Tank-in-series filter behavior: When a parallel L–C tank is placed in series with an output load resistor and driven by a voltage source, what kind of filter response results at the load?

Introduction / Context:
Placing a parallel resonant (tank) circuit in series with a load is a classic way to create a notch or band-stop response. At its resonant frequency, the tank's impedance becomes large, impeding current to the load and thus attenuating the output. Away from resonance, the tank's impedance is lower and the signal passes more freely.

Given Data / Assumptions:

• Ideal parallel L–C tank with finite but high Q.
• Tank is in series between source and load resistor.
• We read the output across the load resistor.

Concept / Approach:
For a parallel L–C at resonance, the inductor and capacitor currents circulate within the tank, making its input impedance very high. Because the tank is in series with the load, this high impedance throttles current, yielding a deep attenuation (notch) at f0. At frequencies well below or above f0, the tank's impedance decreases, so more current flows and the load voltage recovers, which is characteristic of a band-stop filter centered at f0.

Step-by-Step Solution:

Verification / Alternative check:
Frequency sweeps of such a network show a distinct dip at f0 in |V_out/V_in|. Moving the tank to a shunt branch produces complementary band-pass behavior across the load.

Why Other Options Are Wrong:
Low-pass/high-pass: these have monotonic pass/stop behavior, not a notch centered at f0.
Band-pass: would require a topology that maximizes output near f0, not minimizes it (e.g., series RLC with output across R).

Common Pitfalls:
Confusing series vs shunt placement of the tank; overlooking how loading alters Q and notch depth.

Final Answer:
band-stop