Low Phase Noise Amplifiers
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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.
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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.
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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.
System Phase Noise Calculations
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
| Source | S1=-150 | 2nd Doubler D2=-155 |
| 1st Doubler | D1=-155 | 3rd Doubler D3=-155 |
| 1st Amp. | A1=-165 | 2nd Amp. A2=-165 |
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.
What is Amplifier Residual Phase Noise
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.
What is the Difference Between Time Domain and Frequency Domain Regarding Phase Noise?
Difference in the Time Domain
- Phase noise: in the time domain, phase noise performance is introduced into the signal as jitter. The word jitter, which is a perfect term used to describe this metric, is just how it sounds. Better put, it is the random and unpredictable fluctuation and deviation of an RF signal's Y-axis crossing points from their ideal, sinusoidal periodic timing. This causes the signal, in other words, to arrive sometimes earlier and sometimes later than the signal ideally should.
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Noise figure: the overall noise contributed by an amp (noise figure), simply put, is the random increase in the output power, output voltage, or current fluctuations of the output signal over a given period of time. If an amp’s output signal were a perfect sine wave, the noise would be seen as erratic or fuzzy, reflecting the addition of random, broadband noise.
Difference in the Frequency Domain
- Phase noise: in the frequency domain, the unstable nature of an amplifier’s output phase is better illustrated on a graph with noise sidebands. Instead of a single, narrow vertical spike or line at the carrier tone, one of today’s oscillators has noise-related energy that widens out from the carrier tone. The shape of these noise sidebands depends on the noise within the oscillator.
- Noise figure: when looked at in the frequency domain, noise figure is illustrated in the overall noise of the amp. A “perfect” amp would only amplify the existing thermal noise, or the noise generated as electrons move through from the base to the emitter, as in the case of a silicon bipolar transistor. An amplifier with a high noise figure will raise or elevate the output noise power across a wider band of frequencies, effectively raising the noise floor.
What is the Connection between the Time Domain and Frequency Domain for Phase Noise?
Even though phase noise is entirely different from noise figure, they share some commonalities. An amp’s noise figure adds to the overall noise floor of the RF chain. This noise can mix in a way with the nominal RF signal and be converted into phase noise. This effect is apparent in amps when those amps are operating in saturation. For example, a poor LO in a receive side receiver with a high noise figure can produce noise that "mixes" in a way with strong nearby signals. This mixes with the signal of interest through reciprocal mixing. When a Mixer in a chain mixes up from the LO to the IF, the phase noise present in the source creates adjacent bands of energy, which are somewhat pulled into the receiver’s channel of interest, thus raising the noise floor for the entire receiver.
Comparing Phase Noise to Noise Figure
Phase Noise |
Noise Figure |
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Measured in the frequency domain as a plot of spectral density of noise sidebands relative to the carrier. |
A single measurement in the frequency domain. Describes how much an amp degrades the signal to noise ratio. |
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The random fluctuations in the phase of a signal, which causes the signal's energy to spread out around its ideal carrier frequency. |
The amount of noise a component (like an amplifier) adds to a signal, relative to the thermal noise present at its input. |
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Seriously degrades signal fidelity, especially in radar platforms. Potentially contributing to reciprocal mixing with noise figure. Can increase the BER bit error rate, adds to sub-clutter visibility of a forward looking radar and potentially degrades the system’s dynamic range. |
Limits a receiver's sensitivity by raising the noise floor. A high noise figure means the receiver needs a stronger input signal to achieve a specific SNR. |
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A critical parameter for sources and synthesizers where signal purity and integrity is vital. |
A critical parameter for low noise amplifiers (LNAs) in communication applications where the input signal power is low or weak and vulnerable to any extra noise. |
About Amplifiers
Spectrum Control is a leading provider of high-performance RF amplifier solutions. We design, develop, and manufacture small signal amplifiers. Our standard and custom-designed, high-reliability amplifiers meet emerging military and commercial requirements.
Spectrum Control remains on the cutting edge of amplifier technology through out-of-the-box engineering and the manufacture of designs that reach increasing levels of complexity, power and performance. Spectrum Control can tailor a solution to your unique requirements.
Low phase noise, low noise, ultra high linearity, high frequency, and medium power RF amplifier products are available.