Electronics > Beginners
NPN Transistor as a switch: what's the use of the emitter resistor?
gf:
I did not call it emitter follower. I said "amp with emitter feedback" (which is eventually a voltage controlled current source, driving RL). With proper biasing at the input, the circuit could well act as amplifier with a closed-loop voltage gain of approx. RL*beta/(R1+R2*beta). But agreed, if the input voltage is low enough, then the current through RL can be turned off completely - then it becomes a switch. Contrary to the "traditional notion of a switch" (-> low impedance in the "on" position -> low power dissipation at the switch), the transistor and R2 do well dissipate power, when the "switch" (i.e the current source) is turned on.
Jwillis:
Emitter resistor is a way of biasing the transistor to prevent thermal run away.
Some times there will be a base resistors with paralleled transistors to prevent load hogging because not all transistors of like kind will turn on at exactly the same voltage.
Look up transistor biasing for further explanation because the way a transistor is biased depends on application.
Audioguru:
The transistor circuit shown is called a "phase splitter". The phase at the emitter output is the same phase as the base input but the phase at the collector output is -180 degrees.
Benta:
Why do you guys keep going on about this? The headline clearly states is it to be used as a SWITCH.
As an amplifier stage, it's far too simple and has no biasing provisions.
atmfjstc:
I think @soldar has it right. It is indeed a switch configuration, with Re there to ensure maximum switching speed. I remember reading about this trick in school. There is some detail here:
http://www.engineering-bachelors-degree.com/electronic-components/uncategorized/bipolar-junction-transistor-circuitshigh-speed-bjt-switching/
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