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Design >> RF Design >> Help: Relationship between power gain and intrinsic voltage gain of LNA
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Message started by yong_rfic on Aug 14^th, 2012, 1:48pm

Title: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by yong_rfic on Aug 14^th, 2012, 1:48pm

Hi guys,

If the input and output of LNA are both well matched, i.e., input is matched to 50Ohm, output is also matched to 50Ohm load through a source follower. Then the power gain of this LNA, or measured S21 between the input/output ports would be 20dB(Av/2/2), is that right? Av is the voltage gain of LNA without Rs and RL.
And when doing sp simulation in cadence, we can get S21 as well as max. gain, available gain and transducer gain, this makes me confused, I want to know which one can reflect the actual gain most accurately?
Thanks,

Title: Re: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by RFICDUDE on Aug 14^th, 2012, 6:01pm

Quote:

If the input and output of LNA are both well matched, i.e., input is matched to 50Ohm, output is also matched to 50Ohm load through a source follower. Then the power gain of this LNA, or measured S21 between the input/output ports would be 20dB(Av/2/2), is that right?

Yes, if the output and input are well matched to the same impedance (50 ohms) then S21 equals the voltage gain from the input port to the output port.

Quote:

And when doing sp simulation in cadence, we can get S21 as well as max. gain, available gain and transducer gain, this makes me confused, I want to know which one can reflect the actual gain most accurately?

Well, I guess it depends on what you consider to be the "actual gain?"

Power gain definitions are used because the impedances have a significant impact on actual voltages and currents when operating at high frequencies, but the definition of "real" power is a more consistent measure of the input and output signals relative to the real part of the source and load interface to the circuit under test. There are different power gain definitions according to how the circuit is intended to be used or what performance parameters are most important in a given system design.

I would think that "available power gain" is most relevant to actual voltage gain when the real part of the output and input port impedances are the same. Available power gain is defined as the available output power delivered to the load divided by the available power of the source, this implies that the output and input are conjugate matched. Available power gain is still relevant when the real part of the input and output load impedances are different, you just need to account for the impedance transformation (just like a transformer) as part of the voltage gain (output impedance> input impedance) or gain loss (output impedance <input impedance).

Hope this helps.

Title: Re: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by yong_rfic on Aug 14^th, 2012, 6:50pm

Hi RFICDUDE,
Thank you so much for your reply,
Here I attached the schematic to make it more clearly.
the gain of LNA is Av = v2/v1
S21 = vo-/vi+ = v3/v1 = 1/2*Av, is that right? It is a factor of 1/2 instead of 1/4 in my previous post.

Title: Re: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by aaron_do on Aug 15^th, 2012, 10:38pm

Hi,

Quote:

They all have different meanings and uses. For example, Power Gain is not much use if you want to find the stop-band rejection of a low-pass filter. However, if you want to calculate the filter's effect on the efficiency of a PA, Power Gain is more meaningful. If you study the definitions and try a few circuits the meanings should become clear. The concept of available power is very meaningful.

regards,
Aaron

Title: Re: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by RFICDUDE on Aug 16^th, 2012, 4:39am

Yong,

The voltage gain equivalencies you are trying to make depend on whether or not the circuit is loaded when the measurement is made.

For instance, if the buffer is designed properly then v3/v2=1 when the circuit is loaded (otherwise it isn't a good buffer). The gain of the amp is still Av=v2/v1, so the overall gain from the input of the amp to the output is Av=v3/v1=v2/v1. The gain from v0 to v3 does equal Av/2 because the source is delivering power to the input of the amplifier under matched conditions.

Title: Re: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by vivarf on Aug 26^th, 2012, 6:44am

The available gain definition, RFICDUDE provided might not be correct.
In my understanding the available power gain (GA) definition is the available power delivered to the load (in conjugate matching) dividing to the available power from the source (in any matching condition). So the max gain is the available gain when the source is conjugate matching. The transducer gain (GT) the ratio of power delivered to the load dividing to power available from the source. S21 is transducer gain. When both input/output are conjugate matching:
S21=GT=GA=Gmax
In high frequency design, you always use Sparameter to characterize you block. S21 or GT is the real gain of your LNA when you terminate it in a 50 measure system.

Title: Re: Help: Relationship between power gain and intrinsic voltage gain of LNA
Post by aaron_do on Aug 26^th, 2012, 10:30pm

Hi vivarf,

Quote:

S21 or GT is the real gain of your LNA when you terminate it in a 50 measure system.

I think you adding a bit of confusion here. I don't think there is such a term as the "real gain" of the LNA. A signal generator displays the available power, that's why the output power you see on a spectrum analyzer is equal to GT.

BTW, I think you're right about the definition of GA.

Aaron