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Design >> RF Design >> Input or Ouput stage Linearity limitation
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Message started by flywill on Jan 5^th, 2008, 5:40pm

Title: Input or Ouput stage Linearity limitation
Post by flywill on Jan 5^th, 2008, 5:40pm

All,

How can you tell for a system itslinearity is limited by the input stage or output stage?
By simulating IIP3/P1dB vs input power, what is the difference can be observed if input stage limit the linearity or output stage?

Thanks.

Title: Re: Input or Ouput stage Linearity limitation
Post by didac on Jan 6^th, 2008, 2:35am

Hi,
Linearity of a System is limited by the last stage, is where you will find the strongest signals:
1/IPtot = 1/IP31 + Gain1/IP32 + Gain1*Gain2/IP33+...
PS:if gain of the previous stage is Loss(filter) then you improve IIP3.
Hope it helps,

Title: Re: Input or Ouput stage Linearity limitation
Post by Eugene on Jan 6^th, 2008, 12:52pm

It's easier if the gain of the stage is programmable. If as you vary the gain, the input referred compression point remains fixed while the output referred compression point varies with gain, the amplifier input limits linearity. On the other hand, if the output referred compression point remains fixed while the input referred compression point varies with gain, then the amplifier output limits linearity.

Title: Re: Input or Ouput stage Linearity limitation
Post by aaron_do on Jan 6^th, 2008, 8:06pm

Hi,

just to add to what didac said, the signals are larger at the output stages so they need higher linearity, but if you filter off the undesired signals (interference), then the linearity requirements of the output stages reduce.

cheers

Title: Re: Input or Ouput stage Linearity limitation
Post by didac on Jan 7^th, 2008, 12:53am

Hi,
Just to add that I agree with both aaron and Eugene, in my previous post no AGC it's taken into consideration(system in open loop).

Title: Re: Input or Ouput stage Linearity limitation
Post by flywill on Jan 10^th, 2008, 6:59pm

Eugene wrote on Jan 6^th, 2008, 12:52pm:

Eugene,

I agree with the oIP3 part.
But for iIP3, if it is fixed with gain variation, it means iIP3 has 1dB/dB with input signal increase, which means it is not linearity limited. Am I missing anything?

Title: Re: Input or Ouput stage Linearity limitation
Post by Eugene on Jan 10^th, 2008, 10:51pm

I'm not sure I understand your question but I will try to describe a graphical explanation of what I was trying to say. Let's consider the output limited case first and try to visualize the relationship between output voltage and input voltage. Output voltage will increase with input voltage until the amplifier saturates, at which point the transfer curve flattens out. If we increase the gain, the slope of the linear part increases. Because the amplifier is output limited, the higher gain transfer curve flattens out at the same output level but the input referred onset of saturation decreases. The input referred saturation point varies with gain when the nonlinearity is at the output.

Now consider the input limited case. We start with the same initial transfer curve but this time when we increase the gain (i.e. the initial slope of the curve), the output saturates at a higher level because the input referred onset of saturation remains the same.

If you try to draw out these curves I think you'll see my point. I tried to attach a couple of drawings. The top picture shows output limiting. The bottom picture shows input limiting. You need to see two different gains to know which one you have. I hope this helps.

Title: Re: Input or Ouput stage Linearity limitation
Post by flywill on Jan 11^th, 2008, 11:36am

Eugene,
Thanks, it is very helpful.
And the graph matches my point. As the 2nd graph pointed out, for input limited case, as gain increase, at lower input voltage output saturate, which means lower iip3.

cheers.