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26th June 2019, 10:03 #1
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Power Gain and Voltage Gain of LNA
Hi all,
I simulated a source degenerated LNA in ADS and I got S21=7(not in dB for now). Now as far as my knowledge S21 is giving the power gain. If I try to extract voltage gain from PG:
Ri=50ohm, Ro=1.3Kohm(the calculated impedance of the LC tank at fc)
Does it mean that the LNA multiplies the voltage at its input by 13.5?
I ran a transient simulation and I got a different number. Why!?
and what about S21 in dB? dB(S(2,1)) in ADS gives 20log(S(2,1))! shouldn't it be 10log since its about power?

26th June 2019, 10:43 #2
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Re: Power Gain and Voltage Gain of LNA
Your equation is wrong.
See the followings.
https://designersguide.org/forum/Ya...1266168115/1#1
https://www.edaboard.com/showthread.php?343941
Yes.Last edited by pancho_hideboo; 26th June 2019 at 10:52.

26th June 2019, 10:44 #3
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Re: Power Gain and Voltage Gain of LNA
Sparameters are voltage ratios, not power, so it is 20log ()
I would expect an LNA to operate at 50 Ohm load, so why do you look at gain into 1.3kOhm load?

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26th June 2019, 11:47 #4
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Re: Power Gain and Voltage Gain of LNA
No.
Sparameters are ratios of sqrt(power).
https://www.edaboard.com/showthread....powerwaves#17
https://www.edaboard.com/showthread....=Power+wave#26Last edited by pancho_hideboo; 26th June 2019 at 11:52.

26th June 2019, 12:01 #5
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Re: Power Gain and Voltage Gain of LNA
Sweetheart, I knew you would start that discussion again, again, because you love to fill threads with self quotes. Have a great life!

26th June 2019, 13:42 #6
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Re: Power Gain and Voltage Gain of LNA
This thread is very easy thing, since reference impedances are real number.
However S21 is a ratio of sqrt(power) not voltage.
Many people who can not understand S21 correctly report same thing as this thread.
This discussion has been completely concluded like following.
"Power Wave Formulation" is described even in a major standard text book.
Generalized Sparameters are defined by "Power Wave Formulation" in almost all commercial RF Circuit Simulators.
They give S11=0 at conjugate matched condition.
So there is no need to further discuss at all.Last edited by pancho_hideboo; 26th June 2019 at 13:50.
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26th June 2019, 16:32 #7
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Re: Power Gain and Voltage Gain of LNA
Thank you all for your replies "pancho_hideboo" and "volker@muehlhaus"
I've read these topics and many other things on the issue but got more confused and because of time constraints, I can't really go deep into it! So let me say what exactly I want to do and know:
I have an LNA with LC load resonating at 2.4GHz, The inductor gives 1.3Kohm at fc in order to increase the narrowband gain. The S11 of LNA is good enough(14dB), but since there is no matching between LNA and the mixer, do I really need to care about S21? In "RF MicroelectronicsRazavi" there are design examples without mentioning S21 actually. And now, using which analyze of ADS, I should measure the gain of LNA? for the other stages of the receiver chain, I measured "Vo/Vin" as the gain. The type of gain in LNA should be the same as the other blocks to design AGC.

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26th June 2019, 16:45 #8
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Re: Power Gain and Voltage Gain of LNA
There is no time constraint.
You have much time, since you are student.
Simply don’t use Sparameters, if you don’t have much correct RF knowledge.
Instead of SP analysis, use AC analysis.
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26th June 2019, 16:48 #9
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Re: Power Gain and Voltage Gain of LNA
To calculate the gain with source or load impedances different from 50 ohm, you need to take all four S parameters into account. S21 is the gain with 50 ohm source and load impedance (can be expressed as power or voltage gain as you like).
However, as you are apparently designing the circuit yourself rather than applying an oftheshelf LNA, why don't you simply determine the gain with actual load impedance in the simulation. You don't need to take the detour over S parameter.
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26th June 2019, 17:02 #10
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Re: Power Gain and Voltage Gain of LNA
It is true for actual measurement using actual Vector Network Analyzer.
However we can evaluate S21 in simulation even if Rsource is not equal to Rload.
S21=sqrt(Pload/Pa), Pa is Available Power of Signal.
He can not understand that Rout=1.3kohm is not required for S21.
Rsource, Rin, Rout, Rload.
If you evaluate relation between S21 and VGain, Rsource, Rin, Rload are required.
VGain=V2/V1
Pa=E^2/(4*Rsource), Pload=V2^2/Rload
V1=V2/VGain=Rin/(Rsource+Rin)*E
V2/E=Rin/(Rsource+Rin)*VGain
S21=sqrt(Pload/Pa)=2*sqrt(Rsource/Rload)*(V2/E)=2*sqrt(Rsource/Rload)*Rin/(Rsource+Rin)*VGain
If Rsource=Rload=Rin, S21=VGain.
I think so.
Use AC Analysis.Last edited by pancho_hideboo; 26th June 2019 at 17:19.
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27th June 2019, 08:18 #11
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Re: Power Gain and Voltage Gain of LNA
Yes I'm designing the LNA and other blocks myself, that's what I should do. now I realized that I only need Sparameters for S11 of LNA. The confusion was because S21 is expressed as gain in everywhere, then when I simulated in AC analyze gain was something else! So I got stuck between microwave stuff and RFelectronics. And the 1.3Kohm impedance at the output for S21, came from the idea that the output should be high in order to get gain(Lack of microwave information)!
This was the straight forward answer. I calculated the voltage gain from this derivation, campared to the AC analyze, both equal with every Rload.
Thank you "pancho_hideboo" and "FVM" :)

27th June 2019, 09:02 #12

27th June 2019, 09:29 #13
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27th June 2019, 09:45 #14
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Re: Power Gain and Voltage Gain of LNA
RF beginners should not use Sparameters.
See the followings.
https://www.edaboard.com/showthread.php?384906#11
https://designersguide.org/forum/Ya...1237656418/3#3

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27th June 2019, 15:13 #15
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