Low Phase Noise Amplifiers

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 for low noise RF amplifiers?

In a low-noise amplifier (LNA), gain and noise figure are interrelated but distinct parameters. Generally, they exhibit an inverse relationship, improving one can often degrade the other, depending on the design trade-offs involved. Although gain and noise figure are optimized independently, achieving a high gain in the first stage of an amplifier is particularly beneficial, as it minimizes the contribution of noise from subsequent stages.

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

The noise figure (NF) of an amplifier is defined as the ratio of the input signal-to-noise ratio (SNR) to the output SNR. A lower noise figure indicates better noise performance. One way to reduce NF is by operating the amplifier’s transistors near their maximum current capacity, which increases the signal amplitude relative to the amplifier’s intrinsic noise sources, thereby improving the output SNR.

As amplifier gain increases, both the signal and the internally generated noise are amplified. However, when the gain is sufficiently high, the input noise becomes the dominant contributor to the total noise, effectively reducing the relative impact of the amplifier’s own noise. This behavior is quantified by the Friis formula (or Friis equation), which describes how noise figure accumulates in cascaded amplifier stages.

In practical system design, placing a high-gain, low-noise amplifier (LNA) at the front end, ideally as close as possible to the antenna, significantly minimizes the influence of noise from subsequent stages. For this reason, the first-stage LNA typically has the most stringent noise and gain specifications in the entire receive chain.

Download PDF: How are Gain and Noise Figure Related?

Correlated and Uncorrelated Components

When calculating the phase noise of a system there are many considerations. The following illustrates phase noise calculation of a system and the effects of correlated components verses uncorrelated components. Following these calculations there is an illustration
showing consideration of absolute power levels and how they affect the phase noise of a system. Let us assume the phase noise for each component at a single offset frequency, say 100 kHz is the following in dBc/Hz:

Phase Noise of each Component

We will start off with a doubler and amplifier:

If we have a component preceding a doubler or any multiplier, its phase noise will be degraded by 20 log of the multiplication factor.

Let’s consider a mixer. If we have two uncorrelated sources with the same phase noise characteristics conditioned by these components and fed into a mixer the calculations and result would be as follows:

When the output of a single source is split, conditioned and mixed it is considered correlated. This would also be true if the two sources above were phase locked to each other. The calculations for the source would be correlated while the other components are not. The
result would be higher phase noise.

One can also conclude from this that the phase noise of a doubler made by splitting a source and then combining it in a mixer will be the same as using a passive doubler.

Thermal Noise Floor

If the phase noise measurement of a component

is made at the same power levels that will exist in the system the thermal noise floor is already in the result. If the data is recorded at much higher power levels than components will experience in the system or if the engineer would like to know what the power levels must be to maintain a particular phase noise result then the thermal noise floor must be a consideration. The thermal noise floor or kTB is ~-174 dBm/Hz. This is based on Boltzmann’s constant, the temperature, and the bandwidth of the signal.

Thermal noise is specified in dBm. If the phase noise of a component comes close to this power level it will be degraded and there is no method to correct the situation short of filtering for phase noise. The same rules for calculating correlated and uncorrelated components apply as before so let us consider the performance of the following cascade.

Therefore if the source in this system were ideal it would have a phase noise of –189 dBc/Hz (-174-15). Allowing the power level to drop significantly degrades the system noise performance.

Conclusion

When calculating the phase noise of a system one must pay close attention to power levels and precisely how the signal is used and reused.

Download the PDF: System Phase Noise Calculations

Amplifier residual phase noise, also known as Additive Phase Noise, is the additional phase noise or the phase noise contribution that an active component, like an amp, adds to an output signal as the signal passes through the device. Phase noise is a parameter or critical metric for characterizing and defining the performance that differentiates the fundamental noise of the amplifier from the noise of the source at the amp’s input.

What’s the difference between Residual Phase Noise and Absolute Phase Noise?

Understanding, identifying, and recognizing the difference between residual phase noise and absolute phase noise is critical for understanding an amplifier’s contribution to the system's overall noise performance.

• Residual Phase Noise: residual phase noise is the phase noise that is added to the path by the amplifier. When measuring this parameter, the test set uses the same clean signal source for both the device under test (DUT) and a low phase noise input reference. The phase noise test set, like the Agilent 5511 or the Rhode FSWP, then analyzes, but inputs, subtracts out the phase noise from the amplifier, and leaves only the phase noise contributed by the amplifier or the DUT.
• Absolute Phase Noise: the total or complete phase noise of the input signal to the amplifier. For a signal generated by a synthesizer or an oscillator, that signal is measured to determine its absolute phase noise. When that signal passes through an amplifier, for example, the amplifier's phase noise contribution combines with the synthesizer’s or oscillator’s absolute phase noise to produce a new, higher absolute phase noise measurement at the amplifier's output.

Why is Residual Phase Noise so important?

