Having solved the mystery of "pulling" and "pushing" devices, it would be interesting to do the same with "sourcing" and "sinking" devices.

Background

A transistor with a constant base-emitter voltage represents a constant current source (to be more precise, we should also include the power source).

For example, in the CircuitLab schematics below, I have adjusted the base-emitter voltage so that to pass 1 mA current through the load. If you want to change it, open Vin's parameters window, hover the mouse over RL, and adjust Vin while looking at the reading. If the parameters window covers RL, then move the schematic to the right edge of the screen, open and click on parameters to pin it, and return the schematic to the left side of the screen.

Note something interesting in this arrangement - the collector and emitter currents are equal because the base current is not added to the emitter one.

Varieties

Depending on the place of insertion, we call this device by different names.

"Current source"

If we connect the transistor Q with its emitter to the positive supply rail, and the load RL to ground, we call it "current source".

^{simulate this circuit – Schematic created using CircuitLab}

"Current sink"

If we connect an NPN transistor Q with its emitter to ground, and the load RL to Vcc, we call it "current sink".

^{simulate this circuit}

Both?

But what should we call (what is) the transistor when it is "floating" between two loads RL1 and RL2?

^{simulate this circuit}