Checking Amplifier Bandwidth Margin in a High Q Active Filter

A high Q active filter can be sensitive to amplifier behavior even when its resonant frequency is well below the amplifier’s headline bandwidth. The circuit needs sufficient gain and suitable phase behavior where its response peaks. Compare the real implementation with the intended transfer function instead of selecting an amplifier from frequency alone.

Identify the topology’s dependence

The Analog Devices AN649 filter design note explains that multiple feedback and integrator based sections depend on amplifier open loop behavior and that high Q, high frequency sections are demanding. Its guidance connects the required amplifier gain margin to the filter’s resonant response.

Use the selected topology’s equations and operating conditions. A rule developed for a unity gain buffer arrangement does not automatically describe a multiple feedback bandpass section.

Compare ideal and practical response

Start with the intended pole frequency, Q, and gain using ideal elements. Then include a suitable model of the chosen amplifier and compare peak gain, center frequency, bandwidth, and phase. Keep component values and loading identical so the amplifier’s contribution is visible.

Inspect the region around resonance closely. A modest change in damping can produce a meaningful change in peak amplitude or ringdown time even when a broad frequency plot makes the two responses look similar.

Separate bandwidth from large signal limits

A satisfactory small signal response does not establish performance at the largest required amplitude. Evaluate output swing, current, and slew rate independently. Include the stage’s internal peaking when determining that amplitude.

Check supply conditions and the load presented by the next stage. If the amplifier’s model omits a relevant behavior, identify that limitation rather than interpreting a simulated response as a guaranteed bound.

Verify the assembled circuit

Measure response with a source level low enough to remain linear, then repeat at the intended operating amplitude. Compare resonance, bandwidth, and transient decay. Keep source impedance and probe loading controlled because they can also shift a narrow response.

Save the ideal target, practical prediction, amplifier identity, supply and load conditions, and measured deviations. If a faster amplifier is substituted, verify stability and the entire response instead of assuming more bandwidth can only help. A useful selection establishes that the amplifier preserves the required response within the application’s tolerance. Merely quoting a gain bandwidth product that exceeds the center frequency leaves the effect of Q, topology, and operating amplitude unresolved. Repeat the check after changing the following stage, because its input impedance may alter the reviewed loading.