A Look at Low-Noise Amplifiers

Maurizio Di Paolo Emilio, who has a PhD in Physics, is an Italian telecommunications engineer who works mainly as a software developer with a focus on data acquisition systems. Emilio has authored articles about electronic designs, data acquisition systems, power supplies, and photovoltaic systems. In this article, he provides an overview of what is generally available in low-noise amplifiers (LNAs) and some of the applications.

By Maurizio Di Paolo Emilio
An LNA, or preamplifier, is an electronic amplifier used to amplify sometimes very weak signals. To minimize signal power loss, it is usually located close to the signal source (antenna or sensor). An LNA is ideal for many applications including low-temperature measurements, optical detection, and audio engineering. This article presents LNA systems and ICs.

Signal amplifiers are electronic devices that can amplify a relatively small signal from a sensor (e.g., temperature sensors and magnetic-field sensors). The parameters that describe an amplifier’s quality are:

  • Gain: The ratio between output and input power or amplitude, usually measured in decibels
  • Bandwidth: The range of frequencies in which the amplifier works correctly
  • Noise: The noise level introduced in the amplification process
  • Slew rate: The maximum rate of voltage change per unit of time
  • Overshoot: The tendency of the output to swing beyond its final value before settling down

Feedback amplifiers combine the output and input so a negative feedback opposes the original signal (see Figure 1). Feedback in amplifiers provides better performance. In particular, it increases amplification stability, reduces distortion, and increases the amplifier’s bandwidth.

 Figure 1: A feedback amplifier model is shown here.


Figure 1: A feedback amplifier model is shown.

A preamplifier amplifies an analog signal, generally in the stage that precedes a higher-power amplifier.

IC LOW-NOISE PREAMPLIFIERS
Op-amps are widely used as AC amplifiers. Linear Technology’s LT1028 or LT1128 and Analog Devices’s ADA4898 or AD8597 are especially suitable ultra-low-noise amplifiers. The LT1128 is an ultra-low-noise, high-speed op-amp. Its main characteristics are:

  • Noise voltage: 0.85 nV/√Hz at 1 kHz
  • Bandwidth: 13 MHz
  • Slew rate: 5 V/µs
  • Offset voltage: 40 µV

Both the Linear Technology and Analog Devices amplifiers have voltage noise density at 1 kHz at around 1 nV/√Hz  and also offer excellent DC precision. Texas Instruments (TI)  offers some very low-noise amplifiers. They include the OPA211, which has 1.1 nV/√Hz  noise density at a  3.6 mA from 5 V supply current and the LME49990, which has very low distortion. Maxim Integrated offers the MAX9632 with noise below 1nV/√Hz.

The op-amp can be realized with a bipolar junction transistor (BJT), as in the case of the LT1128, or a MOSFET, which works at higher frequencies and with a higher input impedance and a lower energy consumption. The differential structure is used in applications where it is necessary to eliminate the undesired common components to the two inputs. Because of this, low-frequency and DC common-mode signals (e.g., thermal drift) are eliminated at the output. A differential gain can be defined as (Ad = A2 – A1) and a common-mode gain can be defined as (Ac = A1 + A2 = 2).

An important parameter is the common-mode rejection ratio (CMRR), which is the ratio of common-mode gain to the differential-mode gain. This parameter is used to measure the  differential amplifier’s performance.

Figure 2: The design of a simple preamplifier is shown. Its main components are the Linear Technology LT112 and the Interfet IF3602 junction field-effect transistor (JFET).

Figure 2: The design of a simple preamplifier is shown. Its main components are the Linear Technology LT1128 and the Interfet IF3602 junction field-effect transistor (JFET).

Figure 2 shows a simple preamplifier’s design with 0.8 nV/√Hz at 1 kHz background noise. Its main components are the LT1128 and the Interfet IF3602 junction field-effect transistor (JFET).  The IF3602 is a dual N-channel JFET used as stage for the op-amp’s input. Figure 3 shows the gain and Figure 4 shows the noise response.

Figure 3: The gain of a low-noise preamplifier.

Figure 3: The is a low-noise preamplifier’s gain.

