Low Noise Amplifier Symbol
Definition: The Low Noise Amplifier (LNA) symbol represents a two-port active amplifier block placed at the input of a receiver chain that provides signal gain with the minimum possible added noise, characterised by its noise figure (NF, in dB), gain (G, in dB), and input/output impedance (typically 50 Ω), drawn in schematics as a triangular or rectangular amplifier block labelled 'LNA' with an In pin on the left and an Out pin on the right, with no dedicated symbol in IEC 60617 or IEEE 315.
Also known as: LNA, low-noise amplifier, low-noise preamp, RF preamplifier, front-end amplifier, receive amplifier.
What the Low Noise Amplifier symbol means
The Low Noise Amplifier (LNA) symbol identifies the first active gain stage in a radio-frequency (RF) or microwave receiver chain, positioned immediately after the antenna and any RF band-pass filter. Its defining characteristic is a low noise figure — typically 0.3–3 dB — meaning it adds very little noise to the signal it amplifies. According to the Friis formula for cascaded noise, the first amplifier in a chain dominates the system's total noise performance; therefore, the LNA's noise figure and gain determine the receiver's sensitivity and minimum detectable signal level.
In RF system block diagrams and schematics, the LNA symbol marks the point where the tiny received signal (often −90 to −120 dBm) is amplified to a level where subsequent mixing, filtering, and demodulation stages can process it without noise from those stages degrading the signal-to-noise ratio (SNR). The In pin connects to the antenna or RF filter output; the Out pin connects to the subsequent stage (typically a mixer or another amplifier). The LNA must also present a low reflection (well-matched input impedance, typically 50 Ω) to avoid reflecting signal power back toward the antenna.
How to identify the Low Noise Amplifier symbol
The LNA symbol is drawn as either a triangle (pointing right, with the apex at the output) — the standard amplifier glyph used for all analogue amplifiers — or a rectangular function block, labelled 'LNA' or 'Low Noise Amplifier'. The In pin (signal input) connects to the left side of the triangle or the left edge of the block, and the Out pin connects to the right side or right edge. Some representations add 'NF = X dB' and 'G = Y dB' annotations inside or beside the block to specify the key performance parameters. A separate DC supply pin (VCC or VDD) and a bypass capacitor may also be shown connected to the block.
Function in a circuit
An LNA amplifies a weak RF signal received from an antenna while adding the minimum possible thermal noise. The circuit is typically built around a low-noise transistor (GaAs pHEMT, SiGe HBT, or RF MOSFET) biased in its optimal low-noise operating region. Input matching is achieved with an inductive degeneration network or a shunt-series LC network that simultaneously achieves minimum noise figure and acceptable input return loss (S11 < −10 dB). The LNA's voltage gain (typically 15–25 dB) raises the signal level so that the noise contributions of subsequent stages (mixer, IF amplifier) are negligible relative to the amplified signal, improving overall receiver noise figure according to the Friis cascaded noise formula: NF_total = NF1 + (NF2 - 1)/G1 + ...
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated symbol for low noise amplifiers. In IEC-style system block diagrams the LNA is represented as a triangular amplifier symbol (per IEC 60617-09, analogue amplifier convention) or a rectangular function block labelled 'LNA', following the general functional-block drawing standards. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) defines the general amplifier symbol as a triangle; this symbol is used for the LNA in North American RF schematic drawings, with the In pin at the triangle's left vertex and the Out pin at the right apex. The block may be labelled 'LNA' to distinguish it from other amplifier types. |
| Key difference | IEC 60617-09 and IEEE 315-1975 both use the triangle or rectangle for general amplifiers; neither defines a specific glyph for low noise amplifiers. The LNA annotation inside or beside the block is the distinguishing indicator in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Noise figure (NF): 0.3–1.0 dB (GaAs pHEMT, low-noise optimised), 1.0–3.0 dB (SiGe integrated LNA), 3–6 dB (CMOS RF LNA); Gain: 15–25 dB (typical); Input/output impedance: 50 Ω (standard RF system impedance); Operating frequency: 100 MHz–100 GHz (device and technology dependent); Input 1 dB compression point (P1dB): −15 to +5 dBm; IIP3 (input third-order intercept point): −5 to +15 dBm; Supply voltage: 1.5–5 V DC; Current consumption: 5–50 mA.
Where the Low Noise Amplifier symbol is used
- Cellular base station and handset receivers: LNAs in the 700 MHz–5 GHz range amplify received signals from the antenna before the RF mixer stage
- Software-defined radio (SDR) front ends: wide-band LNAs (100 MHz–6 GHz) amplify signals before the wideband ADC or down-conversion mixer in HackRF, RTL-SDR, and USRP receivers
- GPS and GNSS receivers: LNAs at 1575.42 MHz (GPS L1) amplify −130 dBm satellite signals; many GPS antennas include an integrated LNA at the feed point
- Satellite communication (VSAT, SatCom) ground stations: LNAs with NF < 0.5 dB are used in the LNB (low-noise block downconverter) of satellite dish receivers to achieve the lowest possible system noise temperature
- Radar receiver front ends: LNAs amplify the radar return echo before the pulse-compression or Doppler-processing stages
- Radio telescope receivers: cryogenically cooled LNAs (NF < 0.1 dB) are used in deep-space antenna systems to detect extremely weak astronomical signals
Example
In the RF front-end of a 2.4 GHz Bluetooth receiver, an integrated SiGe LNA (NF = 1.5 dB, G = 18 dB, IIP3 = −10 dBm) is shown between the antenna matching network and the quadrature mixer. The LNA block symbol is labelled 'LNA / NF = 1.5 dB / G = 18 dB'; its In pin connects to the antenna-side bandpass filter output (50 Ω) and its Out pin connects to the mixer RF input port. A 10 nF bypass capacitor decouples the VDD supply pin, and a 10 kΩ bias resistor sets the transistor quiescent current. The block-level schematic uses this LNA symbol to abstract the multi-transistor matching and biasing circuitry.
