The Filter Stopband Problem Nobody Talks About — and the Pair That Fixes It

Every engineer learns about passband and stopband in filter theory. But here’s a question that rarely gets asked in textbooks: what exactly is the input impedance doing in the stopband?

The answer is uncomfortable. In a tee-configuration low-pass or high-pass filter, the stopband input impedance doesn’t stay controlled — it rises without limit. And that unmanaged impedance can make a driving amplifier unstable in ways that are hard to diagnose.

The Problem in Practical Terms

John Dunn, an electronics consultant and graduate of the Polytechnic Institute of Brooklyn, breaks it down in an EDN tutorial: in a tee-configuration filter, the input impedance in the passband tends toward the load resistance value. But cross into the stopband and the impedance climbs toward infinity. For many driving sources — especially high-frequency op-amps — this isn’t just a theoretical nuisance. It can cause oscillation.

The issue is particularly acute in active filters where the driving amplifier sees different impedances depending on frequency. If the amplifier expects a relatively constant load and suddenly sees an impedance that’s climbing with frequency, phase margin erodes.

The Partial Remedy: Filter Pair Architecture

Dunn’s solution is conceptually clean: use both a low-pass and a high-pass filter together, each feeding its respective load. The result is that the input impedance becomes controllable both above and below the corner frequency of whichever filter defines your intended signal path.

What’s lost: the corner frequency impedance null doesn’t disappear. But what’s gained: impedance both above and below that null tends toward the chosen load resistance (e.g., 50Ω), which makes the driving amplifier happier and more stable.

When This Matters Most

This technique is most relevant in precision analog signal processing — instrumentation amplifiers, sensor interfaces, and high-resolution ADC front-ends — where the filter often precedes a sensitive amplifier stage. If you’re designing a front-end for a 24-bit ADC, that impedance behavior in the stopband isn’t just inefficient; it can fold noise back into your passband.

The tutorial includes frequency response plots showing the controlled vs. uncontrolled impedance cases, making the difference immediately visible.

The Practical Takeaway

Filter impedance control is one of those topics that feels like textbook theory until you encounter it in the lab and your amplifier is oscillating for no obvious reason. Dunn’s EDN piece is a practical introduction that connects the dots between filter topology and amplifier stability — and offers a concrete architectural fix.