AC filters (RL): A series RL filter has the output taken across the inductor. Which type of frequency-selective behavior will this network exhibit?

Introduction / Context:
First-order RL networks are foundational building blocks in analog electronics. Whether the output is taken across the resistor or the inductor changes the filter's behavior. Understanding this helps in predicting Bode plots, time-domain response, and in designing simple tone controls or sensor front ends.

Given Data / Assumptions:

• Series RL circuit driven by an AC source.
• Output is measured across the inductor L.
• Ideal components; no parasitic effects beyond R and L.

Concept / Approach:
The inductor's reactance is XL = 2 * π * f * L, which increases with frequency. In a series path, the voltage division is frequency dependent: V_L / V_in = XL / sqrt(R^2 + XL^2). At low f, XL is small, so most input appears across R (little across L). At high f, XL dominates, so most input appears across L. Therefore, with output taken across L, higher frequencies are passed better than lower ones—this is a high-pass characteristic (first order).

Step-by-Step Solution:

Verification / Alternative check:
Plot the magnitude response: the -3 dB cutoff occurs near f_c = R / (2 * π * L). Frequency sweep measurements will show increasing output with frequency when probing across the inductor.

Why Other Options Are Wrong:

• Passing lower frequencies / blocking higher frequencies: Describe a low-pass response (which would be seen if the output were across R).
• Blocking a certain range: That suggests a band-stop (not first-order RL).

Common Pitfalls:
Mixing up where the output is taken; swapping L and R or moving the probe changes the apparent filter type.

Final Answer:
passing of the higher frequencies