## 3 Current Status of the Circuit Component in the MMW Regime
### Power Amplifier (PA)
#### 3.2.2 PA in SiGe Technology  
Figure 7 illustrates the saturation power performance of the published MMW SiGe PAs. In [96], the efficiency and gain of the PA are enhanced by using the transformer-coupling current-reuse technique. Also, this inter-stacked transformer improves the output loss owing to the differential-to-single-ended conversion. In [97], the collector-emitter breakdown beyond BV\({}_{\text{CEO}}\) is achieved with the differential common-base pairs and stability and reverse isolation are improved by the cross-coupled, collector-emitter neutralization. Furthermore, the novel transformer power combiner improves the uniformity of the load impedance while transforming to each primary port. Thus, the undesired coupling will be minimized. In [98], the adaptive bias control V-band PA is reported. The main insight is that a fully-integrated on-chip RMS power detector for automatic level control (ALC), build-in self-test and voltage standing wave ratio (VSWR) protection incorporate with the PA to improve the performances. In [84], the distributed active transformer, which is realized with tacked coupled wires as opposed to slab inductors, performs a high coupling coefficient of 0.8 at 60 GHz. The possibility of efficient power combining and impedance transformation for the operation in MMW frequencies is exemplified. In [99], a novel two-dimensional passive propagation media is proposed for power combining and impedance transformation. By exploiting this media, the W-band PA delivers 125 mW at 85 GHz in the SiGe technology. In [100], improvement of the isolation between adjacent lines and better integration of the PA is achieved by utilizing the conductor-backed coplanar waveguide transmission line. As a result, this PA has a compact area and high output power due to the reduction in path losses with this transmission line structure. In [101], the differential cascode topology incorporates with the center-tapped microstrip lines are proposed. The path losses can be reduced by using center-tapped microstrip lines, especially for long quarter-wave RF chokes. In [102], the novel combiner is proposed to enhance the power transformation efficiency (reduce the loss) and eliminate the necessaries of the additional baluns in differential circuits because it behaves like a balanced-to-unbalanced converter. In [103], the 140 GHz and 170 GHz PA are reported. Both of the PAs are measured over 125 \({}^{\circ}\)C. The 140 GHz PA has a measured output power of +1 dBm with a gain of 17 dB whereas the 170 GHz PA has a measured output power of 0 dBm with a gain of 15 dB.  
In [104], the proposed PA utilizes the transformers for power combining and inter-stage matching networks. The bias condition of the proposed cascode HBT configuration is optimized fot highest output power and gain performance. In [105], the proposed Q-band high power-added efficiency stack PAs exhibit the high power-added efficiency operation over the collector-emitter breakdown voltage. The active devices are designed to turn ON and OFF at the same time to make sure the voltage swing across the transistors add up in phase resulting in larger overall output voltage swing and power of the PA. In [106], a state-of-the art PA, which operates beyond 120 GHz, is reported. The highest power performance is achieved by optimizing the design in device sizing, EM modeling and layout techniques. In [107], the D-band amplifier has a measured saturation output power of 8 dBm at 155 GHz. In [108], another state-of-the-art PA is presented by adopting the staggered tuning mechanism. In this manor, the peak gain of the individual or group of individual stages can tuned at offset frequencies, in turn; combining these individual or group stages could perform a broadband operation. This PA has largest operation bandwidth among all silicon D-band PAs.