 

Figure 4: The noise response of a low-noise preamplifier

Figure 4: A low-noise preamplifier’s noise response is shown.

LOW NOISE PREAMPLIFIER SYSTEMS
The Stanford Research Systems SR560 low-noise voltage preamplifier has a differential front end with 4nV/√Hz input noise and a 100-MΩ input impedance (see Photo 1a). Input offset nulling is accomplished by a front-panel potentiometer, which is accessible with a small screwdriver. In addition to the signal inputs, a rear-panel TTL blanking input enables you to quickly turn the instrument’s gain on and off (see Photo 1b).

Photo 1a:The Stanford Research Systems SR560 low-noise voltage preamplifier

Photo 1a: The Stanford Research Systems SR560 low-noise voltage preamplifier. (Photo courtesy of Stanford Research Systems)

Photo 1 b: A rear-panel TTL blanking input enables you to quickly turn the Stanford Research Systems SR560 gain on and off.

Photo 1b: A rear-panel TTL blanking input enables you to quickly turn the Stanford Research Systems SR560 gain on and off. (Photo courtesy of Stanford Research Systems)

The Picotest J2180A low-noise preamplifier provides a fixed 20-dB gain while converting a 1-MΩ input impedance to a 50-Ω output impedance and 0.1-Hz to 100-MHz bandwidth (see Photo 2). The preamplifier is used to improve the sensitivity of oscilloscopes, network analyzers, and spectrum analyzers while reducing the effective noise floor and spurious response.

Photo 2: The Picotest J2180A low-noise preamplifier is shown.

Photo 2: The Picotest J2180A low-noise preamplifier is shown. (Photo courtesy of picotest.com)

Signal Recovery’s Model 5113 is among the best low-noise preamplifier systems. Its principal characteristics are:

  • Single-ended or differential input modes
  • DC to 1-MHz frequency response
  • Optional low-pass, band-pass, or high-pass signal channel filtering
  • Sleep mode to eliminate digital noise
  • Optically isolated RS-232 control interface
  • Battery or line power

The 5113 (see Photo 3 and Figure 5) is used in applications as diverse as radio astronomy, audiometry, test and measurement, process control, and general-purpose signal amplification. It’s also ideally suited to work with a range of lock-in amplifiers.

Photo 3: This is the Signal Recovery Model 5113 low-noise pre-amplifier.

Photo 3: This is the Signal Recovery Model 5113 low-noise preamplifier. (Photo courtesy of Signal Recovery)

Figure 5: Noise contour figures are shown for the Signal Recovery Model 5113.

Figure 5: Noise contour figures are shown for the Signal Recovery Model 5113.

WRAPPING UP
This article briefly introduced low-noise amplifiers, in particular IC system designs utilized in simple or more complex systems such as the Signal Recovery Model 5113, which is a classic amplifier able to obtain different frequency bands with relative gain. A similar device is the SR560, which is a high-performance, low-noise preamplifier that is ideal for a wide variety of applications including low-temperature measurements, optical detection, and audio engineering.

Moreover, the Krohn-Hite custom Models 7000 and 7008 low-noise differential preamplifiers provide a high gain amplification to 1 MHz with an AC output derived from a very-low-noise FET instrumentation amplifier.

One common LNA amplifier is a satellite communications system. The ground station receiving antenna will connect to an LNA, which is needed because the received signal is weak. The received signal is usually a little above background noise. Satellites have limited power, so they use low-power transmitters.

Telecommunications engineer Maurizio Di Paolo Emilio was born in Pescara, Italy. Working mainly as a software developer with a focus on data acquisition systems, he helped design the thermal compensation system (TCS) for the optical system used in the Virgo Experiment (an experiment for detecting gravitational waves). Maurizio currently collaborates with researchers at the University of L’Aquila on X-ray technology. He also develops data acquisition hardware and software for industrial applications and manages technical training courses. To learn more about Maurizio and his expertise, read his essay on “The Future of Data Acquisition Technology.”

High-Voltage Gate Driver IC

Allegro A4900 Gate Driver IC

Allegro A4900 Gate Driver IC

The A4900 is a high-voltage brushless DC (BLDC) MOSFET gate driver IC. It is designed for high-voltage motor control for hybrid, electric vehicle, and 48-V automotive battery systems (e.g., electronic power steering, A/C compressors, fans, pumps, and blowers).

