Hello,

I have been having a discussion with a colleague about how to use an NPN transistor for switching a load.

I've always connected an NPN transistor in Common Emitter (CE) configuration with the load between the collector and V+.

My colleague says he's always connected the load between the emitter and V- i.e. Common Collector (CC).

Which is correct?

 

hi,
Depending upon the application both can be considered correct.
E

 

Could you give examples of which scenarios each of the two configurations would be more appropriate?

 

It is more conventional to use an NPN transistor for low side switching, as you prefer, and to use a PNP for high side switching, as you colleague prefers not to do.

Either can work; it depends on the driving circuit.

 

The CC mode is an emitter follower, so the output is one diode-drop below the base voltage.
If you want to switch a voltage that's greater than the base control signal voltage, you need to use a CE configuration.

 

The there is the less commonly used method where C-E are reversed,
for example NPN where load is in emitter to V+, collector grounded. There
are some transistor that exhibit lower Vcesat in reverse mode operation.

Regards, Dana.

 

hi eyesee,
Look thru these images.
https://www.google.com/search?q=ima...HlzoTbAhXJAMAKHVnNC7kQsAQIJg&biw=1287&bih=894
E

 

The output voltage of CC mode as Vcc-0.2V(Vce)-0.7V(Vbe), so if you want to have a higher output voltage close to the Vcc then you can't use it.

The output of CC mode is the same phase with input, if you need that then to use CC mode is the way to go.

The output of CE mode is the inverting of input, I like to use this way, because it can be match the most of application, you can see the circuit below:
The Transistor's Sink And Source.