Low Noise Amplifiers

What is a Chip and Wire Amplifier?

Advantages of Chip-and-Wire Amplifiers over MMIC Designs for RF Applications

Executive Summary

Selecting the optimal amplifier technology significantly impacts RF system performance across communications, radar, aerospace, and instrumentation applications. This white paper compares traditional chip-and-wire amplifier assemblies with Monolithic Microwave Integrated Circuit (MMIC) designs, highlighting their distinct performance advantages in critical areas such as those found in low noise amps (LNAs), ultra-low phase noise amps, and high linearity amps, supported by real-world examples and quantitative data.

Introduction

The choice of amplifier technology can drastically influence system effectiveness, reliability, and longevity. While MMIC amplifiers are widely adopted for standardized, high-volume applications, chip-and-wire amplifier assemblies offer unmatched customization capabilities and superior performance metrics, especially beneficial in specialized or performance-critical RF systems.

Key Performance Advantages

1. Superior Low Noise Amp (LNA) Performance: Chip-and-wire amplifier assemblies provide engineers the flexibility to select discrete transistors and precisely matched passive components optimized explicitly for minimal noise figures. Through careful hand-selection and meticulous tuning, these assemblies achieve significantly lower noise figures compared to typical MMIC counterparts. This directly translates into enhanced sensitivity and improved system-level performance in radar and more effectively in communication systems.

Example: A radar system utilized chip-and-wire technology, achieving a lower noise figure of 1.2 dB for smaller targeted bandwidth, compared to a MMIC solution at 2.1 dB which was designed for a much broader bandwidth, appealing to a larger audience, sacrificing NF for bandwidth, thereby dramatically enhancing target detection range.

2. Exceptional Ultra-Low Phase Noise Amps: In applications requiring precise signal integrity — such as high-frequency defense related targeting and advanced radar systems—chip-and-wire methodologies more often excel over their MMIC counterparts. Their discrete construction allows detailed optimization of both transistor selection and circuit layout, significantly reducing residual and additive phase noise to the overall RF chain.

Example: In testing, chip-and-wire designs have demonstrated residual phase noise improvements up to 10 dBc/Hz or more (10 kHz offset) compared to comparable MMIC designs.

3. Enhanced High Linearity Amps: Depending on the type of High Linearity performance required, Second Order Harmonic performance might offer distinct advantages over Third Order Two Tone levels, depending on the even order harmonics that require suppression. Ultra-High Linearity Amp designs using chip & wire methodologies frequently incorporate multiple Push Pull and Darlington circuits to cancel even order harmonics, providing out of band harmonic filtering that can both crowd and corrupt the fundamental and thus causing in band spurious from out of band harmonics.

Example: A major defense contractor was upgrading an over the horizon threat detection radar and their system design team needed IP2 performance not normally available with traditional MMIC designs. A standard 16 transistor hybrid chip & wire design met levels of +100 dBm (IP2 Two Tone) allowing them to eliminate additional filters used to remove some even order harmonics.

Customization and Adaptability

Today’s chip-and-wire amplifier circuits incorporate inherent advantages in their extensive customization potential. Unlike MMIC amplifiers, where integration limits optimization, chip-and-wire amplifiers offer adaptability, allowing engineers to fine-tune each parameter precisely to system specifications. This customization ensures maximum compatibility and optimal performance across various RF systems and applications.

Reliability and Thermal Management

Chip-and-wire amplifiers inherently provide superior thermal management capabilities, owing to discrete component to package placement and advanced Surface-mount package engineering. Improved heat dissipation results in enhanced higher MTBF values, especially critical in demanding aerospace and defense applications.

Cost-Benefit Analysis

While MMIC designs may appear on the surface to be economically attractive initially for high-volume standard applications, chip-and-wire amplifiers deliver enhanced performance and reliability over time. Their superior operational lifetime, reduced maintenance requirements, and improved performance metrics often offset initial investment differences, proving highly cost-effective in specialized markets such as defense, aerospace, and critical communication infrastructures.

