Editing Low-noise amplifier (section)

== Design considerations ==
Low noise amplifiers are the building blocks of communication systems and instruments. The most important LNA specifications or attributes are:<ref>{{Cite web|url=https://www.onesdr.com/2020/01/11/an-introduction-to-low-noise-amplifier-specifications/|title=An Introduction to Low Noise Amplifier Specifications|date=2020-01-11|website=Software-Defined Radio Simplified|language=en-US|access-date=2020-01-11}}</ref>
* Gain
* Noise figure
* Linearity
* Maximum RF input

A good LNA has a low NF (e.g. {{val|1|u=dB}}), enough gain to boost the signal (e.g. {{val|10|u=dB}}) and a large enough inter-modulation and [[compression point]] (IP3 and P1dB) to do the work required of it. Further specifications are the LNA's operating bandwidth, gain flatness, stability, input and output [[voltage standing wave ratio]] (VSWR).

For low noise, a high amplification is required for the amplifier in the first stage. Therefore, junction field-effect transistors [[Junction Field-Effect Transistor|(JFETs)]] and [[high-electron-mobility transistor]]s (HEMTs) are often used. They are driven in a high-current regime, which is not energy-efficient but reduces the relative amount of [[shot noise]]. It also requires input and output [[impedance matching]] circuits for [[narrow-band]] circuits to enhance the gain (''see [[Gain-bandwidth product]]'').

=== Gain ===
Amplifiers need a device to provide gain.  In the 1940s, that device was a [[vacuum tube]], but now it is usually a transistor.  The transistor may be one of many varieties of [[bipolar transistor]]s or [[field-effect transistor]]s. Other devices producing gain, such as [[tunnel diode]]s, may be used.

Broadly speaking, two categories of transistor models are used in LNA design: Small-signal models use quasi-linear models of noise and large-signal models consider non-linear mixing.

The amount of gain applied is often a compromise. On the one hand, high gain makes weak signals strong.  On the other hand, high gain means higher-level signals, and such high-level signals with high gain may exceed the amplifier's dynamic range or cause other types of noise such as harmonic distortion or nonlinear mixing.

===Noise figure===
The [[noise figure]] helps determine the efficiency of a particular LNA. LNA suitability for a particular application is typically based on its noise figure. In general, a low noise figure results in better signal reception.

=== Impedance ===
The circuit topology affects input and output impedance. In general, the source impedance is matched to the input impedance because that will maximize the power transfer from the source to the device. If the source impedance is low, then a [[common base]] or [[common gate]] circuit topology may be appropriate. For a medium source impedance, a [[common emitter]] or [[common source]] [[Topology (electrical circuits)|topology]] may be used. With a high source resistance, a [[common collector]] or [[common drain]] topology may be appropriate. An input [[impedance match]] may not produce the lowest noise figure. <!--There is another notion of a noise impedance match. Need a description of noise models. -->

=== Biasing ===
{{Expand section|date=August 2019}}
Another design issue is the noise introduced by [[Bipolar transistor biasing|biasing networks]]. In communication circuits, biasing networks play a critical role in establishing stable operating points for active components, but they also introduce noise. The primary types of noise introduced by these networks are thermal noise and flicker noise. Thermal noise arises from resistive elements in the network, which is inevitable as any resistive component generates noise due to the random motion of charge carriers. This type of noise is especially problematic at high frequencies. Flicker noise, also known as 1/f noise, is related to the current flow through devices like transistors and becomes more significant at lower frequencies.<ref>{{Citation |last1=Honnaiah |first1=Puneeth Jubba |title=Design of a Low Noise Amplifier |date=2019-12-30 |url=https://arxiv.org/abs/1912.13029 |access-date=2024-09-16 |arxiv=1912.13029 |last2=Reddy |first2=Shridhar}}</ref>

For instance, in low-noise amplifiers (LNA), the biasing network must be carefully designed to minimize the impact of noise on the overall performance. Improper biasing can lead to increased noise figures, compromising the signal-to-noise ratio and degrading communication system performance. The design and selection of components within the bias network are therefore crucial to ensuring low-noise operation, particularly in systems that rely on amplifying weak signals.<ref>{{Cite journal |last1=Zhao |first1=Jinxiang |last2=Wang |first2=Feng |last3=Yu |first3=Hanchao |last4=Zhang |first4=Shengli |last5=Wang |first5=Kuisong |last6=Liu |first6=Chang |last7=Wan |first7=Jing |last8=Liang |first8=Xiaoxin |last9=Yan |first9=Yuepeng |date=2022-02-18 |title=Analysis and Design of a Wideband Low-Noise Amplifier with Bias and Parasitic Parameters Derived Wide Bandpass Matching Networks |journal=Electronics |language=en |volume=11 |issue=4 |pages=633 |doi=10.3390/electronics11040633 |doi-access=free |issn=2079-9292}}</ref>

In addition, matching networks and careful biasing techniques, such as using low-noise transistors and optimizing impedance matching, help mitigate the noise effects introduced by bias circuits<!-- Motchenbacher -->