How does a RC tank circuit work?

Answer 1

A tank circuit is an LC filter that, when at resonance, has an near infinite resistance. It is composed of an inductor in parallel with a capacitor. Resonance occurs when the magnitude of the impedance of the cap and inductor are equal. They have a fairly narrow bandwidth, which is why they are used in RF applications. They are able to 'focus in' on the desired frequency, and ignore the others. For example, if I have a simple tank circuit with a resonant frequency at 1 MHz connected to an unregulated ac function generator, and I measure the voltage across the tank circuit, when the frequency I apply is close to the resonant frequency, the amplitude of my output begins to increase. So at 900kHz, say, I might be getting 90% of the signal I apply to the circuit as my output. The impedance of this circuit increases as the frequency nears the resonance frequency of the tank. So if I set the function generator to 1MHz, the tank has extremely high impedance, and functions like an open, so I can expect my entire signal to drop across the tank circuit.

Alternately, they can be used to create oscillators. Another name for a tank circuit which more useful for visualizing how it functions is the slosh circuit. When energy is applied to the slosh circuit, the cap will discharge into the inductor, and vise versa, and it will oscillate like this at the tanks resonant frequency. The energy 'sloshes' from one component to the other. This can be exploited by circuitry to make oscillators, amplifiers, voltage doublers and so on.

I have not seen an RC tank circuit before, but one is theoretically possible using an active circuit called a gyrator, which consists typically of an op amp, a couple resistors, and a capacitor. A gyrator essentially makes one component function like another. If I construct a gyrator with a cap, it will function much like an inductor (not identically, there are a few differences), and likewise, I could make a psuedo-cap using an inductor based gyrator. So one could conceivably make a tank circuit using this gyrator in place of an inductor. But as far as practicality goes, I'm not sure how good of a substitute it would make, as the gyrator doesn't exhibit all of the properties of an inductor, and some active components can be quite sensitive to voltage, and extreme voltages are possible in a tank circuit configuration due to opposing phase shifts from the components. And, just to be pedantic, it would probably be called an RCQ tank circuit.