The A4900’s six gate drives can drive a range of N-channel insulated-gate bipolar transistors (IGBTs) or power MOSFET switches. The gate drives are configured as three high-voltage high-side drives and three low-side drives. The high-side drives are isolated up to 600 V to enable operation with high-bridge (motor) supply voltages. The high-side drives use a bootstrap capacitor to provide the supply gate drive voltage required for N-channel FETs. A TTL logic-level input compatible with 3.3- or 5-V logic systems can be used to control each FET.

A single-supply input provides the gate drive supply and the bootstrap capacitor charge source. An internal regulator from the single supply provides the logic circuit’s lower internal voltage. The A4900’s internal monitors ensure that the high- and low-side external FET’s gate source voltage is above 9 V when active.

The control inputs to the A4900 offer a flexible solution for many motor control applications. Each driver can be driven with an independent PWM signal, which enables implementation of all motor excitation methods including trapezoidal and sinusoidal drive. The IC’s integrated diagnostics detect undervoltage, overtemperature, and power bridge faults that can be configured to protect the power switches under most short-circuit conditions. Detailed diagnostics are available as a serial data word.

The A4900 is supplied in a 44-lead QSOP package and costs $3.23 in 1,000-unit quantities.

Allegro MicroSystems, LLC
www.allegromicro.com

Voltage Regulator Protection (EE Tip #103)

In many cases, the load connected to a voltage regulator is not returned to ground. It goes to an even lower voltage or perhaps even the negative power supply voltage. (Here we make the assumption of using positive voltages, when using voltage regulators with negative output voltages the reverse is true.)

Op-amps and level-shifters come to mind. In such cases, a diode (1N4001 or equivalent) connected across the output of the regulator IC usually provides sufficient protection (see Figure 1).

Source:Ton Giesberts, Elektor, 080943-I, 4/2009

Source:Ton Giesberts, Elektor, 080943-I, 4/2009

Polarity inversions which could occur, for example, during power on or during a short circuit could prove fatal for the regulator IC, but such a diode prevents the output of the IC going lower than ground (well, minus 0.7 V, to be accurate).

A short-circuit proof voltage regulator (such as the 78xx series) will survive such a situation without any problems. It is also possible for the input voltage of a voltage regulator to drop quicker than the output voltage—for example, when there is a protection circuit that shorts the input power supply voltage as a result of an overvoltage at the output.

If the output voltage of the regulator is more than 7 V higher than the input voltage, the emitter-base junction of the internal power transistor can break down and cause the transistor to fail.

You can use a shunt diode to prevent this condition (see Figure 2). This ensures that any higher voltage at the output of the regulator is shorted to the input.

—Ton Giesberts, Elektor, 080943-I, 4/2009

The Future of Very Large-Scale Integration (VLSI) Technology

The historical growth of IC computing power has profoundly changed the way we create, process, communicate, and store information. The engine of this phenomenal growth is the ability to shrink transistor dimensions every few years. This trend, known as Moore’s law, has continued for the past 50 years. The predicted demise of Moore’s law has been repeatedly proven wrong thanks to technological breakthroughs (e.g., optical resolution enhancement techniques, high-k metal gates, multi-gate transistors, fully depleted ultra-thin body technology, and 3-D wafer stacking). However, it is projected that in one or two decades, transistor dimensions will reach a point where it will become uneconomical to shrink them any further, which will eventually result in the end of the CMOS scaling roadmap. This essay discusses the potential and limitations of several post-CMOS candidates currently being pursued by the device community.

