Preamplifiers and Low Noise Amplifiers
When you point a directional antenna at a distant station and turn up the receiver's AF gain, you hear noise — a hiss or roar that represents the combined noise of the receiver's front end amplified to the point where it fills the audio output. The goal of a preamplifier or low noise amplifier (LNA) is to amplify the desired signal before it reaches the noisier stages deeper in the receiver, so that the signal-to-noise ratio (SNR) of the system is limited by the preamplifier rather than by the more noise-producing stages that follow. Done correctly, an LNA dramatically improves the sensitivity of the entire receiver chain.
But preamplification is not always beneficial. On crowded HF bands, or when located near strong transmitters, adding gain before the receiver's first bandpass filter can worsen performance by allowing strong unwanted signals to overload the receiver and create intermodulation products. The correct use of a preamplifier — when to switch it in and when to bypass it — is one of the key operating skills for a competitive DX or weak-signal operator. This lesson explains both the theory and the practice.
- Noise in Amplifiers — The Basics
- Noise Figure
- Noise Temperature
- The Friis Cascaded Noise Formula
- Why the First Stage Dominates
- LNA Design Principles
- JFET and MOSFET LNA Topologies
- Mast-Mounted vs Shack-Mounted Preamplifiers
- When to Use — and When to Bypass — a Preamp
- Noise Figure and Cascaded NF Calculator
- Frequently Asked Questions
Noise in Amplifiers — The Basics
All resistors and transistors generate thermal noise — random voltage fluctuations caused by the random motion of charge carriers at any temperature above absolute zero. The noise power available from a resistor at temperature T (Kelvin) in a bandwidth B (Hertz) is:
where:
k = Boltzmann's constant = 1.381 × 10⁻²³ J/K
T = temperature in Kelvin (room temperature ≈ 290 K)
B = noise bandwidth in Hz
At 290 K in a 1 Hz bandwidth: P_noise = 1.381×10⁻²³ × 290 × 1 = 4.0×10⁻²¹ W = −174 dBm/Hz
This −174 dBm/Hz is the thermal noise floor at room temperature — the absolute minimum noise power that any signal must overcome to be detectable. It is also called kTB or the Johnson-Nyquist noise floor.
In a real amplifier, the output noise is always greater than kTB × G (where G is the power gain), because the amplifier itself adds noise from its transistors and resistors. The ratio of this added noise to the theoretical minimum is what noise figure measures.
Noise Figure
Noise figure (NF) is the most important specification of an LNA or preamplifier. It measures how much noise the device adds to the signal, expressed in decibels above the theoretical minimum:
Equivalently:
NF = 10 × log₁₀(F) where F is the noise factor (linear)
F = 1 + Te / T0 where Te is the equivalent noise temperature and T₀ = 290 K
A perfect noiseless amplifier would have NF = 0 dB (F = 1). Every real amplifier adds some noise, so NF > 0 dB. Some typical values:
| Component / System | Typical NF | Notes |
|---|---|---|
| 50 Ω resistor at 290 K | 0 dB (by definition) | The reference — a matched source resistance adds noise equal to kTB |
| Low-loss coaxial cable (e.g. LMR-400, 30 m at HF) | ≈ 0.5–1 dB | Feedline loss directly adds to system NF |
| GaAs MMIC LNA (e.g. PGA-103+) | 0.5–1.0 dB at VHF/UHF | Best available for consumer applications |
| JFET common-gate LNA (home-built HF) | 1–3 dB at HF | Good for 160 m through 10 m |
| Silicon BJT common-base LNA | 2–4 dB at HF | Simple, inexpensive, widely used |
| Typical HF transceiver front end (no preamp) | 10–20 dB | Dominated by mixer and IF chain noise |
| Typical VHF/UHF transceiver front end | 5–10 dB | Improved over HF due to optimised RF front ends |
Noise Temperature
Noise temperature is an alternative way to express noise performance that is more convenient for radio astronomy, satellite work, and EME (Earth-Moon-Earth) links. The equivalent noise temperature T_e is the temperature to which a hypothetical noiseless resistor would need to be heated to produce the same noise power as the actual device:
Conversely: NF = 10 × log₁₀(1 + Te/290)
Examples:
NF = 1 dB → T_e = 290 × (1.259 − 1) = 75 K
NF = 3 dB → T_e = 290 × (2.0 − 1) = 290 K
NF = 6 dB → T_e = 290 × (4.0 − 1) = 870 K
NF = 0.5 dB → T_e = 290 × (1.122 − 1) = 35 K
Noise temperature is especially useful when dealing with antennas: a yagi antenna pointed at cold sky has an antenna noise temperature of perhaps 30–100 K (because the antenna receives very little thermal radiation from cold empty sky). If the LNA has a noise temperature of 50 K, the system noise temperature is 80–150 K. At HF, the ionosphere and atmospheric noise contribute equivalent noise temperatures of thousands of Kelvin, making a 1 dB vs 3 dB NF difference largely irrelevant — the external noise dominates. At VHF and above, the antenna temperature can be very low and the LNA's noise temperature is critical.