In high-performance systems like ground-based radar, targeting systems, and missile defense applications, every single component in the chain’s noise contribution matters to the overall fidelity of the system.

• Predicting System Performance: by understanding, measuring, and then knowing the residual phase noise contribution of the amplifiers in the chain, design engineers can accurately model and predict the total noise of a complex system.
• Component Level Performance: actually measuring the residual phase noise contribution is the only way to confidently evaluate and compare the fundamental noise performance of both the Source and the amps in the chain. This is especially important for receive-side buffer amplifiers, where the Phase Noise performance is at times slightly degraded when the amp operates past the P1dB point and well into saturation for peak output power performance.
• Identifying Noise Sources: using a specific Residual Phase Noise measurement setup, an engineer can attempt to cancel out the noise contribution from DC sources like a power supply, for example, allowing the design engineer to pinpoint the specific component, or even broader, the chain, that is degrading the system's overall performance.

What contributes to an Amp's Residual Phase Noise?

• Flicker Noise (1/f noise): noise more significant at close-in offsets, the result of low-frequency fluctuations in the amp's transistors.
• White Noise: noise that is a byproduct of the amplifier's thermal noise or Johnson noise. Thermal noise is simply the white noise that is caused by an amp’s transistor as electrons move, as in the case of a silicon transistor, from the base to the emitter.
• AM-to-PM Conversion: an amplifier's linearity can degrade at output power levels beyond the amp’s linear region or perhaps into saturation. Beyond P1dB, amplitude noise or AM noise from the DC supply can register as phase noise or PM, which can increase an amplifier’s residual phase noise contribution. Based on the amps we manufacture here at Spectrum Control, an amp’s best Residual Phase Noise performance comes when it is at, or just shy of, its P1dB point.

Download the PDF: What is Amplifier Residual Phase Noise

Difference in the Time Domain

• Phase noise: In the time domain, phase noise manifests as jitter. The term jitter aptly describes random and unpredictable fluctuations in an RF signal’s zero-crossing points from their ideal, periodic timing. In other words, the signal sometimes arrives earlier or later than it ideally should, introducing timing uncertainty that degrades signal stability and performance.

• Noise figure: The noise figure of an amplifier represents the total noise it contributes to the output. Simply put, it is the random increase in output power, voltage, or current fluctuations over time. If an amplifier’s output were a perfect sine wave, this added noise would appear as small, erratic deviations or fuzziness on the waveform, reflecting random, broadband noise superimposed on the ideal signal.

Difference in the Frequency Domain

• Phase noise: In the frequency domain, phase noise appears as noise sidebands surrounding the carrier tone. Rather than a single narrow spectral line at the carrier frequency, real oscillators exhibit sidebands that spread outward due to random phase variations. The shape and distribution of these sidebands depend on the internal noise sources and characteristics of the oscillator.
• Noise figure: Viewed in the frequency domain, the noise figure is represented by the overall noise floor of the amplifier. An ideal amplifier would amplify only the existing thermal noise (such as that generated by electron motion in semiconductor junctions). However, an amplifier with a high noise figure increases the output noise power across a wide frequency range, effectively raising the system’s noise floor and degrading sensitivity. 

What is the Connection between the Time Domain and Frequency Domain for Phase Noise?

While phase noise and noise figure are distinct parameters, they share a close relationship. An amplifier’s noise figure contributes to the overall noise floor of an RF system, and under certain conditions, this noise can interact with the desired signal to produce phase noise components.

This effect becomes especially apparent when amplifiers operate near saturation. For instance, a local oscillator (LO) with a high noise figure can introduce additional noise that mixes with strong nearby signals, a phenomenon known as reciprocal mixing. In a receiver chain, when a mixer translates signals from the LO to the intermediate frequency (IF), the LO’s phase noise manifests as sidebands that extend into adjacent frequency channels. This effectively raises the receiver’s noise floor and degrades overall system performance.

Download the PDF: Time Domain vs. Frequency Domain

The table below compares Phase Noise and Noise Figure, two fundamental parameters in RF and microwave system design. While both impact overall signal quality and system performance, they describe different aspects of noise behavior. Phase noise characterizes the short-term stability and spectral purity of a signal source, whereas noise figure quantifies how much additional noise a component, such as an amplifier, introduces relative to the thermal noise at its input. Understanding the distinctions between these parameters is critical for optimizing system sensitivity, dynamic range, and signal integrity across communication, radar, and electronic warfare applications.

Download PDF: Comparing Phase Noise to Noise Figure

Spectrum Control specializes in the design and manufacture of hybrid chip and wire amplifiers. Class A amplifiers differ significantly from their Class AB counterparts. In Class A designs, the devices are biased to conduct current throughout the entire input signal cycle. While this approach delivers exceptional spectral fidelity, it comes at the cost of efficiency, as the amplifier operates at a continuous 100% duty cycle. By contrast, Class AB amplifiers represent a middle ground between Class A and Class B operation. They are biased to conduct for more than half (180°) of the waveform, improving efficiency while reducing the distortion that is typical in pure Class B designs.

