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negative impedance for conjugate match?

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db12

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hi.

i use a certain transistor for lna design...after i've stabilized it i wanted to check sm_Z1 in ADS to look at the needed input impedance for maximum gain....but ADS shows me it has to be negaive!!! -1+j... what is going on there???
 

Hi,

For the conjugate matching, just leave along the real part of your input impedance and conjugate the imaginary part.

-1+j is apparently the input impedance of a active component which is not practical when considering matching.

Conjugate mathcing can only ensure maximum power delivery but not the minimum reflection.

In short, for your case if you are going to conjugate match a A+jB impedance, just build a A-jB network.

Added after 2 minutes:

I don't konw if it is correct to say that you are designing a LNA or oscillator.

These are the fileds that we usually talk about conjugate matching when designing the input or output matching network.
 

The negative impedance value indicates that you have a potential oscillator.

Perhaps when you attempted to stabilise the transistor your frequency resolution was too small. Try a greater number of frequency sample points to test.
 

yes, that what i thik too...the thing is, the real part of SmZ1 is actually postive potted on a linear scale (see picture attached)...it just looks weird when i look at it potted in the smith chart...or is there something i don't get?...
 

On a smith chart, a negative resistance port will show up OUTSIDE of the unity circle. So you will have to scale out the smith chart to even see it.
 

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