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Simulators >> Circuit Simulators >> ideal transformer impedance tranformation character varies with frequency
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Message started by dog1 on Aug 7^th, 2014, 6:34am

Title: ideal transformer impedance tranformation character varies with frequency
Post by dog1 on Aug 7^th, 2014, 6:34am

Hi:

I added an ideal transformer in cadence, add a resistor to one end, and is expecting to see the impedance transformation character at the other .

But the resistance I saw on the other end varies with frequency. Why is that? Shouldn't it be the same because I have an ideal transformer with k=1?

When k = 1, I should have
Vo/Vin=N
Io/Iin=1/N

And because Vo/Io=R
Vin/Iin=R/N^2 right?

Thanks!

Title: Re: ideal transformer impedance tranformation character varies with frequency
Post by aaron_do on Aug 7^th, 2014, 6:57pm

Hi,

you can model the transformer like this:

http://en.wikipedia.org/wiki/File:TREQCCTHeyland-to-k.jpg

i.e. you still have an inductance in your model which appears in parallel with the resistance. Since k = 1, you could look at re(Y11) and it should be frequency independent.

cheers,
Aaron

Title: Re: ideal transformer impedance tranformation character varies with frequency
Post by nrk1 on Aug 7^th, 2014, 7:16pm

For a mutual inductor to behave like an ideal transformer, you need k=1, and w*L2 >> R (where R is connected to the side with inductance L2). When w approaches R/L2, the impedance is no longer frequency independent. Some info is at the link below.
http://www.ee.iitm.ac.in/~nagendra/EC1010/201301/lectures/20130402/20130402.html

Title: Re: ideal transformer impedance tranformation character varies with frequency
Post by dog1 on Aug 11^th, 2014, 3:50am

yes, I get it, thanks for both of your help

Title: Re: ideal transformer impedance tranformation character varies with frequency
Post by Ken Kundert on Aug 11^th, 2014, 11:48am

What you have modeled is a 'closely-coupled inductor', which, as you have seen, is not an ideal transformer. If you want an ideal transformer, use the ideal transformer. It is in analogLib.

-Ken