Very low noise preamp (0.4 dB NF) for 435 MHz / 70 cm

Created: Dec. 2021

[For discrete MOSFET pre-amp, go to "BF988 masthead amp for 435 MHz / 70 cm with phantom power"]

Introduction

The compact (2 x 2 x 0.75mm) 8-pin quad flat non-lead (QFN) packaged microwave monolithic integrated circuit (MMIC) consists of a common-source amplifier and an active bias regulator. Its 0.25 µm feature-size GaAs enhancement-mode, pseudo-morphic high electron mobility transistor (ePHEMT) process has a high gain-product bandwidth, fT >30 GHz, that allows the target gain (>17 dB at 0.9 GHz) to be achieved in one stage.

Material & method

A minimal number of external components (C1-L1 and C2-L2) provided the matching and biasing functions that were not feasible to integrate at the chip level (Fig. 1). In addition to DC blocking and RF choking functions, C1-L1 also roll off undesirable gain below the operating frequency (f0). Both L1 and L2 should be operated below their self-resonant frequency (SRF) for effective choking. C3-C5 decouple RF from the bias lines.

Fig. 1: preamp circuit

Table 1: part list

Fig. 2: PCB layout & component position

Fig. 3: Photo of assembled PCB

Fig. 4: Circuit model of PCB assembly

Results

Fig. 5: Simulated (dashes) and measured (solid lines) IRL, ORL and G vs. frequency

Fig. 6: Simulated and measured noise figure (F) vs. frequency. The midband F is <0.4 dB

Fig. 7: Simulated (dash) and measured (solid) wideband gain S21 and stability factor k vs. frequency. The prototype is unconditionally stable over 0.05-20 GHz.

Fig. 8: Fundamental (Po), 3rd order intermodulation product (Pimd3), and, Third order Intercept Point (OIP3) vs. input power (Pi). The measured OIP3 exceeds 37 dBm at 900 MHz. Although not measured, the 450 MHz OIP3 should be within a couple of dB

The above data are a subset of the previously published 900 MHz version.