The Friis Cascaded Noise Formula
When amplifier stages are cascaded — connected in series — the total system noise figure is not simply the sum of the individual noise figures. The Friis formula gives the combined noise factor F_total for N stages in series:
where F₁, F₂, F₃… are the noise factors (linear) of each stage
and G₁, G₂, G₃… are the power gains (linear) of each stage
NFtotal = 10 × log₁₀(Ftotal)
System: LNA → 30 m coaxial cable → transceiver front end
LNA: NF₁ = 1 dB, G₁ = 20 dB
Cable loss: 1.5 dB (= NF₂ = 1.5 dB, G₂ = −1.5 dB, i.e. gain = 10^(−1.5/10) = 0.708)
Transceiver: NF₃ = 12 dB
Convert to linear:
F₁ = 10^(1/10) = 1.259, G₁ = 10^(20/10) = 100
F₂ = 10^(1.5/10) = 1.413, G₂ = 0.708
F₃ = 10^(12/10) = 15.85
F_total = 1.259 + (1.413−1)/100 + (15.85−1)/(100×0.708)
= 1.259 + 0.00413 + 14.85/70.8
= 1.259 + 0.00413 + 0.2097
= 1.4728
NF_total = 10 × log₁₀(1.4728) = 1.68 dB
Result: The system noise figure is 1.68 dB — dominated by the 1 dB LNA. The 12 dB transceiver NF contributes only 0.21 dB to the total because the LNA's 20 dB gain reduces its contribution by a factor of 100.
Without LNA (cable + transceiver only):
F_total = 1.413 + (15.85−1)/0.708 = 1.413 + 20.97 = 22.38
NF_total = 10 × log₁₀(22.38) = 13.5 dB
Adding the LNA improves system NF from 13.5 dB to 1.68 dB — an 11.8 dB improvement in noise figure, equivalent to more than 11 dB improvement in sensitivity.
Why the First Stage Dominates
The Friis formula makes clear that later stages contribute to system noise only in proportion to 1 divided by the product of all preceding gains. With a first-stage gain of 100 (20 dB), the second stage contributes only 1% of its own noise factor to the total. With a gain of 1000 (30 dB), the second stage contributes 0.1%.
This has a crucial practical implication: everything before the LNA matters enormously; everything after it matters very little. Specifically:
- Feedline loss before the LNA directly adds to system NF, dB for dB. A 3 dB cable loss adds 3 dB to system NF.
- Feedline loss after the LNA hardly matters — a 3 dB loss is divided by the LNA's gain before contributing to system NF.
- A lossy input filter, switch, or connector before the LNA degrades system NF. The same component after the LNA has negligible effect.
- A low-noise LNA with 0.5 dB NF and 20 dB gain reduces the contribution of all following stages by 100x.
This is why mast-mounted preamplifiers are so effective for weak-signal VHF/UHF work: placing the LNA at the antenna feedpoint, before any feedline loss, gives the best possible system noise figure. The penalty is the difficulty of powering the LNA (usually via DC on the coaxial cable, called "bias-T" injection) and accessing it for servicing.
LNA Design Principles
A low noise amplifier must be optimised for minimum noise figure, which is a different goal from maximum gain or maximum efficiency. The transistor's noise performance depends on its bias point, input impedance matching, and device selection.
Key Design Rules
- Minimum noise bias point: Each transistor has an optimum collector/drain current for minimum noise figure — typically lower than the current for maximum gain. For a 2N3904 BJT, minimum NF occurs at IC ≈ 0.5–1 mA (much lower than the 2 mA used for maximum gain).
