What happens before and after the saturation of bipolar transistor?

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.

Reply to
nabi
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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.

Reply to
John Popelish

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 :)

Not sure what is the question?

Reply to
lafayetteradio

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 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".

Approximately, yes. The transistor's data sheet will show you Vcesat - that's the collector to emitter voltage at saturation.

Yes, if you account for Vcesat. This is usually called a "low-side switch".

You seem to have answered your own question. :-)

Have Fun! Rich

Reply to
Rich Grise

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