Conclusion

Chip-and-wire amplifier assemblies can outperform MMIC designs across low noise, ultra-low phase noise, and high linearity metrics. By enabling precise optimization and providing superior reliability, chip-and-wire technology clearly emerges as the superior choice for high-performance RF amplifier applications.

Download the PDF: Advantages of Chip-and-Wire Amplifiers over MMIC Designs for RF Applications

How are Gain and Noise Figure Related?

How are gain and noise figure related for low noise RF amplifiers?

For a low noise RF amplifier, gain and noise figure are inversely related. Based on the parameter of interest, either one can be optimized through circuit design trade-offs. Even though gain and noise figure are distinct and separate parameters, a high gain on the first stage of an amplifier lowers the impact of Noise Figure on all the stages that follow.

How does gain compare to noise figure on a single-stage LNA design?

The noise figure is the ratio of the input SNR to the output SNR. One way to achieve a low noise figure, by driving an amplifier’s transistors to maximum current capacity. Raising or increasing the signal level relative to the amp's own internal noise sources improves the output SNR, thereby improving the overall ratio.  As you increase an amp’s gain, any noise added by the RF amplifier is added to the signal and then, unfortunately, amplified like the rest of the signal. However, if you increase the amp’s gain, then any noise present in the input signal to the amplifier dominates and offsets the amp's own internal noise. The Friis formula or Friis equation describes the fundamental relationship between gain and noise in an LNA, where high gain supporting an LNA on an antenna, for example, essentially masks or hides the noise injected by other much noisier elements downstream. Therefore, the cleanest and lowest noise Figure LNAs are placed as close to the antenna as possible.

Understanding Impedance Matching

Load Termination Requirements

How important is a 50 Ohm termination?

A true 50 ohm termination is critical to the health of a small signal RF amplifier because it ensures a free-flowing signal propagation, maximum power transfer, and prevents damage from output reflections. If a 50 ohm amplifier is placed in an RF chain staring at a mismatched impedance, issues, including amp failure, might occur. The importance of impedance matching a 50 ohm, small signal amp with other components surrounding that amp cannot be overlooked; moreover, it is considered an industry standard of sorts.

• Maximum Power Output: when the input signal and load impedances are matched to 50 ohms, nearly all of the amplifier’s output power is delivered to the next cascaded chain component. In an unmatched or poorly matched system, some of that power is either reflected to the amp, as in the case of the amp looking into a mismatch, or reflected to the source, as in the case of a mismatch looking into the amp’s input.

• Preventing Signal Reflections: impedance mismatches cause signal reflections, this we know. This reflected signal then travels back along the signal path line and interferes with the output signal, creating stationary waves.

• Signal Integrity: stationary or standing waves can appear as current or voltage fluctuations along the signal path. This can manifest itself as distortion, a deterioration in the SNR, and seriously affect or degrade the output.

What are the consequences of a mismatch?

When a small signal RF amplifier operates in a chain with a mismatched impedance, the result of this load mismatch can range from a slight reduction in amp performance, all the way to reflections that can cause inoperable harm to the amplifier.

Reduced Performance

• Output Power Loss: looking at a small signal RF amplifier, for example, the output power, which is reflected from the load the amp is connected to, is merely lost energy. This waste reduces the total power output, which was intended to be delivered to the following component downstream. As the load mismatch gets worse, the intended output power loss increases dramatically.
• Distortion: when standing waves are present, as a result of load mismatch, the interference as a result of these stationary waves then distorts a byproduct, which degrades signal fidelity.
• Gain Loss: the gain of an amplifier in an RF chain can be lowered by load-mismatch losses at each gain block socket.

Can impedance mismatches damage an amplifier?

• Transistor Damage: when examining either an open (infinite impedance) or a short (zero impedance), all of the forward-looking output power is then reflected right back into the amplifier's output pin or output connector. This will cause a sudden and dramatic increase in either voltage or current directed at the transistors in the circuit. Designers, at times, will employ a diode to absorb that reflection, or a large cap to ground to drain it away, but not all those measures are 100% effective.
• MTBF: reflected power eventually has to be dissipated as heat if the mismatch is high enough.        This additional heat can affect the MTBF, shortening the life of the amplifier.

