## 3 Current Status of the Circuit Component in the MMW Regime
### Mixer  
Mixer in MMW systems is utilized to offer frequency translation from the input signal to the output signal. It can be used to up-convert the baseband signals to the carrier frequencies, in turn; the signals transmit via antenna [109]. In this case, the input is IF and the output is RF; for down-conversion the opposite is true. In most cases, conversion gain (CG), pumping LO power at optimum CG and port-to-port isolations are more crucial than others such as linearity [110; 111; 112], image rejection [112; 113; 114; 115; 116] and power consumptions [117; 118; 119; 120] among all the specifications of a mixer. The reported works of mixer can be roughly categorized according to its features like wideband, miniature, and so on. Operation bandwidth and CG are important specifications of the mixer for a modern multi-band receiver. Thus, the one of the mixer features wideband performance with flat CG. In this kind of the mixer, the turn-on resistance and the parasitic capacitance of the transistor, which is related to the device size, limit of the bandwidth of a mixer [121]. Consequently, the device sizes have to be selected as careful as possible for the consideration of bandwidth, power consumptions, conversion gain and port-to-port isolations. Another primary limitation of the bandwidth of a mixer is the passive comments [122; 123; 124] in the input or output port of a mixer especially for the case of an active mixer utilizing with the Gilbert-cell [125; 126; 127] in CMOS technologies. Therefore, the passive components should be designed as broadband as possible for achieving wideband performance. Besides the various core circuit configurations of the mixer such as Gilbert cell, anti-parallel diode, single FET, distributed type, and so on; the passive components like balun, power divider, matching networks, filters that might have more impact than that of on the performances of a mixer [128]. For example, low conversion gain (CG) can be attributed to one possibility of lossy passive components. Nevertheless, the narrow band frequency response of the low loss passive components gives rise to the narrow band performances of a mixer under the design requirement of keeping the CG as high as possible. As a result, minimizing the passive components in a mixer design and remaining the wide band performances as possible should be achieved, simultaneously, in an excellent design. In addition, the main idea of miniaturization is producing equivalent electrical length with more compact components in silicon-based technologies [129]. For instance, in silicon-based technologies,  
Figure 7: Saturation power of the recent published MMW SiGe PAs. Data taken from [96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108]. there are multi-metal layers for routing or other applications. With this benefit, the equivalent electrical length can be created by either stacking the baluns, power dividers, inductors, and so on [110]; or enhancing the coupling coefficients, that means smaller inductances, shorter electrical length or novel winding methods than the original design without this technique [111], between the transformers, inductors even transmission lines. Another concept for minimizing the area of the passive components is generating the accurate model for them [110]. In this fashion, the critical design parameters can be observed from the derivation of their frequency responses (S-parameters), in turn, the area of the passive components could be minimized. Namely, some design parameters of the passive components are not so sensitive to their frequency responses like amplitude/phase imbalance, which result in low CG, isolations in some designs that can be minimized for reducing the area. Also, isolation between LO port and RF port is critical because their frequencies are close, in turn, will interfere with each other, especially for a down-conversion mixer design.