Current Source and Sink

Speaking generally, a voltage source is a battery, power supply or other source of voltage with a stiff, low output impedance. With a perfect voltage source the voltage doesn't change no matter what load is placed on it, and whether current is pulled out of it or pushed into it.

In practical terms, voltage sources have some current limit, beyond which the voltage sags or changes, and it fails to be a true voltage source. They also don't can't zero output impedance, they'll have some small equivalent series resistance. They often come with sense leads, allowing you to remotely locate the two controlled voltage-source terminals.

Having explored voltage sources, it'll be easier to understand their complement, current sources. In a general sense, a current source has a fixed output current no matter what load it sees, no matter what voltage it works into, whether positive or negative. Furthermore, just as a voltage source can be positive or negative, a generalized current source can also deliver a positive or negative current, it's still a source of current rather than of voltage, hence a current source.

In practical terms, current sources have some maximum output voltage beyond which they fail to work, this is called the compliance voltage. Some current sources have a compliance voltage that's limited to one polarity, e.g., they cannot work at negative output voltages.

A perfect current source would have an infinite output impedance, or no change in current with output voltage. But for actual current sources the current does change a little with output voltage, and we can define an impedance, or conductance, 1/Zout = dI/dV. Zout is often very high, MΩ or even GΩ. At some high frequency real current sources also fail to maintain their high impedance. Usually this degradation can be expressed as a parallel output capacitance, C. In a high-performance true current source C may be quite small, tens of pF, but many current sources are in fact voltage sources with current-sensing feedback arraigned to make them appear to be current sources. These can have very high equivalent capacitance, even up to uF in value.

We can also think of an output-voltage slew rate. If the load impedance suddenly changes, the output voltage on a perfect current-source should immediately reflect the new impedance, according to Ohm's law, V = Z I. But in reality the voltage will slew at a rate given by dV/dt = I/C, where I is the current-source's output current. Naturally, if a low value of I is programmed, the slew rate will be very slow. Also, naturally, whatever load capacitance is present will slow the slew rate; that can't be considered the fault of the current source.

OK, now to the O.P.'s question about current sources and current sinks. As stated, when we're talking about current sources (or CS), as opposed to voltage sources, it's called a CS whether it sources or sinks current. This means that when you look at "current-source" circuits, you may find a PNP transistor with its collector delivering a current output. Or you may find say an NPN transistor with its collector acting as the current-source output. Now, for the NPN this is actually a current into the collector, most likely going toward ground, or a sinking current. So this can be called a current sink, rather than a current source. But for a CS, such language is optional.

However, if someone tells you they'll let you choose between a current source or a current sink, clearly they're describing the current polarity. And if both of these choices are positive-voltage capable only, we can draw further conclusions. A current source will come from some internal supply, and will deliver a current to your load, which you'll probably return to ground. OTOH, a current sink will draw current from your load, and you will have to supply the positive voltage to source this current on the other side of your load.

Or they may give you an extra terminal with the needed voltage available on it. This option means you might have a chance to damage the device if you inadvertently short this voltage. Yes, they can provide a current-limit protection to their voltage source. But you can see the appeal of a true positive current-source output: one terminal, no external supply needed, simply tie the other side of your load to ground. In the context of the CS sources and sinks we're discussing now, that would be a current source. But be careful, if someone tells you they'll give you a current-source output, they might mean the second type on the right, or a current sink. Be sure to double check what you're actually getting.