Steep transistors: The ability to scale a transistor’s supply voltage is determined by the minimum voltage required to switch the device between an on- and an off-state. The sub-threshold slope (SS) is the measure used to indicate this property. For instance, a smaller SS means the transistor can be turned on using a smaller supply voltage while meeting the same off current. For MOSFETs, the SS has to be greater than ln(10) × kT/q where k is the Boltzmann constant, T is the absolute temperature, and q is the electron charge. This fundamental constraint arises from the thermionic nature of the MOSFET conduction mechanism and leads to a fundamental power/performance tradeoff, which could be overcome if SS values significantly lower than the theoretical 60-mV/decade limit could be achieved. Many device types have been proposed that could produce steep SS values, including tunneling field-effect transistors (TFETs), nanoelectromechanical system (NEMS) devices, ferroelectric-gate FETs, and impact ionization MOSFETs. Several recent papers have reported experimental observation of SS values in TFETs as low as 40 mV/decade at room temperature. These so-called “steep” devices’ main limitations are their low mobility, asymmetric drive current, bias dependent SS, and larger statistical variations in comparison to traditional MOSFETs.

Spin devices: Spintronics is a technology that utilizes nano magnets’ spin direction as the state variable. Spintronics has unique properties over CMOS, including nonvolatility, lower device count, and the potential for non-Boolean computing architectures. Spintronics devices’ nonvolatility enables instant processor wake-up and power-down that could dramatically reduce the static power consumption. Furthermore, it can enable novel processor-in-memory or logic-in-memory architectures that are not possible with silicon technology. Although in its infancy, research in spintronics has been gaining momentum over the past decade, as these devices could potentially overcome the power bottleneck of CMOS scaling by offering a completely new computing paradigm. In recent years, progress has been made toward demonstration of various post-CMOS spintronic devices including all-spin logic, spin wave devices, domain wall magnets for logic applications, and spin transfer torque magnetoresistive RAM (STT-MRAM) and spin-Hall torque (SHT) MRAM for memory applications. However, for spintronics technology to become a viable post-CMOS device platform, researchers must find ways to eliminate the transistors required to drive the clock and power supply signals. Otherwise, the performance will always be limited by CMOS technology. Other remaining challenges for spintronics devices include their relatively high active power, short interconnect distance, and complex fabrication process.

Flexible electronics: Distributed large area (cm2-to-m2) electronic systems based on flexible thin-film-transistor (TFT) technology are drawing much attention due to unique properties such as mechanical conformability, low temperature processability, large area coverage, and low fabrication costs. Various forms of flexible TFTs can either enable applications that were not achievable using traditional silicon based technology, or surpass them in terms of cost per area. Flexible electronics cannot match the performance of silicon-based ICs due to the low carrier mobility. Instead, this technology is meant to complement them by enabling distributed sensor systems over a large area with moderate performance (less than 1 MHz). Development of inkjet or roll-to-roll printing techniques for flexible TFTs is underway for low-cost manufacturing, making product-level implementations feasible. Despite these encouraging new developments, the low mobility and high sensitivity to processing parameters present major fabrication challenges for realizing flexible electronic systems.

CMOS scaling is coming to an end, but no single technology has emerged as a clear successor to silicon. The urgent need for post-CMOS alternatives will continue to drive high-risk, high-payoff research on novel device technologies. Replicating silicon’s success might sound like a pipe dream. But with the world’s best and brightest minds at work, we have reasons to be optimistic.

Author’s Note: I’d like to acknowledge the work of PhD students Ayan Paul and Jongyeon Kim.

New Products: May 2013

iC-Haus

iC-Haus iC-TW8

The iC-TW8 is a high-resolution signal processor designed to evaluate sine/cosine sensors. Its automatic functions help minimize angular errors and jitters. The processor can be used for initial, push-button calibration and to permanently adapt signal-path parameters during operation. The angular position is calculated at a programmable resolution of up to 65,536 increments per input cycle and output as an indexed incremental signal. A 32-bit word, which includes the counted cycles, is available through the SPI.

As an application-specific DSP, the iC-TW8 has two ADCs that simultaneously sample at a 250-ksps rate, fast CORDIC algorithms, special signal filters, and an analog front end with differential programmable gate amplifier (PGA) inputs that accepts typical magnetic sensor signals from 20 mVPP and up. Signal frequencies of up to 125 kHz enable high rotary and linear speeds for position measuring devices and are processed at a 24-µs constant latency period.

The device’s 12-bit measurement accuracy works with one button press. Measuring tools are not required. The iC-TW8 independently acquires information about the signal corrections needed for offset, amplitude, and phase errors and stores them in an external EEPROM.

