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consists of series capacitor with shunt inductor or series inductor with shunt
capacitor. The bandwidth obtained by a single L-C network is not sucient,
however it can be increased by adding another section that forms
ˀ
matching
network. Advantage of the
ˀ
networks is that by using an extra element there is
an extra degree of freedom to control the value of quality factor in addition to
perform impedance matching.
2.2 Selection of Diodes
As the peak voltage of the ac signal obtained from the antenna is generally much
smaller than the diode forward voltage drop[9]. So here is requirement to select
the diode with very low turn on voltage. Since RF energy harvesting is done
in GSM range, therefore the diode with very fast switching speed is required,
schottky diode have metal semiconductor junction in which metal side acts as the
anode and n-type semiconductor acts as the cathode of the diode, which fulfills
the requirement of very fast switching and low forward voltage drop. Because
of its low forward voltage drop, less energy dissipation makes it most ecient
choice for applications sensitive to eciency. In this paper, we use schottky diode
HSMS-2852 from Avago Technologies that has the turn on voltage 150 mV, as
the edges of the schottky contact are fairly sharp, high electric field gradient
occurs around them which limits the reverse breakdown voltage, low forward
voltage and fast recovery time leads to increased eciency. Moreover saturation
current is another parameter that affects the eciency of diode, so for obtaining
high eciency it is desirable to have diodes with high saturation current, low
junction capacitance, and low equivalent series resistance (ESR)[3].
2.3 Selection of Voltage Multiplier Stages
The number of multiplier stages has a major influence on the output voltage
of the energy harvesting circuit. Since only one or two stages are not sucient
to provide a fix amount of voltage that is capable to operate a wireless device,
so we have to increase the number of stages. The eciency, output voltage and
output power are directly proportional to the number of stages, which is shown
in Fig. 3, 4 and 5. For example from Fig.3 it can be seen that when the voltage
multiplier stage is 1 the maximum eciency is around 50%. As we going to
increases the voltage multiplier stages from 1 to 7, it increases gradually. In
Fig.4
V
1
,
V
3
,
V
5
and
V
7
are the output voltages of 1-stage, 3-stage, 5-stage and
7-stage respectively with different values that shows the effect of number of
stages. Fig.3 shows the another effect that, as we increases the order of stages
the peak of eciency is shifted toward the higher input power region.
2.4 Selection of Load Impedance
Every electronic device has a standard condition to operate for a specific range
of load impedance, below and above this particular range the device performance
