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S21 parameter provides different gain than the one provided by AC analysis

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Marios91

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Hi fellow designers,

I know this has been asked before but I was not able to understand the actual difference.

I have designed the following single tuned amplifeir that should provide a 20 dB gain at 30 MHz to a 50 ohm load with a 1 MHz bandwidth.
Capture.JPG

Doing AC analysis proves that the amplifier complies with the specifications. However if I measure the S21 parameter , I get a different value that the AC analysis measured gain. I know that S21 provides power gain, but should the power gain in dB be the same as the voltage gain in dB?

Capture2.JPG

I know that I have not understand correctly what S21 repsesents, so I hope you can help me.

Thank you in advance :)
 

pancho_hideboo said:
Show me your definition of gain in AC analysis.

See https://www.edaboard.com/showthread.php?385110#10
Click to expand...

Alright, so AC analysis gives you the voltage gain, therefore : A_ac_analysis= 20*log(Vout/Vin). However S21 gives you the root of the ratio of the power delivered to the load and the available power form the source ,thus S21=20*log(2*Vout/Vsource) (assuming that both source and load are 50 ohms). Since Vsource will be almost equal to Vin then S21 will be 6 dB more than the voltage gain. Is it what I am saying is right?
 

It looks like you have a 6dB difference, perhaps from Vin measured at the wrong node. I would test both parameters with a simple through connection.
 

But don't forget that a requirement for S-parameters is to have a defined source and load impedance. If you do it right, S21 and voltage gain must agree.
 

If you do it right, S21 and voltage gain must agree.
Click to expand...
That's only the case if the source specification refers to the loaded voltage. But in the test setup, the source voltage is the unloaded (emf) value, thus you get 6 dB difference. If you look sharp, there's also a slighted shifted maximum frequency between both curves. I didn't further investigate, but this might happen with a frequency dependent input amplifier impedance.
 

pancho_hideboo said:
S21 can be equal to VGain, only when Zin(freq)=Rsource=Rload.
Click to expand...

edit: Initially I disagreed, but I think I get your point now. S21 is the gain of the incident wave, but if we have reflection at the DUT that changes the total voltage at the DUT input. Is that what you mean?
 
Last edited:

ADS Help has an clear explanation.
voltage_gain.PNG

If we assume that Gamma(S) and Gamma(L)=0 ( that's the case ) Voltage_Gain=S21(dB)-6dB due to half of S21 in Magnitude.
 
pancho_hideboo said:
Right as far as Rin>>Rsource=Rload.
Click to expand...

Yes, this is true in my case . I had falsely understood that S21 provides the power gain taking as the overall power provided by the source as an input (ie Pin=Eg^2/Rsource) but now it's all clear to me . Thank you for your answers ?
 

volker@muehlhaus said:
S21 is the gain of the incident wave,
but if we have reflection at the DUT that changes the total voltage at the DUT input.
Is that what you mean?
Click to expand...
No.

From definition of S21, denominator of S21 is constant regardless of Zin.
You have to interpret a changing of Vout not Vin.

Marios91 said:
the overall power provided by the source as an input (ie Pin=Eg^2/Rsource)
Click to expand...
I can not understand what you want to mean at all.
What do you want to mean by Pin ?
Such power does not exist for source.
 

pancho_hideboo said:
No.

From definition of S21, denominator of S21 is constant regardless of Zin.
You have to interpret a changing of Vout not Vin.

I can not understand what you want to mean at all.
What do you want to mean by Pin ?
Such power does not exist for source.
Click to expand...
With Pin I mean the maximum power that can enter the 2 port network , in this case my amplifier.
 

Marios91 said:
With Pin I mean the maximum power that can enter the 2 port network , in this case my amplifier.
Click to expand...
No.
Maximum power is Pa=(Eg/2)^2/Rsource.
This is achieved at Zin=Rsource.

Source can not deliver Eg^2/Rsource to any Zin value.
Delivered power is zero, if Zin=0 or Zin=infinity.
 
Last edited:
BigBoss said:
ADS Help has an clear explanation.
View attachment 158790
If we assume that Gamma(S) and Gamma(L)=0 ( that's the case ) Voltage_Gain=S21(dB)-6dB due to half of S21 in Magnitude.
Click to expand...
Completely wrong.
Can you understand why Γin is not included ?

When Rsource=Rload, S21=Vout/(Eg/2).
Voltage gain is defined as Vout/Eg in ADS’s function.

Definition of this voltage gain is different from definition of VGain in this thread.
Voltage gain is defined as Vout/Vin in this thread.

Still there are many people who can not understand S21 correctly in this forum.
Surely see https://www.edaboard.com/showthread.php?385110#10
 
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