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Old 16th June 2003, 04:27 AM   #1
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Default input impedance of active filters.q

i was looking around at the demo of "filter solutions" and noticed each circut type has a "Zin" which has a phase, magnitude, and group-delay. so how does this affect the phase, magnitude, and group-delay of the filter?
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Old 25th June 2003, 11:07 PM   #2
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Active filters are usually Sallen & Key designs. They work by applying positive feedback, and their input impedance is often a lot lower than casual inspection would suggest. They need to be fed from a low impedance source. In op-amp terms, that means a buffer, in transistor terms, an emitter follower, and valves need a cathode follower. Buffers should not be shared.

Failure to buffer properly is likely to cause frequency response errors and distortion in the preceding stage. That's the effect of their input impedance...
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