# Design Techniques for Low-Voltage RF/mm-Wave Circuits in Nanometer CMOS Technologies
## 4 Circuit Topologies for Low-Voltage Operation of RF/mm-Wave Blocks  
Geometrical scaling enabled RF/mm-wave applications for nanometer CMOS thanks to transition frequency, \(f_{\text{T}}\), and maximum oscillation frequency, \(f_{\text{MAX}}\), values close to, or even higher than, 200 GHz. Such performance improvements have been obtained at the cost of a considerable reduction in the transistor breakdown voltage (BV), and, consequently, of the maximum supply voltage in comparison with equivalent HBT BiCMOS technologies. This poses new challenges for the design of RF/mm-wave CMOS front ends that are still the very bottleneck for several applications (e.g., 5G, automotive radar, and imaging). The turning point is to properly adjust the well-established RF/mm-wave topologies to mitigate low-voltage operation drawbacks. To this aim, the following simple guidelines can be applied to the main circuit blocks, such as LNA, mixers, and PAs:  
* Reduce the operative threshold voltage, \(V_{\text{T}}\),
* Increase the transconductance, \(g_{\text{m}}\), by means of positive feedback;
* Use only one transistor between the supply voltage, \(V_{\text{DD}}\), and ground;
* Adopt transformer-based power combining techniques. Figure 4: The 28-nm FD-SOI CMOS technology: (**a**) simplified cross-section of LVT flipped-well transistors, (**b**) \(V_{\text{T}}\) variation with respect to the body voltage for the LVT flipped-well devices. Reprinted with permission from Ref [25]. Copyright 2019, IEEE. A reduction in the \(V_{\rm T}\) is strongly suggested to operate the transistor with a lower gate-source voltage, \(V_{\rm GS}\)[26]. It can be achieved by exploiting the body biasing technique in standard CMOS, and more profitably in FD-SOI CMOS technologies, as discussed in Section 3. This approach is very effective, but often requires the implementation of additional circuitry, namely, body biasing generators [27; 28]. Moreover, reducing \(V_{\rm T}\) by means of body biasing could be in contrast with the exploitation of such a technique to compensate for the process-voltage-temperature (PVT) variations, which are typically quite large in nanometer CMOS, especially for automotive applications [29; 30]. To further increase the gain performance of a transistor operated at low voltage, positive feedback configurations can be exploited, while trading off degradations of other performance parameters (e.g., noise or linearity) [20]. Several implementations of positive feedback in RF/mm-wave blocks adopt reactive coupling (i.e., inductive or capacitive) or an integrated transformer [31; 32; 33]. The more effective strategy for the low-voltage operation of RF/mm-wave circuits is to adjust standard topologies to have only one transistor between \(V_{\rm DD}\) and the ground. In this perspective, the classical n-MOS cascode topology, largely adopted in high-frequency amplifiers (e.g., LNAs and PAs), thanks to its intrinsic advantages in terms of gain, stability and input-output isolation, cannot be used below a supply voltage of 1 V (i.e., \(V_{\rm DD}>2V_{\rm DS,SAT}\)). Viable circuit alternatives could include the following:  
* Neutralized common source (CS) topology;
* Folded cascode topology;
* Reactive resonant coupling: capacitive coupling;
* Reactive resonant coupling: transformer coupling. Figure 5 depicts the low-voltage topologies for an RF/mm-wave LNA stage. It is worth noting that all the reported low-voltage topologies are fully compliant with the standard simultaneous noise/input impedance matching, based on gate, \(L_{\rm G}\), and source, \(L_{\rm S}\), inductance. Moreover, they can also be used to implement a PA stage. The choice of the most suitable solution depends on both the application (e.g., frequency band and current consumption/silicon area specs) and the adopted CMOS technology node. Figure 5: (**a**) Cascode; (**b**) neutralized common source (CS); (**c**) folded cascode; (**d**) capacitive-coupled (CC) cascode; (**e**) transformer-coupled (TC) cascode.