Active low pass filter

[conchunchun]

active non inverting low pass filter

How to design a 1st order low pass non-inverting filter with all the condition given?
Input impedance of the active filter is larger than 10k ohm fc=1khz, DC gain=2.
How to deal with the imput impedance?
How to calculate R and C value of the non-inverting active filter?
How to calculate the input impedance? Thanks

 

[IanP]

non inverting low pass filter

See example on page 6:
http://alpha.science.unitn.it/~bassi/Signal/TInotes/sloa049.pdf

Regards,
IanP

 

[conchunchun]

I also have downloaded n read the link before...But i still dont get it...How do I deal with input impedance to determine my R and C value at non-inverting side? My filter is 1st order.

 

[IanP]

I gues this is your configuration .. see picture below ..
If you don't use buffer use Rf=10kΩ or more .. this will ensure input impedance of ≥10kΩ ..
If you use buffer, place 10kΩ (or more) resistor between Vin and ground ..

Regards,
IanP

 

[crossbow_123]

Hi, You can refer to razavi book for active low pass filter


best regards
crossbow

 

[conchunchun]

To IanP,

The circuit given is correct with no buffer. So the first step I do is assume Rf=10k ohm then calculate the C value by using 1/2pi*R*fc... How do you know when Rf=10k, the value of input impedance will larger than 10k ohm...can show me some calculation.

Secondly, how to determine the input impedance...is it like this: Zin=Xc/(Xc+Rf)

To crossbow
Do u mean this book? Design of Analog CMOS Integrated Circuits..I search through it I do not find any info useful.
If not could you please provide the links...Thanks a lot

 

[IanP]

The input impedance Zin is the sum of RF and connected in parallel (+) input impedance and Zc, and is, of course, frequency dependant ..

Zin ≈ Rf + ( 1/ωC // [+ input] )

At high frequecy (1/ωC=>0) Zin will be very close to 10kΩ, whereas at very low frequency (1/ωC=>∞) Zin = Rf + input impedance of (+) pin ..

Regards,
IanP

 

[conchunchun]

I think the input impedance is Zin ≈ Rf + 1/ωC. Why we need to make 1/wc parallel with [+input]. There is only capacitance and resistance only at diagram.

 

[IanP]

In some circumstances you have to take into account Rin (or Zin) of real opamps ..
http://www.ecircuitcenter.com/Circuits/opmodel1/opmodel1.htm
An ideal opamp has Ri=>∞ ..
But, we are using real opamps, and, for example, µA741's Rin is in the range of 0.3MΩ (min) to 2MΩ (max) ..
So, when you write down formulas, you have to include Rin[+] ..
In the most cases Rin (Zin[+]) is >> (much , much bigger) than surrounding resistors (impedances) and therefore does not have any real meaning, but it is still there ..

Regards,
IanP