The iC-TW8 has two configuration modes. Preset functions and interpolation factors can be retrieved through pins and the device can be calibrated with a button push. No programming is required for initial operation.

The device’s functions—including an AB output divider for fractional interpolation, an advanced signal filter to reduce jitter, a table to compensate for signal distortion, and configurable monitors for errors and signal quality—can be accessed when the serial interfaces are used. Typical applications include magnetic linear displacement measuring systems, optical linear scales, programmable magnetic/optical incremental encoders, high-resolution absolute/incremental angle sensors with on-axis, Hall scanning, and the general evaluation of sine/cosine signals (e.g., PC measuring cards for 1 VPP and 11 µAPP).

The iC-TW8 operates on a 3.1-to-5.5-V single-ended supply within a –40°C-to-125°C extended operating temperature range. It comes in a 48-pin QFN package that requires 7 mm × 7 mm of board space. A ready-to-operate demo board is  available for evaluation. An optional PC operating program, in other words, a GUI, can be connected with a USB adapter.

The iC-TW8 costs $7.69 in 1,000-unit quantities.

iC-Haus GmbH

www.ichaus.com


ULTRASOUND RECEIVERS

Analog Devices AD9675

The AD9675 and the AD9674 are the latest additions to Analog Devices’s octal ultrasound receiver portfolio. The devices and are pin compatible with the AD9670/AD9671.

The AD9675 is an eight-channel ultrasound analog front end (AFE) with an on-chip radio frequency (RF) decimator and Analog Devices’s JESD204B serial interface. It is designed for mid- to high-end portable and cart-based medical and industrial ultrasound systems. The device integrates eight channels of a low-noise amplifier, a variable-gain amplifier, an anti-aliasing filter, and a 14-bit ADC with a 125-MSPS sample rate and a 75-dB signal-to-noise ratio (SNR) performance for enhanced ultrasound image quality. The on-chip RF decimator enables the ADC to be oversampled, providing increased SNR for improved image quality while maintaining lower data I/O rates. The 5-Gbps JESD204B serial interface reduces ultrasound system I/O data routing.

The AD9674 offers similar functionality, but includes a standard low-voltage differential signaling (LVDS) interface. Both devices are available in a 144-ball, 10-mm × 10-mm ball grid array (BGA) package.

The AD9674 and the AD9675 cost $62 and $68, respectively.

Analog Devices, Inc.

www.analog.com


LOW-VOLTAGE DIGITAL OUTPUT HALL-EFFECT SENSORS

Melexis MLX92212

Melexis MLX92212

MLX92212 digital output Hall-effect sensors are AEC-Q100-qualified devices that deliver robust, automotive-level performance. The MLX92212LSE-AAA low-hysteresis bipolar latch and the MLX92212LSE-ABA high-hysteresis unipolar switch are optimized for 2.5-to-5.5-V operation. They pair well with many low-power microcontrollers in embedded systems. The sensor and specified microcontroller can share the same power rail. The sensors’ open-drain outputs enable simple connectivity with CMOS/TTL. They exhibit minimal magnetic switch point drift over temperature (up to 150°C) or lifetime and can withstand 8 kV electrostatic discharge.

The MLX92212LSE-AAA is designed for use with multipole ring magnets or alternating magnetic fields. It is well suited for brushless DC electric motor commutation, speed sensing, and magnetic encoder applications. Typical automotive uses include anti-trap/anti-pinch window lift controls, automatic door/hatch systems, and automatic power seat positioning. The MLX92212LSE-ABA enables the use of generic/weak magnets or larger air gaps. It can be used in simple magnetic proximity sensing and interlocks in covers/hatches or ferrous-vane interrupt sensors for precise position and timing applications.

Both MLX92212 devices utilize chopper-stabilized amplifiers with switched capacitors. The CMOS technology makes this technique possible and contributes to the sensors’ low current consumption and small chip size.

The MLX92212 sensors cost $0.35 each in 5,000-unit quantities and $0.30 in 10,000-unit quantities.