Can I prevent damage to an amplifier which has a poor mismatch?

• Share your Incident Impedance Measurements with the RF Amplifier Supplier: Knowing the mismatch issues before selection can help mitigate issues down the road. Spectrum Control can, at most times, place elements like diodes and pads on the output to help absorb, reflect, or condition those harmful mismatch reflections.
• Add a Pad: for reflection-sensitive amplifiers, a pad, maybe just ¼ dB to ½ dB (if there’s margin in the output power for a pad), can be strategically placed at the amp output or at the next component’s input to help reduce the degree of reflected power.
• Use Proper Load During the Test Phase of the Design: always terminate an amplifier with a load that matches a 50 ohm impedance. In test sessions, this means using a 50 ohm dummy load. In a system-level evaluation, this requirement extends to the entire RF chain, including connectors, cables, and any other element in the chain.

GaAs Transistors

Why GaAs Offer Better Noise Figure Performance than Silicon Transistors

Gallium arsenide (GaAs) transistors offer better noise figure performance than silicon transistors, due to several material influences, which include:

• High electron mobility
• Low intrinsic carrier concentration
• Semi-insulating substrate

Let’s examine these in greater detail.

1. Higher Electron Mobility

Electron mobility is a measure of how fast electrons move through a semiconductor material when excited by way of an electric field. gallium arsenide (GaAs) has a particularly high electron mobility over its silicon competitor (up to 6x greater) due to several distinct advantages. GaAs has a lower electron effective mass, which allows electrons to accelerate at a much higher speed through both the material and the electric field, which means they spend less time exposed to these “noisy regions” and are less likely to be influenced by its side effects. 

The way GaAs atoms are arranged influences mobility as well. Gallium arsenide’s crystal lattice is somewhat open compared to its silicon competitor, which has a denser diamond-packed cubic structure. This open structure allows electrons to move with fewer electron scattering impacts and events. Collisions with imperfections in the structure cause electrons to lose energy, which slows their forward progress, leading to higher thermal noise.

Gallium arsenide is also a direct band gap semiconductor; unlike its Silicon competitor is an indirect bandgap semiconductor. With a direct band gap advantage, electrons enter and then move in the conduction band without a loss of energy, which boosts mobility. Silicon, on the other hand, requires additional energy to move in the conduction band, which impedes free electron flow and lowers mobility (higher noise).

2. Low Intrinsic Carrier Concentration

Gallium arsenide offers a smaller band gap than its silicon competitor, thus requiring less thermal energy to move an electron from the valence band to the conduction band. This lower energy requirement for gallium arsenide electrons means that random thermal vibrations (temperatures above cold) create a much larger number of electron-hole pairs compared to a material with a larger band gap like silicon.

This process of silicon electrons being thermally excited and then recombining with holes creates an unstable and constantly fluctuating number of carriers. This unstable and constantly changing population is and of itself, electrical noise. A higher population of carriers leads to an inconsistent and greater current instability, and thus higher noise for the silicon transistor.  

3. Semi-Insulating Substrate

Gallium arsenide has a low parasitic capacitance compared to silicon. This coupling minimizes noise that would normally travel from the transistor to other parts of the circuit. Silicon, on the other hand, has a higher parasitic capacitance over gallium arsenide, which offers the opposite effect: higher capacitance equals higher noise figure. Gallium arsenide is, by nature, a semi-insulating substrate. GaAs substrates have lower capacitance to ground, thus lower noise figure.

GaAs Derivatives

Derivatives of a pure gallium arsenide structure include high electron mobility transistors (HEMTs), including a blend of gallium arsenide and aluminum gallium arsenide to channel electron movement into a further confined narrow layer, by design, improves electron flow, reduces electron scattering, and thus improves electron mobility.

