RL integrator topology: In an RL integrating circuit (a first-order low-pass using RL), the output voltage is taken across the resistor, not the inductor. True or false?

Difficulty: Easy

Correct Answer: False

Explanation:


Introduction / Context:
Integrator and differentiator realizations with first-order networks depend on where the output is measured. Confusing the output node leads to the wrong frequency response (HP vs LP) and erroneous time-domain behavior.



Given Data / Assumptions:

  • Series RL network driven by a source.
  • Goal: integration-like (low-pass) behavior.
  • Steady-state sinusoidal analysis is valid; time constants define transient response.


Concept / Approach:

An RL low-pass (integrator-like) response is obtained when the output is taken across the resistor: H(jω) = R / (R + jωL). This passes low frequencies (|H| → 1 as ω → 0) and attenuates high frequencies with the 20 dB/dec slope. Taking the output across the inductor yields a high-pass, not an integrator.



Step-by-Step Solution:

Define ZR = R and ZL = jωL in series.For output across R: HLP(jω) = R / (R + jωL) ⇒ low-pass (integrator-like in time domain).For output across L: HHP(jω) = jωL / (R + jωL) ⇒ high-pass (differentiator-like).Therefore, stating that an RL integrator takes output across L is incorrect.


Verification / Alternative check:

Apply a step input: across R (low-pass), voltage rises exponentially toward the step (integration of input slope); across L (high-pass), you see a decaying spike (differentiation of the step).



Why Other Options Are Wrong:

  • Choosing “True” swaps the node definitions: output across L produces a high-pass response, not an integrating low-pass.


Common Pitfalls:

Equating “integrator” strictly with ideal 1/(s) without acknowledging that first-order RL/RC circuits provide approximate integration only when time-constants are chosen appropriately relative to input waveforms.



Final Answer:

False

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