Transistor as a switch- configuration of BJT to driving LEDs

[rezaxyz]

Transistor as a switch

Hi dear friends,
In your opinion what's the best configuration of BJT to driving LEDs (DotMatrix, big 7-segments, …) ?
Why common-collector is not suitable for switching heavy loads?
Is there any AN or reference to study more about this subject?

Regards,
Reza

 

[btbass]

Re: Transistor as a switch

Common collector or emitter follower is actually very good at driving heavy loads.
This is the normal configuration for a power amplifier output that drives the speaker load. It has high current gain, but no voltage gain.
If you have enough voltage swing on the input, you could drive your leds with it.

Common emitter is usually used because mostly you are switching with a logic level signal and the supply to the load is higher than logic level. It is used to translate the voltage levels.
As far as the transistor is concerned, makes no difference if the load is in the collector or emitter, the same current is passing through it if the transistor is saturated, ie, turned hard on and not in it's linear region.

 

[FvM]

Re: Transistor as a switch

As far as the transistor is concerned, makes no difference if the load is in the collector or emitter, the same current is passing through it if the transistor is saturated, ie, turned hard on and not in it's linear region.

A common collector amplifier is operating in unsaturated respectively linear mode, if you don't use an auxilary supply to raise Vbe above Vce. This implies a slightly higher Vce and may be a minor disadvantage with a heavy load. But it may be usable as well. In terms of low saturation voltage and easy control, a MOSFET may be the better alternative.