Current from I/O PIN

Hardware Arduino Due
15

Grumpy_Mike:
The whole point of a BJT is that it is driven by current, not voltage. That current might be small but it is current never the less because it is current that makes the transistor work. FETs on the other hand are voltage devices and draw negligible current apart from the initial charging of the gate source capacitance.

I never stated that a BJT could work without current! I originally recommended using FETs over on the 3 mA pins because I know the difference on how they operate.

Grumpy_Mike:
Lets look at each in turn:-

  1. Common emitter - the normal way I would recommend, base current determines collector current

  2. Common base - note the page says this is not suitable for a TTL voltage buffer, it has a low gain and typically I use this for video coupling.

  3. Common collector better known as an emitter follower. - This has no voltage gain but a current gain, you do not need a base resistor because of the feedback on the emitter keeping the emitter / base voltage stable. However this is 0.7V so this means when you use it as a voltage buffer you loose 0.7V. Used with the Due that will reduce the voltage output from 3V3 to 2.6V. When lighting an LED this will not be high enough for blue and white LEDs although it is enough for red and green. Again I would not recommend this due to the voltage drop that it produces.

Although you are correct that I should have stated, "common emitter" and not "common collector". I... wasn't in a frame of mind conducive to self proof-reading when I wrote it. I took your comment in a way you probably didn't mean, and furthermore let it irritate me more than I should have even if you did. :blush:

Regardless, with NPN a common emitter circuit the voltage difference between the base and emitter will be the VIN. Since VOUT can be expressed as follows VOUT = AV * VIN; how is VIN not effectively controlling VOUT? Yes, I intentionally didn't mention that the current will be amplified as well and that is fundamental to the operation of the BJT. However, in my mind at least, it wasn't necessary because the maximum current is so low and all the pins except DAC0 and DAC1 are either outputting digital or PWM signals. So it's just easier when biasing the circuit to concentrate on the voltage, either setting it up to be an actual voltage amplifier or a switch, and only address the current in the context of ensuring it doesn't go above 3 mA.