Electronic – What exactly is a current source

The voltage across a capacitor is the integral of the current through it. If you feed a constant current to a capacitor, its voltage ramps up linearly, which is exactly what you want for a sawtooth waveform generator.

Yes, you're correct that this cannot continue forever; the complete waveform generator circuit will be discharging the capacitor periodically in order to prevent the constant-current circuit from saturating.

In the circuit you show, D1, D2 and R1 provide a voltage reference, in this case, approximately 1.2V, which is the total forward drop across the two diodes. This establishes the voltage across the B-E junction of Q1 and R2. Since the voltage across the B-E junction is also a diode drop, or 0.6V, this means that the remaining voltage, 0.6V appears across R2.

From Ohm's law, we now know the current through R2, 0.6V/2200Ω = 273µA. This is the current that is charging C1.

The voltage across the capacitor is a function of time: V = I×t/C. Let's rewrite this as V/t = I/C, which means that the rate of change of the voltage is the current divided by the capacitance. In this case, 273µA/0.1µF = 2730 V/s, or equivalently, 2.73 V/ms.

Both walk together. Imagine voltage to be, as the name says, potential. In other words, is what your source can potentially causes in your circuit. It is its potential to generate a current. As the ohms law states (U=Ri), we can think of current being a consequence of a resistence connected to this source. This is almost a rule for almost every source of electricity in our days, but there are exceptions (below)! So, whenever you think of your wall outlet or a car battery or a cellphone battery, they have a voltage potential and the current will be calculated by which material or what are you connecting to the power source (by its resistance). Generally, the current specs tells us what is the MAXIMUM current that this power supply can handle. But it does not tell you that this source will be always +V volts and A amperes!

But note that this is the case for voltage sources. As the name says, it guarantees a constant voltage (ideally). So you calculate current because you know that the nominal voltage will remain the same.

So the excepetions will be current sources. Now everything is inverted! This kind of power supply guarantee's that the current will remain fixed. So you can calculate what voltage it is applying to your circuit so it can delivery that amount of current (also by ohms law) but generally we do not need that. Although a current source concept is very useful inside electronic devices, we are not used to see them in our days. But one good example is your telephone line comming from the wall. Those are current sources. Note that you cant damage the wires or the telephone company by making a short circuit to these wires. You can connect a multimeter and you will see that the current will be stable at some point (here in Brazil at 24mA). I can connect a 10Ohms resistance or a 300Ohms resistance and the current will be the same. Of course the voltage applied will be different, and that is how a current source works.

So it all depends on what type of source you are dealing with. If it guarantees a current fixed, you can calculate the voltage difference between terminals. If it guarantees a voltage value, you can calculate its current depending on which load you put there. In most cases, those power supplies from computers, cellphones, etc are all voltage sources and its specifications guarantee a nominal voltage and a maximum current. But don't expect to have that current regardless of the connected load!