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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.
Amplifier Residual Phase Noise Amplifier AM Noise Performance
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.
Free Modifications to Standard Amplifiers
Modern chip-and-wire amplifier circuits deliver inherent advantages through their extensive customization potential. Unlike MMIC amplifiers, where integration can restrict optimization, chip-and-wire amplifier designs provide unmatched adaptability, enabling engineers to fine-tune every parameter to exact system requirements. This flexibility ensures maximum compatibility and improved performance across a wide range of RF systems and applications.
Example:
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?
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.
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 free-flowing signal propagation, maximum power transfer, and prevents damage from output reflections. If a 50-ohm amplifier is placed in an RF chain facing a mismatched impedance, issues, including amplifier failure, may occur. The importance of impedance-matching a 50-ohm small-signal amplifier with surrounding components cannot be overstated; moreover, it is considered an industry standard.
- 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 reflected, either back to the amplifier (when the amplifier sees a mismatch) or back to the source (when the mismatch is at the amplifier’s input).
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Preventing Signal Reflections: Impedance mismatches cause signal reflections, this is well known. The reflected signal then travels back along the signal path and interferes with the output signal, creating standing (stationary) waves.
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Signal Integrity: Standing waves can appear as current or voltage fluctuations along the signal path. This can manifest as distortion, a deterioration in the signal-to-noise ratio (SNR), and can seriously affect or degrade the amplifier’s output.
What are the consequences of a mismatch?
When a small-signal RF amplifier operates in a chain with a mismatched impedance, the result can range from a slight reduction in performance to reflections that cause serious or even permanent damage to the amplifier.
Reduced Performance
- Output Power Loss: In a small-signal RF amplifier, any output power reflected from the load is lost energy. This reduces the total power output intended for the downstream component. As the load mismatch worsens, the intended output power loss increases dramatically.
- Distortion: When standing waves are present due to load mismatch, their interference produces distortion, degrading signal fidelity.
- Gain Loss: The gain of an amplifier in an RF chain can be lowered by load-mismatch losses at each gain block stage.
Can impedance mismatches damage an amplifier?
- Transistor Damage: In cases of an open (infinite impedance) or short (zero impedance), all forward-directed output power is reflected back into the amplifier’s output pin or connector. This can cause a sudden and dramatic increase in voltage or current directed at the transistors. Designers sometimes employ diodes to absorb that reflection or large capacitors to ground to drain it away, but such measures are not always 100% effective.
- MTBF: Reflected power must eventually dissipate as heat if the mismatch is severe enough. This additional heat can reduce the mean time between failures (MTBF), shortening the amplifier’s lifespan.
Can I prevent damage to an amplifier which has a poor mismatch?
- Share your Incident Impedance Measurements with the RF Amplifier Supplier: Knowing potential mismatch issues before amplifier selection can help mitigate problems later. Spectrum Control can often include elements such as diodes or pads on the output to help absorb, reflect, or condition harmful mismatch reflections.
- Add a Pad: For reflection-sensitive amplifiers, a small pad, perhaps ¼ dB to ½ dB (if there’s sufficient output power margin), can be strategically placed at the amplifier output or at the next component’s input to reduce reflected power.
- Use Proper Load During the Test Phase of the Design: Always terminate an amplifier with a 50-ohm load. During 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 all other components.
Comparing High Linearity Topologies
High linearity is a critical requirement in RF front-end design, ensuring low distortion, high intercept points, and improved signal integrity. This note compares three common amplifier topologies, Darlington, Push-Pull, and Parallel, in terms of distortion characteristics, intercept point (IP2, IP3) performance, and other operational trade-offs. It also provides guidance for selecting the appropriate topology for different RF applications.
| Darlington | Push-Pull | Parallel | |
| Distortion | A Darlington circuit does not inherently cancel distortion. Distortion products are directly influenced by transistor design and biasing. | A push-pull circuit naturally cancels even-order harmonics due to balanced configuration, reducing total distortion. | A parallel circuit offers no intrinsic distortion cancellation; distortion depends on device matching and operating point. |
| IP2 (Second Order Intercept) Performance | A Darlington circuit is fundamentally inferior to push-pull due to lack of distortion cancellation. | A push-pull design is excellent IP2 performance, making it ideal for demanding RF systems. | A parallel circuit does not, does not inherently improve IP2. |
| IP3 (Third Order Intercept) Performance | A Darlington design can offer respectable IP3 with careful design. | A push-pull design supports higher output power, better efficiency, and improved load handling. | A parallel design IP3 performance is comparable to Push-Pull when properly implemented. |
| Other Performance Metrics | Lower output impedance and supply current improvements; generally lower efficiency | A push pull circuit supports higher output power, better efficiency, and improved load handling. | A parallel design provides higher output power by combining devices but requires careful current sharing to maintain linearity |
Key Takeaway
- Push-pull designs are advantageous when harmonic suppression is a priority
Application Guidelines
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When IP2 performance is critical. select push-pull topology
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For general gain block requirements where linearity is important, Darlington can be a cost-effective option
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When output power is the sole requirement, parallel topology is best suited
Summary
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Darlington: simple implementation, moderate IP3 performance, but limited IP2
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Push-Pull: best for linearity and harmonic suppression, suitable for high-performance RF systems
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Parallel: best for output power scaling but requires careful design for thermal and current balance; the choice of topology should align with the primary performance requirement: linearity, gain, or output power
Figure 1: Harmonic Distortion - Darlington vs Push-Pull
Figure 2: Relative IP3 Performance by Topology
Download the PDF: Comparing High Linearity Amplifier Topologies for RF Systems
Class A vs. Class AB Operation
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.
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Performance
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Class A Amplifier | Class AB Amplifier |
| Heat | Class A amplifiers generate more heat due to their constant DC power consumption and may require a copper bus to dissipate the excess thermal energy. | Class AB amplifiers produce less heat than typical CW Class A designs because their transistors are not continuously operating at 100%. |
| Power | Because Class A amplifiers are less efficient, they generally deliver lower power output compared to Class AB designs. | Class AB amplifiers typically achieve higher output 1 dB compression points due to their more efficient use of the DC supply. |
| Bias | A Class A amplifier conducts DC current throughout the full 360° of signal propagation. The transistors remain in an “always-on” state, even when no RF input signal is applied. | Class AB amplifiers conduct current for more than 180° of the signal cycle. Typical designs employ a push-pull configuration, in which one transistor conducts during the positive half of the waveform while the other transistor conducts during the negative half. |
| Efficiency | Class A amplifiers are inherently inefficient, with typical efficiencies ranging between 20% and 25%. This low efficiency is a direct result of their continuous-wave (CW) operation, which dissipates a large portion of the DC supply power as heat. | Class AB amplifiers are significantly more efficient than Class A designs, with typical efficiencies reaching up to 60%. |
| Fidelity | Class A amplifiers provide the lowest distortion and highest signal fidelity. Since the transistors operate in the linear region at all times, they avoid the “crossover distortion” that can result from clipping or switching transitions. | Class AB amplifiers deliver higher fidelity than pure Class B designs but fall short of the performance achieved by Class A amplifiers. A small overlap is introduced during the conduction phase between transistors, which effectively eliminates the crossover distortion that is characteristic of simple Class B amplifier designs. |
IP3 vs. IP2
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?
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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.
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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.
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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.
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.