Melexis Microelectronic Integrated Systems

www.melexis.com


POWERFUL SPI ADAPTERS

Byte SPI Storm

Byte SPI Storm

The SPI Storm 50 and the SPI Storm 10 are the latest versions of Byte Paradigm’s SPI Storm serial protocol host adapter. The adapters support serial peripheral interface (SPI), Quad-SPI, and custom serial protocols in the same USB device.

The SPI Storm 50 and the SPI Storm 10 support serial protocols and master up to 50 and 10 MHz, respectively. The SPI Storm 10 features an 8-MB memory, while the higher-end devices are equipped with a 32-MB memory.

The SPI Storm adapters enable system engineers to access, communicate, and program their digital board and digital ICs, such as field-programmable gate array (FPGA), flash memories, application-specific integrated circuit (ASIC), and

system-on-a-chip (SoC). The SPI Storm 10 is well suited for engineering schools and universities because it is a flexible, all-around access device for hands-on digital electronics. The 50- and 100-MHz versions can be used in mid- and high-end testing and debugging for telecommunications, medical electronics, and digital imaging industries.

The SPI Storm 50 and the SPI Storm 10 cost $530 and $400, respectively.

Byte Paradigm

www.byteparadigm.com


ANALOG-BASED POWER MANAGEMENT CONTROLLER WITH INTEGRATED MCU

Microchip MCP19111

Microchip MCP19111

The MCP19111 digitally enhanced power analog controller is a new hybrid, digital and analog power-management device. In combination with the expanded MCP87xxx family of low-figure-of-merit (FOM) MOSFETs, it supports configurable, high-efficiency DC/DC power-conversion designs for many consumer and industrial applications.

The MCP19111 controller, which operates at 4.5 to 32 V, integrates an analog-based PWM controller with a fully functional flash-based microcontroller. This integration offers the flexibility of a digital solution with the speed, performance, and resolution of an analog-based controller.

The MCP19111 devices have integrated MOSFET drivers configured for synchronous, step-down applications. The MCP87018, MCP87030, MCP87090, and MCP87130 are 25-V-rated, 1.8-, 3-, 9-, and 13-mΩ logic-level MOSFETs that are specifically optimized for switched-mode-power-supply (SMPS) applications.

The MCP19111 evaluation board includes Microchip’s high-speed MOSFETs. This evaluation board includes standard firmware, which is user-configurable through an MPLAB X IDE graphical user interface (GUI) plug-in. The combined evaluation board, GUI, and firmware enable power-supply designers to configure and evaluate the MCP19111’s performance for their target applications.

The MCP19111 controllers cost $2.81 each and the MCP87018/030/090/130 MOSFETs cost $0.28 each, all in 5,000-unit quantities.

Microchip Technology, Inc.

www.microchip.com


ELASTOMER SOCKET FOR HIGH-SPEED QFP ICs

Ironwood SG-QFE-7011

Ironwood SG-QFE-7011

The SG-QFE-7011 is a high-performance QFP socket for 0.4-mm pitch, 128-pin QFPs. The socket is designed for a

1.6-mm × 14-mm × 14-mm package size with a 16-mm × 16-mm lead tip to tip. It operates at bandwidths up to 10 GHz with less than 1 dB of insertion loss and has a typical 20 mΩ per I/O contact resistance. The socket connects all pins with 10-GHz bandwidth on all connections. The small-footprint socket is mounted with supplied hardware on the target PCB. No soldering is required. The small footprint enables inductors, resistors, and decoupling capacitors to be placed close to the device for impedance tuning.

The SG-QFE-7011’s swivel lid has a compression screw that enables ICs to be quickly changed out. The socket features a floating compression plate to force down the QFP leads on to elastomer. A hard-stop feature is built into the compression mechanism.

The sockets are constructed with high-performance, low-inductance gold-plated embedded wire on elastomer as interconnect material between a device and a PCB. They feature a –35°C-to-100°C temperature range, a 0.15-nH pin self inductance, a 0.025-nH mutual inductance, a 0.01-pF capacitance to ground, and a 2-A per pin current capacity.

The SG-QFE-7011 costs $474.

Ironwood Electronics

www.ironwoodelectronics.com