Download the PDF: Class A vs. Class AB

RF Amplifier Linearity and Noise

IP3 and IP2 values are measurements of an amplifier’s linearity. They indicate how much non-linear distortion the amplifier produces and how that distortion contributes to the output signal.

Although both IP3 and IP2 quantify distortion, they differ in the type of distortion measured and the impact on overall system performance. In general, higher IP3 or IP2 values correspond to greater linearity and better amplifier performance.

A “linear” amplifier increases the strength of the input signal without altering its original shape. In other words, the output signal is an exact scaled replica of the input. This characteristic is critical in applications that require high spectral fidelity, such as communication systems and precision instrumentation.

To better understand linearity and non-linearity, consider a real-world analogy using a guitar and a frequency generator.

A frequency generator is a highly linear system designed to produce a simple, “pure” tone. Playing a Middle C produces a single sine wave at approximately 261 Hz. In this system, the output amplitude is directly proportional to the input voltage, raising the input simply increases the output level. The resulting waveform is clean and consists of only one frequency.

By contrast, a guitar is inherently non-linear. When a string is plucked to produce Middle C, it does not vibrate solely at its fundamental frequency. Instead, it also generates vibrations at integer multiples of that frequency, known as harmonics. The pluck (input) produces a complex output waveform containing both the fundamental and its harmonics, resulting in a rich and natural tone.

In this analogy, the amplifier’s role is similar to how the intensity of the pluck affects the sound. A harder pluck increases loudness but also changes the harmonic balance, introducing additional distortion. These harmonics, integer multiples of the fundamental, create the overtones that shape the timbre of the sound.

Nonlinearity in the guitar arises from several factors that interact in complex ways: the stiffness of the strings, the material of the body, and the playing technique. Each introduces deviations from ideal harmonic behavior. This nonlinearity makes the guitar sound warm and expressive, whereas the frequency generator produces an artificial, sterile tone.

In this analogy, the amplifier’s role is similar to how the intensity of the pluck affects the sound. A harder pluck increases loudness but also changes the harmonic balance, introducing additional distortion. These harmonics, integer multiples of the fundamental, create the overtones that shape the timbre of the sound.

Nonlinearity in the guitar arises from several factors that interact in complex ways: the stiffness of the strings, the material of the body, and the playing technique. Each introduces deviations from ideal harmonic behavior. This nonlinearity makes the guitar sound warm and expressive, whereas the frequency generator produces an artificial, sterile tone.

In amplifiers, similar effects occur. Nonlinearities cause the output signal to deviate from a perfectly scaled version of the input. These distortions can originate from the amplifier’s fundamental design, component quality, or operating environment. When an amplifier operates nonlinearly, frequencies within the input can mix, producing new frequencies that are the sums and differences of the original ones. These intermodulation products are unwanted because they are not harmonically related to the original signal.

No amplifier is perfectly linear, every RF component exhibits some degree of nonlinearity and will eventually reach a point of saturation where the output can no longer increase proportionally with the input. For this reason, linear amplifiers are designed to operate within a specified range where their linearity and thus IP2 and IP3 performance is optimized.

What is an Intercept Point?

• The intercept point is a mathematical construct used to describe an amplifier’s linearity. It represents the theoretical input power level at which the power of the desired output signal and the power of the distortion products (typically intermodulation products) would be equal.

• A higher IP2 or IP3 value indicates a more linear device and greater resistance to distortion, meaning the amplifier can handle stronger input signals before nonlinear effects become significant.

• In practice, amplifiers reach their 1 dB compression point (P1dB) and begin limiting output power well before the intercept point is actually achieved. The intercept point is therefore used as a figure of merit rather than a directly measurable condition.

Why is IP3 more important than IP2?

Third-order distortion products typically appear very close to the fundamental signal frequencies, making them difficult to suppress using standard filtering techniques. These unwanted signals can fall directly within the receiver’s operating band, degrading sensitivity and overall performance.

For this reason, IP3 is often considered the most critical linearity parameter for a receiver. It directly reflects the system’s ability to maintain clean signal integrity and operate effectively in environments with strong, closely spaced signals, such as crowded or high-interference frequency bands.

When is IP2 more important than IP3?

IP2 is generally less critical than IP3 in superheterodyne receivers because second-order distortion products typically fall outside the desired signal band and can be filtered out more easily.

However, IP2 becomes crucial in direct-conversion (zero-IF) receivers, where second-order distortion can create unwanted DC offsets or low-frequency artifacts that directly interfere with the desired signal. In such architectures, maintaining a high IP2 is essential to ensure accurate demodulation and minimize baseband distortion.

Download the PDF: IP3 vs. IP2