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IIP3 and 1db compression point(P1dB) of LNA

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sureshnaidu

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From the nonlinearity analysis we know that there is a proportional relationship beetween iip3 and P1dB(1dB compression point).i.e., if IIP3
is high P1dB also high.But when I simuting my LNA for various IIp3 values iam getting same P1dB.IIP3 varying from 12dBm to 19dBm but P1dB remains at -8dBm.what may be the reason.iam using cadence tool and spectre simulator.can any one help? Thanks in advance.
 

bluetooth

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Did you use a bipolar transistor and vary the collector current and nothing else?
 

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P1dB is measured at output and not at input? P1dB - at output measured - should be proportional to P-DC. If this is not the case your mosfet goes into voltage saturation or current saturation and not both simultaneously. This is due to output mismatch.
 

prcken

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i think the formula P1dB=IIP3-9.6dB is derived from Taylor series analysis
maybe in spectre simulation, the IIP3 and P1dB is derived from its transform function accordingly, not pure Taylor series.
 

pavel_adameyko

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From the nonlinearity analysis we know that there is a proportional relationship beetween iip3 and P1dB(1dB compression point).i.e., if IIP3
is high P1dB also high.But when I simuting my LNA for various IIp3 values iam getting same P1dB.IIP3 varying from 12dBm to 19dBm but P1dB remains at -8dBm.what may be the reason.iam using cadence tool and spectre simulator.can any one help? Thanks in advance.
Hi
Could you clarify more the structure of your circuit?
What is the input power of tones in iip3 analysis?
I think the proportional relationship appears only if we assume that coefficients of power series of transfer function don't change with input power level - it's not the case when we measure P1db - dc bias point may change significantly.
So may be the reason of such behaviour is that during iip3 and p1db measurements input power levels seen by circuit differs very much - so there are different sources of nonlinearity.
Regards.
 

sureshnaidu

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Hi
Could you clarify more the structure of your circuit?
What is the input power of tones in iip3 analysis?
I think the proportional relationship appears only if we assume that coefficients of power series of transfer function don't change with input power level - it's not the case when we measure P1db - dc bias point may change significantly.
So may be the reason of such behaviour is that during iip3 and p1db measurements input power levels seen by circuit differs very much - so there are different sources of nonlinearity.
Regards.


iam using a power level of -30dbm.the structure is common source cascode amplifier.thanks and regards.
 

pavel_adameyko

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iam using a power level of -30dbm.the structure is common source cascode amplifier.thanks and regards.
Just for curiosity, with +12 or even +19 IIp3 you get extremely linear LNA. How much is it gain? Have you used special linearization methods?
 

sureshnaidu

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Just for curiosity, with +12 or even +19 IIp3 you get extremely linear LNA. How much is it gain? Have you used special linearization methods?

gain is 10dB.Derivative superposition is used for linearity improvement.input matching is provided through resistor at the gate.
 

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Did you measure P-DC when measuring P1dB? Is P-DC constant or raises your drain current by probably two times or three times when you raise your input RF power? I don´t know know what derivative superposition is. Can you please explain?
Thanks.
 

Ruritania

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I guess the question really is "why derivative superposition (DS) does not help P1dB?"

DS is used to cancel out the 2nd derivative of transconductance, and the P1dB is also caused by this 2nd derivative (generally). So it should help both the IP3 and P1dB, shouldn't it?

Any follow-up?
 

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