# A Review on Vibration Control Using Piezoelectric Shunt Circuits
## 2 Types, Applications, and Recent Advances of Piezoelectric Shunt Circuits
### Negative Capacitance Shunt Circuits  
Piezoelectric materials have intrinsic capacitance due to their dielectric characteristics. In a fundamental electrical equivalent model, a piezoelectric patch operates similarly to a capacitor. In addition to the piezoelectric patch's intrinsic capacitance, an external capacitor can be added to the shunt circuit to change its resonance properties and improve performance in vibration control, while the resonant RL circuit operates similarly to the classical dynamic vibration absorber. Using capacitive circuits changes the stiffness of the piezoelectric element [53; 54]. The addition of an external capacitor modifies the circuit's total impedance, which can change the resonance frequency and increase the system's damping capabilities. [PERSON] et al. [55] and [PERSON] and [PERSON] [56] suggested an upgraded shunt circuit consisting of a capacitor C and an RL branch in parallel (RL-C parallel circuit), which allows the ideal value of the inductance L to be reduced for tuning the circuit with a structure that have a low natural frequency. [PERSON] [40] investigated the former RL-C circuit by comparing it with the RL series and RL parallel, although the RL-C circuit allows for reducing the inductance value, the results indicate that it does not perform as good as the RL series circuit. [PERSON] et al. [57] introduced an (LRLC) circuit, as shown in Figure 4, that has two inductors, one capacitance, and one resistance, resulting in two resonances. The presence of supplementary resonance enhances performance and robustness. According to the results of the numerical simulation, the LRLC shunt had better attenuation values than the LR shunt and a lower amplitude in the FRF, as shown in Figure 5.  
Figure 4: Circuit diagram of the parallel LR shunt [57]. Figure 5: Frequency response plot for the LRLC shunt [57]. Moreover, the role and shape of the capacitors were modified by introducing the negative capacitance in piezoelectric shunt circuits. Negative capacitors are different than the ordinary capacitors, where the former are not considered as a passive component because they contain operational amplifiers. The negative capacitor can increase the electromechanical coupling factor and also increase the broadband of the absorber by canceling the reactive impedance and the inherent capacitance of the piezoelectric patch. [PERSON] and [PERSON] (2018) proposed a shunt circuit that utilizes piezoelectric patches to attenuate multi-mode vibration amplitudes. The piezoelectric patches are connected in series and parallel to resistor-negative capacitor branch circuits, as shown in Figure 6. The theoretical analysis provides evidence that this system is capable of significantly minimizing the amplitudes of multiple-mode vibrations throughout the entire frequency range of the structure. The implementation of a negative capacitance is investigated by [PERSON] et al. [PERSON] _et al._ (2016). They demonstrated that the shunt negative capacitance controller effectively attenuates multiple vibration modes. It is less sensitive to environmental changes, easier to implement, and functions as a broad-spectrum vibration absorber. They showed the effectiveness of this method in damping the first six vibration modes. [PERSON] et al. [PERSON] _et al._ (2016) presented a piezoelectric suspension that is connected to an RC\({}_{\text{N}}\) shunt circuit. Negative capacitance shunting offers potential for broadband performance but is limited by the operational amplifiers' lower voltage capacity compared to the higher driving voltages of piezoelectric transducers. To address this, [PERSON] _et al._ (2016) introduced a specialized amplification circuit designed for higher voltage applications. Also, to solve the problem of capacitance mistuning, they introduced an adaptive negative capacitor that can be electronically tuned to the value of the inherent capacitance of the patch to the electromechanical coupling between the structure and the circuit. [PERSON] et al.