# Interpreting NF Circle ang Gain circle for matching network

1. ## Interpreting NF Circle ang Gain circle for matching network

Hello,i have built a circuit shownbellow,from the S-param simulation i got the Gain circle and NF circle
how to know from the circles to what input impedance network should i use and to what output impedance network should i use?
Thanks.

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update:

Why the point we take on the smith chart in the Gamma_s and not the Gamma_L?
Thanks

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2. ## Re: Interpreting NF Circle ang Gain circle for matching network

NFmin is a single point and NF Circles are rounding this point by sacrificing NF value.So the best NF can be obtained at that point only, then first circle covers more points ( impedance values ) by cost of degradation of NF by xdB ( more flexibility but more NF ) so on..
Gain circles are also interpreted as such that way,,
Noise is a function of Source Impedance which is seen by active device so Gamma_S is used.Gamma_L has very little contribution onto this.

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3. ## Re: Interpreting NF Circle ang Gain circle for matching network

for whar kind of circle Gamma_L is used?
Thanks.

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4. ## Re: Interpreting NF Circle ang Gain circle for matching network

Originally Posted by yefj
for whar kind of circle Gamma_L is used?
Thanks.
Gamma_L is used for Power Gain ..

5. ## Re: Interpreting NF Circle ang Gain circle for matching network

Hello, i am trying to produce a gain circles on a simple transistor as shown bellow.

i have defined the range from 1 til 30 dB and i dont get any circle just dots in infinity?

Where did i go wrong?
Thanks.

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6. ## Re: Interpreting NF Circle ang Gain circle for matching network

Is it a joke ?? Everything is short circuited..
Read on simulation techniques much.There are millions app notes and papers about it.

7. ## Re: Interpreting NF Circle ang Gain circle for matching network

Hello , yes i had a little biasing problem where i put Vdc in port 0.8 and for some reason it biased 1.6V on the gate as shown bellow.
i Have succseeded building those circles and i picked a point in between the NF circle an GAIN circle and the GAMMA is GAMMA_S(as you said)
Next i calculate from GAMMA_S yo GAMMA_L extract the impedance using the following matlab code

My question is, what is the best tool you reccomend for building a matching network from 50 to the load impedance and source impedance we calculated?
z=gamma2z(gamma,50)/50;

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I have calculated the Zs and ZL as shown bellow so i need to do a matching network from 50 to Zs and ZL to 50 so i will get minimum S11 and minimum S22?

Updated matlab code:
z_s=gamma2z(gamma_s,50);
s11=0.99875-0.03202*i
s12=721.33*10^(-6)+8.622*10^(-3)*i
s21=-188.37*10^(-3)+30.611*10^(-3)*i
s22=875.51*10^(-3)-100.72*10^(-3)*i
gamma_L=conj((s22+(s12*s21*gamma_s)/(1-s11*gamma_s)))
z_L=gamma2z(gamma_L,50)

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Do i need to build the network to the conjugate value of Zs and ZL
?

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