# Why is a parallel RLC circuit usually driven by a current source?

Almost always when I see an example of a parallel RLC/LC circuit diagram online, the circuit is driven by a current source instead of a voltage source. On the other hand, the series RLC is always driven by a voltage source. Why is this? Why don't we drive parallel circuits with a voltage source?

• I've seen plenty of examples of parallel circuits with voltage sources. I think what you're seeing is just a coincidence. Dec 16 '18 at 20:19
• As @knzhou is saying we have all seen plenty of parallel circuits driven by a voltage source but then there is no really interesting question one can ask. IOW it is not why they are driven that way but rather what happens when driven that way. Since we know what the voltage is on each R,L or C terminal pair there is nothing to ask unlike if they driven by a current source. In the latter case you have to work your way through the problem. Dec 16 '18 at 20:35
• @hyportnex That's a tempting answer, but not quite right. You could still ask "how much current comes from the voltage source as a function of frequency?" Dec 16 '18 at 20:41

Well actually, yes, we can because of the equivalence between current and voltage sources. A current source $$I_s$$ in parallel with an impedance $$Z$$ is equivalent to a voltage source $$V = I_s Z$$ in series with an impedance $$Z$$. With a parallel RLC, it's better to use the equivalent current source because it's easier to understand the effect that the source's own internal impedance has on the resonator. For example, consider the case where the source impedance $$Z$$ is just a resistor $$R$$. In the parallel representation we have the circuit shown below with an ideal current source connected to our RLC. The source's resistor simply adds in parallel with the resonator's own resistor! Therefore, computing e.g. the damping time for the combined circuit is particularly simple in this representation.