How Do You Match an LNA for Optimum Noise Figure?

 

Last month’s newsletter noted that that we provide de-embedded s-parameters for Guerrilla RF’s portfolio of ultra-low noise figure (NF) devices. These s-parameters enable accurate simulation of gain, return loss and NF. This month, we’ll see how de-embedded s-parameters inform the design of your input matching network to achieve the optimal NF for your design.
 
Our de-embedded s-parameter files with noise data for the GRF207X/8X family of ultra-low noise amplifiers allow the simulation of noise circles. Noise circles map the degradation of NF as the input match moves away from the optimum. Take a look at the noise circles below.

 

 

The NF of an LNA is at its absolute lowest value (NFmin) when the LNA input is terminated with Gamma Opt (Γopt ). This source impedance is depicted in the graphic above as point m1.

Note that the NFmin associated with m1 is extremely low at 0.18 dB. Furthermore, the noise circles surrounding m1 represent small NF increases of only 0.02 dB. Thus, simply presenting a 50 Ω source impedance (near m3) using a high-Q series cap on the LNA input results in a NF penalty of only around 0.04 to 0.06 dB.
 
Here's the big takeaway: With modern pHEMT LNAs such as GRF2073, matching to Γopt versus matching to 50 Ω yields little benefit. If you were to tune for Γopt, the resistive losses associated with the extra matching components would often outweigh any NF improvement. This is because the NF of an LNA application circuit strongly depends on both the device NF and the resistive losses from the input-matching network.

For these pHEMT devices, minimal high-Q matching often results in a near optimal NF implementation. An additional bonus is that you end up with fewer external components and lower cost. The appropriate matching is often just a high-Q series DC blocking capacitor that is an RF short at the band of interest, and thus it essentially presents 50 Ω to the input of the LNA.

 

Regardless of your design task, the Guerrilla RF applications engineering team is ready to help. Our goal is to make your product successful.

For more information, contact your local Brennan Associates representative.