- Noise-matching the input: Maximum gain and minimum noise figure require different input matching conditions. A simultaneous noise-and-power match is only possible at one specific source impedance (the optimum source impedance, Z_opt). Most LNA designs use input matching for minimum NF rather than maximum gain.
- Low-noise bias network: The bias resistors themselves add noise. R2 (lower bias resistor) appears directly in parallel with the transistor's base, adding thermal noise. To minimise this, R2 should be large, or RF bypass capacitors should effectively short-circuit it at signal frequencies.
- Common-gate/common-base topology: Common-gate (FET) and common-base (BJT) configurations have lower input impedance and different noise characteristics from common-source/common-emitter. Common-gate JFETs are popular for HF LNAs because they provide low noise at HF and have good reverse isolation (low Miller capacitance).
- Feedback: Resistive feedback (shunt or series) can be used to set the input impedance to 50 Ω but it also adds noise. Inductive source degeneration (a small inductance in the source/emitter lead) sets the input impedance without adding resistive noise — this technique is used in the lowest-noise LNAs.
JFET and MOSFET LNA Topologies
For HF and VHF ham radio work, JFET and MOSFET LNAs are preferred over BJT designs because:
- JFETs have very low 1/f noise at audio frequencies (important for receive audio) and comparable HF noise to BJTs
- MOSFETs (especially dual-gate MOSFETs) offer a convenient second gate for gain control (AGC)
- Both types have high input impedance, making them easy to match to antenna systems
- JFETs tolerate static discharge better than MOSFETs (though antenna lightning protection is still essential)
Common Topologies and Their Applications
| Topology | Device | Typical NF | Application |
|---|---|---|---|
| Common-source (CS) | J310, BF245, MPF102 JFET | 1–3 dB at HF, 1–2 dB at VHF | HF/VHF receive preamplifiers, general-purpose LNA |
| Common-gate (CG) | J309, BF245 JFET | 2–4 dB at HF | HF LNA where 50 Ω input is needed without matching network |
| Dual-gate MOSFET | BF998, 40673, 3SK88 | 1–4 dB at HF/VHF | HF/VHF preamplifier with AGC capability |
| GaAs MMIC | PGA-103+, SPF5189Z, ERA-5SM | 0.5–1.5 dB at VHF/UHF | VHF/UHF/microwave LNA — best NF readily available |
| Common-emitter (CE) BJT | BFR92, AT-41435 | 1–4 dB at HF, 2–5 dB at VHF | Simple HF preamp, driver stages |
The Dual-Gate MOSFET — Ham Radio Favourite
The dual-gate MOSFET (e.g. BF998, 40673) deserves special mention because it combines low noise with voltage-controlled gain. Gate 1 receives the signal through the input matching network. Gate 2 receives the AGC (automatic gain control) voltage from the receiver. Increasing the Gate 2 voltage increases drain current and gain; decreasing it reduces gain. This allows the receiver's AGC system to control the preamplifier's gain continuously, reducing the risk of strong-signal overload while maintaining sensitivity on weak signals. Dual-gate MOSFETs are found in virtually every HF/VHF receiver designed between 1970 and 1990 and remain popular in home-built circuits today.
Mast-Mounted vs Shack-Mounted Preamplifiers
| Parameter | Mast-Mounted | Shack-Mounted |
|---|---|---|
| System NF improvement | Maximum — amplifies before feedline loss | Limited — feedline loss appears before the preamp and directly degrades system NF |
| IMD / dynamic range | Worse — all strong signals arrive at full antenna level | Better — feedline loss attenuates both desired and interfering signals equally |
| Powering | DC via bias-T injector on coax cable | Standard mains or 12 V from shack supply |
| Transmit protection | Essential — TR switching relay or PIN diode limiter must disconnect LNA during transmit | Easier — can use standard antenna relay in shack |
| Maintenance | Requires climbing or lowering the mast | Easily accessible in shack |
| Best applications | EME, satellite, meteor scatter, weak-signal VHF/UHF/microwave | HF DX listening, general-purpose sensitivity boost |
When to Use — and When to Bypass — a Preamp
Adding a preamplifier always improves sensitivity (the ability to hear weak signals). But it also always reduces dynamic range (the ability to handle strong signals without distortion). The correct choice depends on which factor is limiting performance at any given moment:
- Use the preamplifier when: The noise floor is limiting you — the desired signal is too weak to decode even with the receiver's gain turned up fully. This is common during meteor scatter, EME, or working weak DX on VHF. Also use it when the band is quiet and you are noise-limited, not signal-limited.
