# Filter w/ seemingly higher output voltage than input?

I'm working on a simple LC filter as part of an AM radio project. I was intending to use a ferrite antenna (modeled roughly by the voltage source in series w/ inductor which is then connected to a capacitor that will be parallel to the rest of the circuit), but according to my simulations and calculations the output voltage not only peaks at w0, but peaks higher than then input voltage which seems conceptually impossible to me. Is this right or am I missing something?

• That's entirely possible with resonance. Dec 24, 2020 at 4:00
• Dec 24, 2020 at 4:03
• Keep in mind that a gain in voltage is accompanied by a loss of current in a passive-component circuit (L&C). Dec 24, 2020 at 14:04

## 1 Answer

That is common behavior for a resonant LC circuit.

Near resonance, the input impedance nears zero, because the capacitive reactance is equal and opposite from the inductive reactance ($$\X_C\approx-X_L\$$). Therefore even a small input voltage can produce a large current through the LC series combination. However the voltage across each individual element in the circuit is still $$\V=jIX\$$, so the voltage across each element can be larger than the input voltage.

It may be more accurate to model the source in your antenna as a current source rather than a voltage source, and to include a loading effect due to whatever circuit the antenna is feeding.