Electronics > Beginners

NPN Transistor as a switch: what's the use of the emitter resistor?

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soldar:
Not an emitter follower. The resistor limits the current in the device and prevents the bipolar transistor from going into saturation. Switching out of saturation takes longer time so fast counters and other switches will often prevent going into saturation by using such a resistor. Today we just use FETs, which don't have this problem.

bionic:
The emitter resistor will decide what Beta or amplification for that specific transistor will be!

Remember that:
IC/IE  = Alfa

Where Alpha gives:
Beta=(alpha/alpha-1)=IC/IB

Benta:
bionic, I don't think you've quite understood this circuit.
But welcome to the Forum.

gf:
Per se - as drawn - it just looks like an amp with emitter feedback...
But where/how is the load supposed to be connected? :-//
Or if really anything is supposed to be switched with the transistor, where/how is it connected to the given circuit?
[ If nothing else is connected to the given circuit, then I'd consider it rather useless. ]

EDIT: Or is RL itself supposed to the load? If yes, what kind of load is it?

Benta:

--- Quote from: gf on March 19, 2019, 08:36:50 pm ---Per se - as drawn - it just looks like an amp with emitter feedback...
But where/how is the load supposed to be connected? :-//
Or if really anything is supposed to be switched with the transistor, where/how is it connected to the given circuit?
[ If nothing else is connected to the given circuit, then I'd consider it rather useless. ]

EDIT: Or is RL itself supposed to the load? If yes, what kind of load is it?

--- End quote ---

RL is the load, yes. And it could be anything. Another input. A pullup resistor. An LED.
The collector of the transistor is a current source.
Let's say you have a 3.3 V logic output driving the transistor base. 3.3 - 0.7 = 2.4 V across the emitter resistor, equals 24 mA. Wonderful for driving an LED as load.
Or another input through a cable that might be shorted to plus through a connector short cut: no damage to the transistor. And, and, and...

Forget the emitter follower picture you have in your mind. It's a switch.

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