- Bypass the preamplifier when: Strong signals from nearby transmitters are causing IMD or blocking. On crowded HF bands during contests, a preamplifier often makes performance worse, not better. When S-meter is already reading S5 or above on the noise, you are externally-noise-limited and a preamp adds no benefit.
- Use the attenuator instead: When S-meters are pinned and signals are extremely strong, switch in a 10–20 dB attenuator to protect the receiver front end and improve signal-to-IMD ratio.
A useful rule of thumb: at HF below 30 MHz, external noise usually dominates above S3–S4 signal levels, making a preamplifier unnecessary and potentially harmful. At VHF and above, a good mast-mounted LNA almost always improves performance because the antenna temperature is very low (cold sky).
Noise Figure and Cascaded NF Calculator
Noise Figure and Cascaded System NF Calculator (Friis Formula)
Enter NF and gain for up to 5 stages in sequence (LNA first, then any losses, then receiver). The calculator applies the Friis formula and shows system NF and noise temperature. Leave unused stages blank or at 0 dB gain.
Frequently Asked Questions
Why does feedline loss before a preamplifier hurt so much, but loss after it barely matters?
The Friis formula shows that each stage's noise contribution is divided by the total gain of all preceding stages. Loss before the LNA has no gain preceding it (gain = 1) to divide its noise contribution, so it adds directly to system noise figure dB for dB. After the LNA, there is 20 dB (or more) of gain dividing the loss's noise contribution — 1 dB of loss after a 20 dB gain LNA contributes only 0.01 dB to system NF. This is why mast-mounted preamplifiers are so effective: they place high gain before the feedline loss, dramatically reducing the feedline's contribution to system noise figure.
Why is a preamplifier sometimes harmful on crowded HF bands?
A preamplifier amplifies everything — both the desired weak signal and strong interfering signals on adjacent frequencies. If two strong signals at frequencies f1 and f2 arrive at the receiver after passing through the preamplifier, the preamplifier's own non-linearity generates intermodulation products at 2f1−f2 and 2f2−f1. These products can fall right on the frequency you are trying to receive. Without the preamplifier, the interfering signals arrive at lower level and the receiver's first mixer or IF amplifier generates less IMD. Additionally, if you are in a high-noise RF environment (urban area, nearby broadcast transmitters), external noise dominates and more gain simply raises both the signal and the noise equally — no SNR benefit. In such cases, bypassing the preamp and possibly switching in an attenuator gives cleaner reception.
What is a bias-T and why is it needed for mast-mounted LNAs?
A bias-T (bias tee) is a device that allows DC power to be sent through a coaxial cable to power a remote device, while keeping the DC separated from the RF signal path at each end. The bias-T has three ports: RF in/out, DC in, and combined RF+DC on the cable. An RF choke (high impedance at RF) passes DC but blocks RF; a blocking capacitor passes RF but blocks DC. In a mast-mounted LNA installation, the shack end sends 12 V DC through the coaxial feedline to power the LNA. The LNA's built-in bias-T extracts the DC power while passing the amplified RF signal. Without a bias-T, you would need a separate power cable to the mast — impractical in most installations.
At what frequency does an external preamplifier become necessary in ham radio?
The answer depends on the noise environment. At HF (below 30 MHz), the ionosphere, galactic noise, and man-made interference create an external noise temperature of thousands to tens of thousands of Kelvin — vastly exceeding any practical LNA's noise temperature. This means the receiver's noise figure has negligible effect on SNR when receiving typical signals. An LNA only helps at HF when signals are weaker than the receiver's noise floor, which is rare except in deep nulls or on quiet bands at night. At VHF and above (especially above 144 MHz), external noise is much lower and the receiver's noise figure matters greatly. A 1 dB improvement in NF is easily heard as an S-meter improvement on 2 metres or 70 cm. For EME and satellite work, where the antenna sees cold sky at 10–30 K, even 0.1 dB of NF improvement translates to a meaningful SNR gain.
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