# Question about BJT in saturation

1. ## Question about BJT in saturation

Hi,
i have some question about biasing the bjt into saturation, and its switch configuration
1. Is it necessary to have Vce <= 0.2 in order for bjt to saturate?
2. So what if a very low resistive load is connected to it, since I think usually it will then have Vce greater than 0.2, so that means bjt can never be saturated under low load condition?
3. is the switch configuration is just common emitter?
4. can I have the bjt as a switch in emitter follower?

your help will be greatly appreciated!

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2. ## Re: Question about BJT in saturation

We can say that BJT act as switch , if it works only in cutoff and saturation states.

Transistor Switches

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3. ## Re: Question about BJT in saturation

1. Not necessary
2. saturation is the condition Ic< Beta *iB
3. yes with Rc* Beta > Rb
4 yes switch is on when base is tied to vcc ; off when base is tied to ground

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4. ## Re: Question about BJT in saturation

Originally Posted by hayowazzup
Hi,
1. Is it necessary to have Vce <= 0.2 in order for bjt to saturate?
You don't set the Vce to saturate but you have a low Vce as a result of the saturation, the Vce level is not exact it can be higher too

Originally Posted by hayowazzup
2. So what if a very low resistive load is connected to it, since I think usually it will then have Vce greater than 0.2, so that means bjt can never be saturated under low low condition?
Again the low Vce is not a requirement for the transistor to saturate but the result of a saturated transistor, the Vce voltage can have a higher level too.

Originally Posted by hayowazzup
3. is the switch configuration is just common emitter?
4. can I have the bjt as a switch in emitter follower?
You can either have the load connected to the collector or the emitter.
When the load is connected to the collector then you can control a load that has a higher voltage then the base.
When the output is the emitter then the output will always have a 0.7v difference from the base, so for example with a 5v to the base of a NPN you will only get 4.3v to the emitter ot with 0v to the base of a PNP you will get 0.7v to the emitter.

Alex

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