Microwave Journal

Sorting Circuit Materials For Low-Noise Amplifiers

May 8, 2014

Low-noise amplifiers (LNAs) are essential in many high-frequency receivers, delivering gain where needed while keeping noise levels to a minimum. Designing an effective LNA circuit often comes down to a critical choice of active device, such as a transistor or integrated circuit (IC). But selecting the right printed-circuit-board (PCB) material can also have a lot to do with achieving LNA performance goals, since circuit laminates can contribute a great deal to final amplifier noise-figure performance. It can help to know what to look for when selecting PCB materials for RF/microwave LNAs.

In general, consistency is important for any material intended for an LNA circuit, whether the consistency is in terms of dielectric constant, in dielectric thickness, or in conductor thickness. Tight impedance matching must be achieved with the circuits and devices within an LNA circuit to minimize noise. Excessive variations, for example, in the dielectric constant or substrate thickness of a PCB can result in inconsistent passive circuit elements and difficult impedance matching for low-noise transistors and ICs, yielding variations in noise figure with frequency.

What do LNA designers look for when selecting PCB materials? Creating a successful LNA circuit design involves a number of different factors, including the use of a PCB material with minimal conductor losses and dielectric losses, a circuit material that enables the fabrication of the passive circuit elements required to achieve the close impedance matching needed between the circuitry and active devices for excellent low-noise circuit conditions. Quite simply, circuit impedance mismatches will raise the noise figure of an LNA, so noise-figure performance can be optimized by minimizing those mismatches to the active devices. Circuitry that is not optimally impedance matched for a particular active device can also lead to unstable operating behavior, since low-noise active devices can oscillate at frequencies beyond their fundamental-frequency ranges when improperly matched.

Low-loss dielectric materials are usually the foundation for RF/microwave LNAs, to minimize transmission losses to and from the LNA’s active circuitry. In particular, it is critical to minimize transmission-line losses leading to the LNA input port, since such losses add directly to the amplifier’s noise figure.

One of the more popular circuit “starting points” for LNA designers is RO4350B™ circuit laminate from Rogers Corp. (www.rogerscorp.com), which is suitable for LNAs through millimeter-wave frequencies. This material exhibits the many characteristics that appeal to LNA designers, including tightly controlled dielectric constant, low dissipation factor, and high thermal conductivity. The RO4350B material is characterized by a dielectric constant of 3.48 at 10 GHz in the z-axis of the material, with dielectric-constant consistency across the board maintained within ±0.05. For an LNA designer, this characteristic helps translate into an LNA noise figure that remains consistent with frequency, input level, temperature, and other operating conditions.

The RO4350B material also exhibits low conductor and dielectric losses, both essential to achieving low LNA noise figures. In PCB materials, loss performance can usually be compared by means of dissipation factor, and in the RO4350B material, this parameter is typically 0.0037 in the z-axis at 10 GHz, dropping to 0.0031 in the z-axis at 2.5 GHz.

This material has many other characteristics that make it attractive to RF/microwave LNA designers and is even available in a version with low-profile copper, as RO4350B LoPro™ materials, for reduced conductor losses when searching for extremely low LNA noise figures. Both RO4350B and RO4350B LoPro circuit materials feature low dielectric loss and both circuit materials are formulated to allow circuit fabrication with standard FR-4 epoxy/glass circuit-material processing approaches. These standard circuit processing approaches include fabrication of reliable plated through holes (PTHs) for an LNA without special preparation steps as typically required with PTFE-based circuit materials.

In line with the FR-4 processing methods, the RO4350B and RO4350B LoPro circuit materials are often combined with low-cost FR-4 as a hybrid multilayer circuit. The hybrid PCB allows the use of the RO4000® circuit materials for the critical electrical performance layers and the FR-4 layers can be electrical layers which are less critical. The different circuit materials are often combined by means of a low-loss bondply material such as RO4450F™ prepreg from Rogers Corp. to preserve the low-loss conditions favorable for excellent LNA low-noise performance. The RO4450F prepreg features a dielectric constant of 3.52 in the z-axis at 10 GHz, with low dissipation factor of 0.004 in the z-axis at 10 GHz and outstanding dimensional stability to preserve the mechanical integrity of the joined circuit materials in an LNA design.

LNA designers in search of the optimum noise-figure performance from their devices will often seek circuit materials with stable dielectric constant and mechanical integrity and the lowest possible dissipation factor to minimize noise-figure contributions from the circuit-board material. One of the circuit laminates that serves as a building block for LNAs with the lowest possible noise figures is the RO3035™ circuit laminates from Rogers Corp. With almost negligible dissipation factor of 0.0017 in the z-axis at 10 GHz, these circuit materials contribute very little in terms of dielectric losses to LNA circuits, maintaining a dielectric constant that remains within ±0.05 of 3.50 in the z-axis at 10 GHz. These ceramic-filled PTFE composite materials are formulated with an in-plane expansion coefficient that is closely matched to that of copper, for outstanding mechanical stability in temperature-sensitive circuits such as LNAs. They exhibit good thermal conductivity of 0.50 W/m/K in support of stable, LNA circuitry through millimeter-wave frequencies.

These different circuit materials provide the stable performance characteristics with the low dielectric and conductor losses needed to achieve the low noise figures with frequency for LNAs based on different active devices. Some general circuit practices can help reach those lower LNA noise figures, such as using high-Q rather than low-Q capacitors in matching networks, and using thinner rather than thicker circuit materials when possible. But, in general, the choice of a circuit laminate with stable dielectric constant across frequency and low dissipation factor such as the several circuit materials noted above can only help in the quest for the lowest possible LNA noise figures at RF/microwave frequencies. 

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.