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Why do I get a negative Re(Zin) while performing SP in ADS?

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Please refer to the pix attached for the simulated circuit.

the real part of Zin I got is -52.450@1GHz, -22.039@2GHz.

I'm wondering why does this happen.

Best Regards.
 

Re: Why do I get a negative Re(Zin) while performing SP in A

Perhaps the ckt is not unconditionally stable.
 

Re: Why do I get a negative Re(Zin) while performing SP in A

Why does that happen? So that microwave engineers can get paid! It is a good thing.
 

Re: Why do I get a negative Re(Zin) while performing SP in A

Hi,
I think you have biasing problem.
check your dc bias poins.
your transistors aren't in active region.

Regards
 

Re: Why do I get a negative Re(Zin) while performing SP in A

Hi,

I surely have checked the bias conditions, they're definitely in the active regions.

I apprecitate if you have a shot at this circuit with SP simulation.

Thank you.
 

Re: Why do I get a negative Re(Zin) while performing SP in A

I agree with g579: it's the stability problem.

Rgs.

Ruri
 

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