Is it right to understand the Zopt（optimized noise impedance）of ind degenarated lna

Is it right to understand the Zopt（optimized noise impedance）of ind degenarated lna

Is it right to understand the Zopt（optimized noise impedance）of ind degenarated lna

Today i took part in an interview.The interviewer asked me to saysomething about lna.At first,I wrote the power matching equations of the inductively source degenerated lna.Then I wanted to say something about the optimized noise impedance Zopt.But I was not sure if what I had said was right.the following is what I said:

At first,we determine the NFmin by the spec nf of lna.The NFmin is only related to the Cgs and Cex of main mosfet of lna.It has no relation with the value of Ls(source inductor) or Lg(gate inductor).so the W/L of main mosfet of lna and the Cex are determined.And the Zopt(optimized noise impedance ) is also only related to the Cgs and Cex of main mosfet,and
has no relation with the value of Ls(source inductor) or Lg(gate inductor).Then we determin the Ls by the 50ohm matching equation gm.Ls/Cgs=50ohm,and the Lg by the imaginary part canceling of the input impedance so as to get a pure real impedance of 50ohm.

Is what i said about the Zopt right?

you brilliant ,please help me.

Re: Is it right to understand the Zopt（optimized noise impedance）of ind degenarated l

Why we use Ls ?? To bring the Zin to Zopt..
Zopt is a intrinsic parameter and is not related to external component.Those components are used to match these two impedances.

Re: Is it right to understand the Zopt（optimized noise impedance）of ind degenarated l

BigBoss said:

Why we use Ls ?? To bring the Zin to Zopt..
Zopt is a intrinsic parameter and is not related to external component.Those components are used to match these two impedances.

Click to expand...

thanks a lot ,Sir.

I got some equations of Zopt from the paper

**broken link removed**

The value of real part of the Zopt is the first equation. A and B1 are demenstrated by the last 2 equations. Cgs is the gate cource capacitance of the main mosfet of lna, Cex

is the externally added capacitor paralel to the gate and source of the main mosfet.α、γ、δ 、c are the process paramter ,they are intrinsic and unchangable.
If the is no Cex, then k =1,and the Re(Zopt) will be much larger than 50 ohm.So we increase the Cex.when the Cex increase,then the k increase , and so the B1 increase.Then the

Re(Zopt) decrease,
.when it decreased to 50ohm,the noise imepadence is matched.And then we justify the Ls to match input impedance of the lna to 50 ohm.If there is no Cex,we cannot get the power

matching and noise matching simultaniously.
In this method,we use Cex to justfy the real part of the optimized noise impedance to 50 ohm?

Re: Is it right to understand the Zopt（optimized noise impedance）of ind degenarated l

Attachment is not opened..
I repeat here again: External/Extrinsic components such as Cex ( as you said ) does not contribute into Zopt.Your target is to match Zopt=Zin.That's it.
Essentially, Power Matching ( for max. power transfer) and Noise Matching cannot be maintained simultaneously in practice so that Impedances may be very far away from each other.
You will decide at that point whether you need a Low Noise or High Gain..