The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> LNA output matching
https://designers-guide.org/forum/YaBB.pl?num=1457102080

Message started by Ata Sarrafi on Mar 4^th, 2016, 6:34am

Title: LNA output matching
Post by Ata Sarrafi on Mar 4^th, 2016, 6:34am

Hello guys,

I have been busy with a problem for couple of days. I will be more than grateful if you help me with this.

I am new in RF design and I am trying to simulate a simple LNA with ADS. My problem is, I do not want to match my LNA's output to 50ohm because, it is in the chip and it will be connected to mixer.

My question is, when I put terminal at the output of LNA what should be the Z of terminal. Without output terminal I am not able to observe noise and some other parameters. I can just plot S11 >:(

best,

Ata

Title: Re: LNA output matching
Post by loose-electron on Mar 4^th, 2016, 10:38am

why don't you figure out what the impedance of the next stage looks like and use it?

#commonsense

Title: Re: LNA output matching
Post by Ata Sarrafi on Mar 4^th, 2016, 10:46am

Yes i tried. The problem is, I want to use mixer after lna, therefore my next stage will be the gate of transistor. I found rf nmos transistor input impedance at 1Ghz and it is like a+bj. But problem is, here a equals 600 ohm which again requires matching. I cant understand why they say inside chip we dont need matching. In high frequencies like 1GHz input impedance drops from 1 meg to 600 !!!! Im totally confused.

Title: Re: LNA output matching
Post by loose-electron on Mar 5^th, 2016, 2:17pm

Impedance matching is more important in a situation where you don't control the other part of the device (like when you design an output to drive 50 ohms)

If you are designing the two sides of the system (one as an output and the other as an input) just determine that the devices are compatible. (efficient transfer of signal of interest) Does that involve impedance matching? Sure. Does that involve reactive elements being used to cancel out other reactive elements at a particular frequency? Sure.

Title: Re: LNA output matching
Post by Ata Sarrafi on Mar 6^th, 2016, 1:02am

I am confised about something. In courses and books, they match input but they say because its inside chip, we dont need output matching ! However when i check output impedance ( input impedance of mixer) in 1GHz, real part is 600 ohm, which effects our design highly !