Cascading Active Filters

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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)

 

[John Popelish]

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

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?

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.

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

 

[Pooh Bear]

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?

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

Graham