SUMMARYThe paper presents the structure and the principle of operation of the 'improved' Howland current pumps (or voltage-controlled current sources (VCCSs) for a grounded load). In particular, under review is the VCCS employing power operational amplifier (op amp) and the VCCS using low power op amp and an additional power transistor, extending working dynamic range. On the basis of analysis of the operational principle, the equations for transfer functions of both circuits and formulas for the related dynamic electrical parameters are obtained. Moreover, using these formulas, a design procedure is developed, and recommendations for simulation modelling are given. The efficiency of the proposed procedure is verified by simulation modelling and experimental testing of sample electronic circuits of VCCSs.

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The aim of the study is the preparation and electrical characterization of lead-free ferroelectric oxide BaSrTiO3 in the composition with a piezoelectric polymer. The properties of the deposited films were compared with pristine oxide. Atomic force microscopy showed a smooth surface, and a regular and homogeneous distribution of particles of both components in the composite films. The dielectric properties (electric permittivity and dielectric loss) were investigated at different temperatures ranging from 5 to 130 °C. Impedance spectroscopy was applied in the frequency range 100–100 kHz. The dielectric constant increase with the addition of a piezoelectric polymer to the ceramic phase was demonstrated. It can be seen that the interface conditions at the electrodes are improved after inserting a piezoelectric polymer. The interpretation of the plots of the complex impedance vs. frequency, and the real part of the impedance vs. the imaginary part, give information about the polarization process revealed in the structures.

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Interface circuits with low power dissipation is proposed and implemented, which is useful for efficient AC/DC voltage conversion of thin-film piezoelectric micro-power stack mounted harvesters. The focus is on low-power (< 1 μW) elements consisting of printed PVDF-TrFE piezoelectric polymer on BaSrTiO3 coated flexible substrate with total thickness of the stack 3.1 μm. Using silver thin films as bottom and top electrodes, the samples exhibited stable piezoelectric rms voltage between 200 mV and 400 mV produced from functional area of ∼1.5 cm2 when stimulates it with sine-wave vibration with frequency of 50 Hz and intensity equivalent to mass loading between 1–80 g. The current registered from a single harvesting element is not greater than 1 μA. For this type of harvesters two power processing circuits intended are designed and built. The first circuit is a voltage doubler, for which the rectified output voltage is approximately equal to twice of the amplitude of the input voltage. The second circuit is a voltage quadrupler; as compared to the first one it provides higher voltage for charging a small and thin-size chip supercapacitor connected to the output port, but due to diodes voltage drops a smaller value of the energy efficiency can be obtained. For the implementation of the electronic circuits, low-power Schottky diodes with a forward voltage below 100 mV at current up to 0.1 mA are chosen. An experimental study to verify the efficiency of the proposed circuits is performed with laboratory made thin-film piezoelectric harvesters.

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