A second generation of GaAs derivatives is a Pseudomorphic High Electron Mobility Transistor (pHEMT) that uses a blended layer of Gallium arsenide and Indium Gallium arsenide to achieve an even higher electron mobility performance. The InGaAs layer is grown onto the GaAs substrate, but doesn’t perfectly match the adjacent lattice. This results in a higher energy “barrier” between the channel and the donor layer. That higher barrier yields improved electron confinement and electron channeling, restricting electron movement to a much more tightly controlled channel this lowering electron scattering, fewer collisions, and lower thermal noise over HEMTs and traditional GaAs solutions.

Download the PDF: Why GaAs Offer Better Noise Figure Performance than Silicon Transistors

Transistor Technologies for LNAs

Different transistor technologies for low noise amplifiers (LNAs) exhibit distinct performance characteristics, primarily driven by their material properties and device structures. The key tradeoffs involve noise figure, frequency capability, cost, and power handling.

Silicon-Based Transistors

Silicon transistors remain the most widely used due to their abundance, mature fabrication infrastructure, and low cost. High-purity silicon wafers with very few defects are produced at scale, enabling integration into complex circuits.

• Performance: silicon-based LNAs typically operate at lower RF frequencies, up to the L-band; widely used in consumer applications such as Wi-Fi and Bluetooth
• Limitations: silicon bipolar transistors suffer performance degradation at higher frequencies due to lower electron mobility compared to compound semiconductors such as GaAs; higher parasitic capacitances and poorer noise figures further limit their suitability in high-frequency, ultra-low-noise circuits
• Advantages: silicon devices are highly cost-effective and allow seamless integration into system-on-chip (SoC)and system-in-package (SiP)solutions

Gallium Arsenide (GaAs) FETs

GaAs is a compound semiconductor with significantly higher electron mobility than silicon, making it well-suited for microwave circuits.

• Performance: GaAs MESFETs provide lower noise figures and higher frequency capability than silicon devices, making them ideal for cellular and satellite communications
• Limitations: GaAs devices are more expensive and mechanically brittle; lower bandgap reduces power handling capability and makes them more prone to voltage breakdown

PseudomorphicHigh-Electron-Mobility Transistors (pHEMTs)

pHEMTs are a class of HEMTs typically grown on GaAs substrates, incorporating a strained ("pseudomorphic") layer that enhances electron mobility.

• Performance: pHEMTs deliver superior noise figure performance compared to standard GaAs FETs or silicon devices, particularly above the HF band; have become the industry standard for LNAs requiring ultra-low noise performance from VHF through Ka-band
• Substrate Variations: some advanced pHEMTs incorporate InP channels on GaAs substrates, further improving electron mobility and noise performance

Indium Phosphide (InP) Transistors

InP-based devices represent the highest-performing option among the four technologies, offering unparalleled noise and frequency characteristics.

• Performance: with extremely high electron mobility, InP transistors achieve the lowest noise figures and highest gains at millimeter-wave and sub-THz frequencies, exceeding 100 GHz
• Specialized Use: due to their cost and complexity, InP LNAs are typically deployed in demanding applications such as radio astronomy, military radar front ends, and high-frequency test and measurement equipment
• Drawbacks: despite their superior performance, InP devices are fragile, costly to manufacture, and prone to lower breakdown voltages compared to GaAs or silicon technologies

Metallization and Fabrication Effects

Beyond the base semiconductor material, metallization choices and fabrication techniques strongly influence transistor noise performance.

• Parasitic Resistance: The resistance of the contact points, especially gate resistance, can be a significant source of thermal noise. Bu using low-resistivity metals like gold, one can minimize this extrinsic added noise source.
• Layout and Design: Careful layout design in order to minimize parasitic contributions is crucial in low noise figure circuit design. Reducing trace and hybrid bond wire lengths and ensuring proper ground planes using thick gold traces all help to reduce noise figure performance. Smaller gate lengths on some modern transistor designs can also improve the noise figure performance.
• Feedback and Matching Networks: Improved impedance matching as well as loop and feedback circuits around the transistor based circuit also affects the overall noise figure of the low noise amp.

Technology Comparison Summary

Download the PDF: Transistor Technologies for Low Noise Amplifiers (LNAs)