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Output impedance of a transistor from load pull characteristics

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nsarimin

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Dear all,

I am a student. If anyone can help me to answer this question, I would greatly appreciate it.

I am designing an output matching network. Is there a way to determine the output impedance of the transistor from its load pull characteristics? As far as I know, we can extract optimum load impedance from load pull simulation. But how to determine the output impedance of the transistor itself?

Thanks
 

Since you can extract the Optimum Load Impedance of the amplifier, you don't need the Output Impedance anymore.
 

Thanks for your reply bigboss.

Actually let me rephrase it, I am trying to model the transistor impedance as source impedance as the equivalent of parallel R and C. Since I know the value of optimal load impedance, is the source impedance just the conjugate of it?

Thank you
 

Yes, the source impedance determined in load pull is the conjugate of optimal load impedance. If it can be assigned to transistor output impedance depends on the circuit.

In any case it's a large signal quantity, valid only for a specific output level.
 

FvM said:
Yes, the source impedance determined in load pull is the conjugate of optimal load impedance.
Click to expand...
This would imply that an optimal match is always an effective (large signal) conjugate match. If "optimal" is defined as anything but maximum small signal gain, then this won't be true in general.
 

nsarimin said:
Thanks for your reply bigboss.

Actually let me rephrase it, I am trying to model the transistor impedance as source impedance as the equivalent of parallel R and C. Since I know the value of optimal load impedance, is the source impedance just the conjugate of it?

Thank you
Click to expand...

According to Maximum Power Theorem, the Load Impedance should be complex conjugate of the Source Impedance and it must be fixed so the theorem is valid under ideal small signal conditions so that the Output Impedance is fixed and does not change by signal amplitude or something.

But Optimum Load impedance as you know well Large Signal metric and it "might be" quite different than complex conjugate of the Output Impedance due to large signal swings do not allow to make it fixed.So the theorem looses its validity under this circumstance.
 

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