What Are Analog RF-CMOS Circuits?

Analog RF-CMOS circuits are radio-frequency electronic networks designed and fabricated using complementary metal-oxide-semiconductor (CMOS) process technology. They encompass the analog and mixed-signal building blocks, including low-noise amplifiers, mixers, voltage-controlled oscillators, power amplifiers, and phase-locked loops, that are integrated in standard digital CMOS processes alongside digital baseband circuits on a single chip. The field emerged in the 1990s as CMOS transistor feature sizes shrank to the point where device transit frequencies exceeded 10 GHz, making CMOS competitive with silicon bipolar and gallium arsenide processes for applications up to several gigahertz.

The principal attraction of CMOS for RF circuits is cost and integration density. Because the same foundry process used for microprocessors and memory can be used for RF front ends, a complete system-on-chip combining an RF transceiver with a digital processor and memory is achievable without the packaging complexity and cost of multi-technology hybrid assemblies. As documented in CMOS technology characterization for analog and RF design from UCLA, achieving this integration required developing models and measurement techniques for CMOS transistors at gigahertz frequencies, where parasitic gate resistance, substrate coupling, and non-quasi-static effects become important.

CMOS Device Physics at Radio Frequencies

At radio frequencies, the performance of a CMOS transistor is characterized by two figures of merit: the transit frequency fT, at which the current gain falls to unity, and the maximum oscillation frequency fmax, at which the power gain falls to unity. Both increase as gate length shrinks, and modern 28 nm and 16 nm FinFET processes achieve fT and fmax values exceeding 300 GHz, enabling CMOS amplifiers and oscillators at millimeter-wave frequencies. The principal noise mechanism at RF is channel thermal noise, supplemented by noise from the distributed gate resistance and substrate parasitics. Matching networks are required to transform device impedances to system reference impedances of typically 50 ohms, and their quality factor limits the achievable noise figure and gain.

Key RF Circuit Building Blocks

The low-noise amplifier (LNA) is the first active stage in a receiver and must amplify a weak antenna signal while adding minimal noise. Common CMOS LNA topologies include the inductively degenerated common-source amplifier, which achieves simultaneous input matching and minimum noise figure through resonance between a source inductance and the transistor input capacitance. The downconversion mixer translates the amplified RF signal to a lower intermediate or baseband frequency by multiplying it with a local oscillator signal, exploiting transistor nonlinearity in a controlled way. The voltage-controlled oscillator (VCO) generates the local oscillator signal, typically using an LC-tank resonator with a varactor for frequency tuning. Research on analog and RF circuit design techniques for nanometer CMOS from IEEE Xplore surveys the evolution of these building blocks as process geometries have scaled.

Transceiver Architectures

The choice of transceiver architecture determines how the RF building blocks are connected and what additional analog functions are needed. The superheterodyne architecture uses two frequency conversion steps, relaxing the requirements on individual filters but requiring an off-chip intermediate-frequency (IF) filter that prevents full integration. The direct-conversion (zero-IF) architecture downconverts directly to baseband in a single step, eliminating the IF filter and enabling higher integration, but introduces sensitivity to LO self-mixing, DC offset, and IQ gain and phase imbalance. Low-IF architectures use a non-zero IF low enough to be integrated but high enough to avoid DC-related problems. The direct-conversion receiver tutorial in Microwave Journal provides a detailed comparison of these architectures and their analog circuit requirements. Each architecture places different demands on the linearity, phase noise, and power consumption of the analog RF-CMOS building blocks.

Applications

Analog RF-CMOS has applications in a range of fields, including:

• Cellular handsets and base stations operating across 2G through 5G frequency bands
• Wi-Fi and Bluetooth short-range wireless transceivers in consumer and IoT devices
• GPS and GNSS receivers requiring low noise and accurate phase detection
• 60 GHz and millimeter-wave short-range links for wireless data transmission
• Radar and sensing systems for automotive and industrial applications