Key facts
- A Low Noise Amplifier (LNA) is the first active gain stage in an RF receiver chain; its primary specifications are noise figure (NF, dB), gain (dB), and input/output impedance (typically 50 Ω).
- The Friis cascaded noise formula shows that the system noise figure is dominated by the first stage: NF_total ≈ NF_LNA + (NF_2 - 1) / G_LNA; therefore the LNA must have both low NF and sufficient gain to suppress noise contributions from subsequent stages.
- The LNA symbol in schematics is a triangular amplifier glyph (pointing right) or a labelled rectangular block; IEC 60617 and IEEE 315-1975 define no dedicated LNA glyph and use the generic amplifier triangle or rectangle.
- LNA noise figure ranges from < 0.3 dB (cryogenically cooled GaAs pHEMT) to ~3 dB (CMOS integrated), with 1–2 dB typical for silicon-germanium (SiGe) consumer RF ICs.
- The LNA must present a well-matched input impedance (typically 50 Ω, S11 < −10 dB) to avoid reflecting signal power back toward the antenna, which would degrade the effective noise figure.
- An LNA has two RF signal pins in schematics: In (connected to antenna/filter output) and Out (connected to subsequent receive chain stage); supply voltage and ground pins are also present but often omitted from block-level diagrams.
- The gain-noise trade-off in LNA design is fundamental: biasing for minimum noise figure (NF_min) does not coincide with biasing for maximum gain; the optimum source impedance for minimum noise (Zopt) differs from the impedance for maximum gain (conjugate match).
Frequently asked questions
What does the LNA symbol look like in an RF schematic?
The LNA symbol is a right-pointing triangle (the standard amplifier glyph) or a rectangular block labelled 'LNA' or 'Low Noise Amplifier'. The In pin connects to the left side (antenna/filter input) and the Out pin connects to the right apex or right edge (mixer or next stage). Performance annotations — noise figure (NF) and gain (G) in dB — may appear inside or alongside the symbol.
What does LNA stand for and what does it do?
LNA stands for Low Noise Amplifier. It amplifies a very weak RF signal received from an antenna while adding the minimum possible thermal noise, preserving the signal-to-noise ratio (SNR) before the signal enters the noisier stages of the receiver chain (mixer, IF amplifier, ADC). The LNA's noise figure and gain determine the overall sensitivity of the radio receiver.
What is noise figure and why does it matter for an LNA?
Noise figure (NF) is the increase in signal-to-noise ratio degradation caused by an amplifier, expressed in dB; a perfect noiseless amplifier has NF = 0 dB. For an LNA, a lower NF means less noise is added to the received signal, enabling the receiver to detect weaker signals. Because the Friis formula shows that the first stage dominates total system noise, the LNA's NF has the greatest impact on receiver sensitivity compared to any other component in the chain.
What is a typical LNA noise figure?
Typical LNA noise figures range from < 0.3 dB for cryogenically cooled GaAs pHEMT amplifiers used in radio telescopes, to 0.5–1.0 dB for room-temperature GaAs or InP discrete LNAs, 1.0–2.0 dB for SiGe HBT integrated LNAs in commercial RF chipsets, and 2.0–4.0 dB for CMOS RF LNAs in low-cost consumer chips. GPS receiver LNAs typically achieve 1.0–1.5 dB NF.
Where is the LNA placed in a receiver circuit?
The LNA is placed as the first active stage immediately after the antenna, following any passive components such as a band-select filter, antenna matching network, or RF switch. It must be as close to the antenna as physically possible to minimise cable losses between the antenna and the LNA input, since any loss before the LNA directly adds to the system noise figure (1 dB of cable loss adds 1 dB to system NF).
What standard defines the LNA schematic symbol?
Neither IEC 60617 nor IEEE 315-1975 (ANSI Y32.2) defines a dedicated symbol for low noise amplifiers. The LNA is represented using the general amplifier triangle symbol from IEC 60617-09 or IEEE 315-1975, with the label 'LNA' added inside or beside the triangle or rectangular block to identify the function.
What is the difference between an LNA and a general RF amplifier?
An LNA is a specialised RF amplifier optimised for the lowest possible noise figure, typically at the cost of some linearity (lower IIP3) and gain compared to a general-purpose RF amplifier. A general RF amplifier or power amplifier (PA) is optimised for gain, output power, and linearity rather than noise. The LNA is used only in the receive signal path where signal levels are very small; RF power amplifiers are used in the transmit path where output power is the priority.
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