RF Front-End · Low-Noise Amplifier

137 MHz LNA

A two-stage low-noise amplifier for weather-satellite downlinks, with a SAW filter between the stages. 32 dB of gain at a 1.89 dB noise figure, and unconditionally stable from 1 MHz to 6 GHz.

Tools

LTspice · scikit-rf · Altium

Board

2-layer FR-4

Gain

32 dB on a VNA

Noise figure

1.89 dB cascaded

Input

137 MHz, 50 Ω

Stage 1

+18.4 dB / 1.77 dB NF

SAW filter

−4.3 dB IL

Stage 2

+18.4 dB

Output

+32 dB total

Schematic of the SMA antenna input with a 10 nF DC blocking capacitor — RF input

Schematic of the first BFP540ESD amplifier stage with its bias divider, emitter degeneration and 680 ohm shunt feedback branch — Stage 1, with the shunt feedback branch

Schematic of the TB1103A SAW filter with a shunt capacitor and series inductor matching network on each side — SAW filter, L-matched on both sides

Schematic of the second amplifier stage, identical in topology and values to the first — Stage 2

Schematic of the bias tee: DC blocking capacitor to the SMA, RF choke and ferrite bead to the supply rail, Schottky reverse-polarity diode — Bias tee, supply up the output coax

Gain at 137 MHz	32 dB measured · 33 dB simulated peak
Cascaded noise figure	1.89 dB
Stability	μ_min = 1.63, 1 MHz to 6 GHz
Device stability, unstabilised	K = 0.03
−1 dB bandwidth	1.16 MHz · covers NOAA-15/18/19 and Meteor-M2
Bias	V_CE = 2.0 V, I_C = 5.0 mA per stage
Board	2-layer FR-4, powered over coax inside the SDR’s 180 mA budget

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