New-Tech Europe Magazine | Dec 2017

MMIC Amplifiers: Current Limitations below 2 GHz The noise figure and linearity of low noise amplifiers are critical factors in maximizing sensitivity and dynamic range in RF receiver design. The amplifier noise figure determines the weakest signal the amplifier can discern, and the IP3 determines the degree to which intermodulation products from nearby signals interfere with the desired signal. The lower the noise figure and the higher the IP3 of the amplifier at the receiver input, the greater the sensitivity and Spurious Free Dynamic Range (SFDR) of the receiver. Today, the market availability of single-function MMIC LNAs with Radha Setty, Guangyue He and Brandon Kaplan, Mini-Circuits high dynamic range is concentrated around the ISM frequency bands between 2 GHz and 6 GHz. These amplifiers are predominantly PHEMT or HBT designs on GaAs, and both come with inherent advantages and drawbacks. HBT designs are capable of achieving very wide bandwidths and excellent power density with saturated output power up to 1W through about 6 GHz. However HBT designs generally aren’t conducive to low noise, typically achieving about 3 dB noise figure at best, making them unsuitable for receivers where higher sensitivity is required. PHEMT designs are capable of realizing noise figures below 1 dB and IP3 above 40 dB but existing approaches are limited in frequency MMIC Amplifiers Stretch the Boundaries of Dynamic Range in VHF/UHF Communications

coverage to roughly 40 MHz and above. External matching networks and feedback can be applied to extend coverage down to 1 MHz, but this has been known to degrade noise figure performance. While PHEMT designs generally provide outstanding noise figure performance and very high IP3 for popular wireless applications from Wi-Fi (2.4 GHz) through WLAN (5.6 GHz), options for applications below 2 GHz almost invariably sacrifice noise figure performance, leaving the densely utilized HF, VHF and UHF application bands from 3 to 2000 MHz underserved. MMIC amplifiers currently on the market within the HF/VHF/UHF region are limited by noise figure performance no less than 2 dB.

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