## 1 Introduction
[15] investigated mixed (parallel/series) connection of PE oscillators and showed that the bandwidth of the array capable of switching connection patterns can be significantly enhanced in comparison to a conventional single (parallel or series) array configuration. The authors of [15] also provide a comprehensive overview of research on PE oscillator arrays. Although the general analytical solution to the problem of calculating multi-frequency responses of multiple electromechanical piezoelectric bimorph beams loaded by variable load resistance was presented five years ago [16], the more specific questions of optimization of particular PEC arrays taking into account the power-transfer circuitry have not been completely solved yet. In particular, [PERSON] and [PERSON] [17] used the idea of equivalent impedance to investigate the electrical response of an array of piezoelectric oscillators using distinct energy harvesting circuits. Three interface electronics systems were considered, including standard AC/DC (single full wave rectifier) and parallel/series-synchronized switch harvesting on inductor (SSHI) circuits. [PERSON] et al. [18] investigated the electrical response of a series connection of PE vibration energy harvesters attached to various interface electronics, including standard and parallel-/series-SSHI circuits. They showed that the parallel-SSHI array system exhibits higher power output with moderate bandwidth improvement, while the series-SSHI system gives a pronounced widening of the bandwidth at the cost of peak harvested power. The standard array system with a full wave bridge rectifier shows a capability of power harvesting between these two SSH systems. Frequency tuning of a PEC can be done either by changing the effective mass of the beam or its stiffness. This can be achieved by variation of the cantilever geometry or material properties. In practical applications, commercially available PECs are likely to be resorted to. Such PECs are normally available in predefined steps of beam stiffness. Therefore, tuning by the tip mass is often advantageous and desirable. Hitherto, most of the papers dealing with parallel or series connections of PECs have used the same realization of the transducer (implying the same amplitude and phase of output voltages). For the mass-tip tuning, this is not the case, and this feature leads to the different behavior of the previously studied circuits, in particular parallel and series connections. In this paper, we address the limits of tip-mass tuning and the analysis of its optimization. In the following considerations, each PEC will be supplied by an individual full wave bridge rectifier circuit. By doing so, we largely avoid the interference between different oscillators due to the different phases of output voltages. Such an idea was proposed earlier by [PERSON] and [PERSON] [19], who obtained analytic estimates on harvested power for various interface circuits, including the standard interface and parallel-/series-SSHI circuits. In the present paper, it will be shown that the power output depends both on the load and the resonance frequency detuning between individual PECs. The considered power-transfer circuits (denoted as standard interfaces in [19]) are simple for practical implementation. It will be also demonstrated that with such power-transfer circuit, the output power is quite close to the theoretically achievable maximum power. However, the modeling approach can be easily extended to other power transfer circuits where the consequences are known, as shown in previous publications [18]. In particular, [PERSON] and [PERSON] have pointed out that implementation of control circuits for SSHI techniques in addition to respective electrical rectification can be a challenging task [19].