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Design >> Analog Design >> Second-order low-pass Chebyshev filter
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Message started by Shlapenka on Nov 21^st, 2009, 4:29am

Title: Second-order low-pass Chebyshev filter
Post by Shlapenka on Nov 21^st, 2009, 4:29am

Greetings everyone :)

So where shoul i start... I am making a course project at my univercity and im having problems with it. So here is the deal:
I've got a scheme asembled (chebyshev filter) in Orcad like this one:

Transistor models are: http://www.mosis.com/cgi-bin/cgiwrap/umosis/swp/params/ibm-035/t75w_5hpe_4lm-params.txt

As we can see Vt0 for P type transostor is -0.4450562V, and for N type 0.5551144 V.

Top M5 and M6 transistors are "load" and must act like resistors. Their L is 35um and W is 0.7um (100/2 ratio).

Middle transistors M3 and M4 are "ladder" and their L is 0.35um, W is 7um (20/1 ratio).

Lowest transistors M1 and M2 are "in" transistors and their L is 0.7um and W is 7um (20/2 ratio).

This is made according to the datasheet i got, iput it in atached archive as Filtrai.pdf, there is also a orcad project. Inside is a charakteristika.png picture, this is what i get in output (AC sweep, from 10Hz to 10GHz, 20pointds decade). So when i make c1 and c2 31.5pF and 30pF, i dont get the peak as i get in charakteristika.png...
Why is that? How can i meke it apear? And in pdf page 422 fig. 5 i need to get currents ~80uA but as u can see i get only in nA, how can i achieve uA?

Thank you so much for your help!

Title: Re: Second-order low-pass Chebyshev filter
Post by buddypoor on Nov 21^st, 2009, 6:49am

Hi, Shlapenka !

I didn´t check the active part of the circuit - however, certainly the whole circuit will not behave like a Tschebysheff filter.
You have two simple RC-elements which are isolated from each other by an amplifier. Thus. the transfer function will have two REAL poles. As you probably know, Tschebysheff reponse requires a complex pole pair instead.

Title: Re: Second-order low-pass Chebyshev filter
Post by aaron_do on Nov 23^rd, 2009, 5:23am

Hi,

it appears that you have forgotten to "GROUND" the node at the bottom. There is no path to ground for your current to go which is why you only have nA.

Hi Buddypoor,

it does seem to be able to provide complex poles. See the pdf. in her attachment.

cheers,
Aaron

Title: Re: Second-order low-pass Chebyshev filter
Post by buddypoor on Nov 23^rd, 2009, 7:37am

Hello Aaron,

thank you for pointing my attention to the attachment.
As I have mentioned before, I didn´t check the active part in detail -assuming it would be any kind of amplifier. As I have learned from the attachement, the transistor arrangement instead works as a NIC (negative impedance converter) - and in this case the whole circuit, indeed, can produce a complex pole pair which is the basis for a Chebyshev response. Thanks.
LvW