LNA Before vs After Mixer: Receiver Architecture
In receiver design, the placement of the LNA relative to the mixer dramatically affects system noise figure and dynamic range. The Friis formula shows that early gain (LNA first) reduces the noise contribution of subsequent stages, but also compresses the dynamic range. Choosing the right architecture depends on the application's sensitivity and linearity requirements.
LNA Before Mixer (Standard Receiver)
The standard superheterodyne receiver places the LNA as the first active stage, before the mixer. The LNA's gain suppresses the noise contribution of the mixer and subsequent stages, giving the best possible noise figure.
Advantages
- Best noise figure — LNA gain reduces mixer noise contribution by G_LNA
- Friis formula: NF_sys ≈ NF_LNA + (NF_mixer − 1)/G_LNA
- Required for weak-signal applications (satellite, radio astronomy)
- Standard in all cellular, WiFi, and Bluetooth receivers
Disadvantages
- LNA gain reduces input dynamic range — large signals saturate the LNA
- LNA IIP3 limits system IIP3 — intermodulation products appear early
- More susceptible to blocking by strong adjacent channels
When to use
Use LNA-first for any receiver where sensitivity is the primary concern: satellite ground stations, IoT, WiFi, cellular uplink, and radio astronomy.
Mixer Before LNA (High-Linearity Receiver)
Some specialized receivers place a filter and mixer before the LNA to down-convert the signal first. This preserves dynamic range at the input but increases system noise figure by the mixer's loss divided by its noise figure.
Advantages
- Better dynamic range at the antenna port
- Useful when strong blockers are present — mixer handles them before amplification
- Used in direct-conversion and some SDR architectures
Disadvantages
- Much worse noise figure — mixer has 6–8 dB conversion loss added to NF
- Only viable when the front-end filter provides sufficient image rejection
- Rarely used in commercial receivers due to sensitivity penalty
When to use
Use mixer-first only in high-dynamic-range, strong-signal environments (HF/shortwave receivers near broadcast transmitters, radar) where blocking resistance outweighs sensitivity loss.
Key Differences
- ▸LNA-first: best NF, limited by LNA IIP3; Mixer-first: worst NF, better dynamic range
- ▸Friis shows first-stage gain directly suppresses subsequent noise by G1
- ▸LNA-first NF ≈ NF_LNA; mixer-first NF ≈ NF_LNA + L_mixer (6–8 dB penalty typical)
- ▸Nearly all commercial receivers use LNA-first; mixer-first is niche (HF, radar)
- ▸Attenuator before LNA is a compromise — increases NF by attenuator value, improves IIP3
Summary
LNA-before-mixer is correct for virtually all sensitivity-constrained receivers. Mixer-before-LNA is a niche choice for environments dominated by strong blockers where dynamic range matters more than sensitivity. When in doubt, put the LNA first and use a high-IIP3 LNA if blocking is a concern.
Frequently Asked Questions
Why does the first stage dominate system noise figure?
From the Friis formula, each subsequent stage's noise contribution is divided by the cumulative gain of all preceding stages. A 20 dB gain LNA reduces the mixer's noise contribution by 100×. This is why LNA selection is the most critical noise decision in a receiver chain.
What is a typical LNA noise figure?
Commercial LNAs for 1–6 GHz achieve 0.3–1.5 dB NF. At 2.4 GHz (WiFi), 0.8–1.2 dB is common. At 28 GHz (5G mmWave), 3–5 dB is typical. Cryogenic LNAs for satellite achieve < 0.15 dB NF.
Should I put a filter before or after the LNA?
A bandpass filter before the LNA reduces out-of-band blockers and improves LNA IIP3 margin, at the cost of insertion loss that directly increases system NF. A filter after the LNA avoids the NF penalty. The correct placement depends on blocker levels. High-performance designs use both (pre-select + post-LNA image-reject filter).
What is the IIP3 of a typical LNA?
LNA IIP3 typically ranges from −5 to +15 dBm. Higher IIP3 means better linearity but usually higher current. For cellular applications, +10 to +20 dBm OIP3 is typical. The system IIP3 is dominated by the first nonlinear stage (usually the LNA or mixer).