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# Calculating Resistance

11/13/2006 1:30 AM

In an inverting amp circuit with a resistor of 51 kilo-ohms and a capacitor of .005 microfarens, how do i calculate the total resistance for feedback?

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Guru

Join Date: Oct 2006
Location: India, 200 Km. North of Delhi.
Posts: 1386
#1

### Re: HELP!! calculating resistance

11/13/2006 2:25 AM

I guess you should call it impedance. "Z"

Z= ( R^2 + Xc^2 )^1/2

Xc = 1/(2*pai*f*C)

f is frequency

C capacitance

Xc capacitive reactance

R resistance

Z is Impedance

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Anonymous Poster
#2

### Re: Calculating Resistance

11/14/2006 1:15 AM

If you are talking about the feedback RC network of and op amp, then the total impedance in the Laplace domain of R || C is R/(RCs +1); So if you where trying to make a low pass filter the cutoff frequency would be fc=1/(2*pi*R*C). Hope this helps.

Guru

Join Date: Sep 2006
Posts: 4513
#3

### Re: Calculating Resistance

11/14/2006 3:51 PM

Would you please elaborate? For example, is the 51k resistor on the inverting input with the 0.005 μF cap as the sole feedback element, ie, are we talking about an integrator? If not, would you please describe the circuit configuration in more detail?

-e

Power-User

Join Date: Sep 2006
Posts: 367
#4

### Re: Calculating Resistance

12/03/2006 10:26 AM

Right. What is the diagram (or "configuration with C').

Also R f (feedback resistance) is a function of desired amplification factor