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Cascading Active Filters

Hi all, i have 2 active filters with a gain of 21 each..cascaded to give me an overall gain of 441. input into the into the non-inverting, feedbak on the inverting. using an NE5534N amp with a feedback of 560K and 27K.. giving me an overall gain of around 21.

i have stuck in a 4.7nF after the 27K going to ground to give me a low pass filter with a cut off frequency of 1.2KHz

my frequency of interest is 40KHZ.

so im using the frequency response of the amp to give me my low pass..

But on testing the circuit my cut- off frequency is not 1.2KHz any more.

can anybody help me as to why this is?

Nick

(analogue electronics rank -Amateur)

Reply to
akarsh000
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Does this mean you have lifted the ground end of the 27k, and inserted the capacitor in series across this break? Or is the capacitor in parallel with the 27k resistor?

Better if you tell us what you find, rather than what you do not.

Reply to
John Popelish

That's because 27k + 4n7 gives 1.2kHz. How were you calculating it ? It's not a very good filter btw.

Graham

Reply to
Pooh Bear

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