An Examination of Low-Noise Amplifiers

An LNA, also known as a preamplifier, is an electronic amplification that is used to amplify weak signals. It is normally positioned close to the signal source to avoid signal power loss (antenna or sensor). Low-temperature measurements, optical detection, and audio engineering are only a few of the applications for which an LNA is suitable. This article addresses LNA systems and integrated circuits.

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

  • Gain is the ratio of output to input power or amplitude, which is generally expressed in decibels.
  • Bandwidth: The frequency range in which the amplifier operates properly.
  • The amount of noise introduced during the amplification process is referred to as noise.
  • The maximum rate of voltage shift per unit of time is known as the slew rate.
  • Overshoot: When a production bounces past its final value before calming down.

A preamplifier is a system that amplifies an analogue signal until it is sent to a higher-power amplifier.


As AC amplifiers, op-amps are commonly used. The LT1028 or LT1128 from Linear Technology and the ADA4898 or AD8597 from Analog Systems are also excellent ultra-low-noise amplifiers. The LT1128 is a high-speed, ultra-low-noise op-amp. It has the following main characteristics:

  • At 1 kHz, noise voltage is 0.85 nV/Hz.
  • 13 MHz bandwidth
  • 5 V/s slew rate
  • The offset voltage is 40 volts.

The Linear Technology and Analog Devices amplifiers both have a voltage noise density of about 1 nV/Hz at 1 kHz and have excellent DC accuracy. Texas Instruments (TI) produces low-noise amplifiers. The OPA211, for example, has a noise density of 1.1 nV/Hz at 3.6 mA from a 5 V supply current and the LME49990 has very low distortion. The MAX9632 from Maxim Integrated has noise below 1nV/Hz.

The op-amp can be implemented using either a bipolar junction transistor (BJT), as in the LT1128, or a MOSFET, which operates at higher frequencies, has a higher input impedance, and consumes less energy. The differential structure is used in applications where undesired generic components between the two inputs must be omitted. Low-frequency and DC common-mode signals (e.g., thermal drift) are removed at the output as a result of this. A differential gain is equal to (Ad = A2 – A1), whereas a common-mode gain is equal to (Ac = A1 + A2 = 2).

The common-mode rejection ratio (CMRR), which is the ratio of common-mode gain to differential-mode gain, is a significant parameter. This parameter is used to determine the efficiency of the differential amplifier.

The SR560 low-noise voltage preamplifier from Stanford Research Systems has a differential front end with 4nV/Hz input noise and a 100-M input impedance. A front-panel potentiometer, accessible with a small screwdriver, is used to null the input offset. A rear-panel TTL blanking input, in addition to the signal inputs, helps you to easily turn the instrument’s gain on and off.

This article briefly addressed low-noise amplifiers, including IC system designs that can be used in simple or complex systems, such as the Signal Recovery Model 5113, a classic amplifier capable of obtaining various frequency bands with relative gain. The SR560 is a high-performance, low-noise preamplifier that can be used for a range of applications such as low-temperature measurements, optical detection, and audio engineering.

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

A satellite communications system is an example of a typical LNA amplifier. Since the received signal is small, the ground station receiving antenna will be connected to an LNA. Typically, the obtained signal is slightly above background noise. Since satellites have little capacity, low-power transmitters are used.

A synchrodyne receiver is a radio receiver that uses synchronous detection powered by a local oscillator to demodulate an incoming radio signal. A single-frequency conversion is used to convert the signal (RF to IF) to baseband. To block signals outside of the reception band, the RF signal from the antenna is pre-filtered by a bandpass filter (BPF). The signal is amplified by the low-noise amplifier (LNA) stage and then down-converted by the mixer stage to zero intermediate frequency (IF). The difference between the RF and local oscillator signal frequencies is the IF signal frequency. The down-conversion process for phase and frequency-modulated signals should be done in quadrature to avoid signal sidebands from aliasing on each other. Since the local oscillator is located in the desired channel, both the upper and lower sidebands must be occupied by signal and noise. Since the down-conversion architecture creates an image with zero-IF frequency, there is no need for an image filter.

The fact that amplification and filtering are often done in the baseband rather than at the RF is a key feature of direct-conversion receivers. A low-pass-type baseband filter is used to pick the appropriate signal (BBF). At baseband, a low-pass filter with a bandwidth of half the symbol rate removes the neighbouring channels. Filters can be realised in on-chip without the use of external high-Q components since the filtering is done at low frequencies. The bulk of signal processing happens at low frequencies, which decreases power consumption. The DCR addresses the image rejection issue that other radio architectures have [2]. Insufficient amplitude and phase balance between in-phase and quadrature-phase signals, on the other hand, will increase the bit-error rate [3, 4]. It has its own drawbacks, such as being particularly vulnerable to flicker noise and DC offsets. High-pass filters can be used to address these problems in the wideband system architecture. The DCR prevents the superheterodyne’s two or three frequency conversions, IF stages, and image rejection issues by using a digital converter. Because of its simplicity, less off-chip components, and low power, the zero-IF is often common and commonly used for RF applications, according to recent research. The RF front-end of most receivers consists of an LNA, a mixer, and an oscillator. Direct conversion receivers are best suited to meet the following constraints in terms of construction and performance: simplicity, integration speed, fewer off-chip parts, and power dissipation. All of the front-end circuits mentioned here are intended for direct conversion receivers.

CMOS is one of the best technologies for implementing analogue designs without any adaptations due to its low processing cost. It can also improve the integration of high-performance analogue circuits with digital circuitry. It also allows for the implementation of the entire device on-chip, including the analogue front-end and optical demodulator, on the same die. The CMOS technology can run at a lower supply voltage than the BiCMOS technology. This is because each transistor has a standard turn on voltage of 0.7–0.8 V, requiring a minimum supply voltage of around 2–3 V for BJT circuits. Modern CMOS processes, on the other hand, deliver a range of threshold voltages (Vt), including high, moderate, and low standards. MOS transistors with a lower threshold, for example, can be used in analogue or digital circuits where speed is critical.

Devices with a higher threshold, on the other hand, are useful when the low-power consumption of digital circuits is impacted by leakage currents. Even when technology is scaled down to deep submicron CMOS processes, this feature helps circuitry design under low operating voltage.

Maurizio Di Paolo Emilio is an Italian telecommunications engineer who was born in Pescara. He helped design the thermal compensation system (TCS) for the optical system used in the Virgo Experiment while working primarily as a software developer with a focus on data acquisition systems (an experiment for detecting gravitational waves) he enjoys delving down in to the inner workings at one of the best Low Noise Amplifiers.

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