[sdiy] Transistor Sallen-Key lowpass filter question

Tom Wiltshire tom at electricdruid.net
Mon Nov 19 00:59:37 CET 2018


Can anyone tell me why a transistor-based Sallen-Key filter like this one would have such a heavy roll-off at the bass end?

	https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterSchematic.png

According to the sim (LTSpice), the response looks like this:

	https://electricdruid.net/wp-content/uploads/2018/11/DelayFilter.png

There’s a significant drop with a -3dB cutoff of about 200Hz. What’s causing that?

Using the same passive components, but with a op-amp in place of the tranny, with the tools at..

	http://sim.okawa-denshi.jp/en/Sallen3tool.php

..doesn’t give the same effect. Am I seeing a simulation effect or a real phenomena?

Thanks,
Tom






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