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Gm C filter transfer function urgent help please

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cleo claudiu

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can someone help me with the transfer function of the filter below? image112.gif
 

LvW

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Hi cleo,

here is the transfer function of your circuit: H(s)=N(s)/D(s)

N(s)=(+ H3 H1)

D(s)=(+ H4 H3) + ( - H2 C2) s + (C2 C1) s^2

The negative sign in the denumerator indicates that something is wrong since all expressions in D(s) must be positive.
A visual inspection of the circuit shows that you have positive dc feedback (see the second unit) thereby inhibiting linear operation.
Please check the circuitry.
 
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LvW

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Correction of the circuit: By changing the output polarity of the 2nd unit the minus sign will disappear.
Then, you will have a second order lowpass function.
 

cleo claudiu

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Correction of the circuit: By changing the output polarity of the 2nd unit the minus sign will disappear.
Then, you will have a second order lowpass function.


thanks verry much, it's very useful, i figured it out about how can i reach that result...and yes, u were right, that polarity is wrong....thanks again
 

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The second unit with 100% feedback acts as a grounded resistor (R=1/g2=1/H2) in parallel with X=1/sC1
With this in mind you can apply the classical rules for OTAs with Vout/Vin=g*Xload.
The whole circuitry is the OTA representation of the classical opamp lowpass configuration consisting of a damped and an undamped integrating device connected in a closed loop.
 
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