector current load becomes base current that poured
directly into the collector.

The swing from base collector junction reverse biased to
forward biased in the defining change that causes saturated
operation to be different from unsaturated operation.
 
If base current keeps increasing, then the collector current increases
upto the saturation point. Books say.
What happens if the base current does not decrease at the saturation
point?
The transistor blows up or there will be no collect current flow?

The transistor will blow up. If you connect your voltage to the base
without a limiting resistor, poof, your transistor will be blown, you
should try it :)

Secondly, when the saturation happens at the collector side, the
volate of the collector point becomes zero volt (ground), so it can be
used to be a temporary ground for another path? A flash light circuit
- which has two LEDs and two bipolar transistor and capacitors on each
collector side, runs this way.

Not sure what is the question?
 
R

Rich Grise

Jan 1, 1970
0
If base current keeps increasing, then the collector current increases
upto the saturation point. Books say.
What happens if the base current does not decrease at the saturation
point?

This is kind of ambiguous. "AT" the saturation point, increasing the
base current can't saturate the transistor any more, it will just
dissipate power in the base, eventually melting it.
The transistor blows up or there will be no collect current flow?

The transistor can be damaged if there's excessive emitter-base flow,
but other than that, the transistor will just stay in saturation.

When you reduce the base drive, of course, the collector current
will decrease - this is called the "linear region".
Secondly, when the saturation happens at the collector side, the
volate of the collector point becomes zero volt (ground),

Approximately, yes. The transistor's data sheet will show you Vcesat -
that's the collector to emitter voltage at saturation.
so it can be
used to be a temporary ground for another path?

Yes, if you account for Vcesat. This is usually called a "low-side switch".
A flash light circuit
- which has two LEDs and two bipolar transistor and capacitors on each
collector side, runs this way.

You seem to have answered your own question. :)

Have Fun!
Rich
 
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