**3.2 Circuit design and simulation**

In previous section, RF signal with maximum power is identified and extracted from spectrum. Now, this signal represents the input of our charging circuit. We assume a1 mW signal with the frequency of 915 MHz as our input to execute our

**101**

**Figure 9.**

*Rectifier circuit return loss (s 11) at 0 dbm input RF power.*

**Figure 8.**

*Rectifier circuit efficiency.*

*RF Energy Harvesting System and Circuits for Charging of Wireless Devices Using Spectrum…*

simulation in ADS. First, we need to increase the DC level of our signal using a DC voltage multiplier circuit as it is shown in **Figure 7**. Note that we have a 4-stage and a 6-stage voltage multiplier in our proposed circuit. The connection between these

In **Figure 8**, Efficiency of the rectifier circuit is shown versus different values of *Pin*. As it can be seen, when the input power is 0 dbm, highest efficiency is obtained at

*DOI: http://dx.doi.org/10.5772/intechopen.84526*

the frequency of 915 MHz.

two multipliers results in increasing of the output voltage.

**Figure 7.** *Proposed voltage multiplier circuit.*

*RF Energy Harvesting System and Circuits for Charging of Wireless Devices Using Spectrum… DOI: http://dx.doi.org/10.5772/intechopen.84526*

simulation in ADS. First, we need to increase the DC level of our signal using a DC voltage multiplier circuit as it is shown in **Figure 7**. Note that we have a 4-stage and a 6-stage voltage multiplier in our proposed circuit. The connection between these two multipliers results in increasing of the output voltage.

In **Figure 8**, Efficiency of the rectifier circuit is shown versus different values of *Pin*. As it can be seen, when the input power is 0 dbm, highest efficiency is obtained at the frequency of 915 MHz.

**Figure 8.** *Rectifier circuit efficiency.*

*A Guide to Small-Scale Energy Harvesting Techniques*

**3.2 Circuit design and simulation**

modulation.

**Figure 6** shows power for filter, spectrum and the output of filter (Antenna

In previous section, RF signal with maximum power is identified and extracted from spectrum. Now, this signal represents the input of our charging circuit. We assume a1 mW signal with the frequency of 915 MHz as our input to execute our

*Frequency response of power spectrum, filter and filter output for OFDM transmitter with QPSK modulation.*

input signal). We use OFDM transmitter as mentioned earlier with QPSK

**100**

**Figure 7.**

**Figure 6.**

*Proposed voltage multiplier circuit.*

**Figure 9.** *Rectifier circuit return loss (s 11) at 0 dbm input RF power.*

**Figure 9** indicates the return loss (*s*11) for different frequencies and minimum return loss is obtained at 915 MHz frequency at a circuit input power of 0 dbm. This is resulted from our designed matching circuit and shows its desirable performance.

In **Figure 10**, rectifier circuit output voltage and current are indicated with the maximum at 915 MHz.

For frequencies of 580, 650, 760 and 915 MHz, the outputs of voltage multiplier circuit are shown in **Figure 11**. As it can be seen, we obtain a 8.8 V DC voltage for our input signal.

**Figure 10.** *Rectifier circuit output voltage and current.*

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**Figure 13.** *Charger circuit.*

**Figure 12.**

*Output voltage versus input power.*

*RF Energy Harvesting System and Circuits for Charging of Wireless Devices Using Spectrum…*

**Figure 12** shows the higher efficiency of our proposed circuit comparing to three other methods. That is because of exploiting a 10-stage voltage multiplier (a 4-stage

Complete charger circuit is proposed in **Figure 13**. Output currents for aforementioned four frequencies are given in **Figure 14**. Also, output voltages are

*DOI: http://dx.doi.org/10.5772/intechopen.84526*

connected to a 6-stage).

**Figure 11.** *Voltage multiplier circuit output.*

*RF Energy Harvesting System and Circuits for Charging of Wireless Devices Using Spectrum… DOI: http://dx.doi.org/10.5772/intechopen.84526*

**Figure 12** shows the higher efficiency of our proposed circuit comparing to three other methods. That is because of exploiting a 10-stage voltage multiplier (a 4-stage connected to a 6-stage).

Complete charger circuit is proposed in **Figure 13**. Output currents for aforementioned four frequencies are given in **Figure 14**. Also, output voltages are

**Figure 12.** *Output voltage versus input power.*

*A Guide to Small-Scale Energy Harvesting Techniques*

maximum at 915 MHz.

our input signal.

**Figure 9** indicates the return loss (*s*11) for different frequencies and minimum return loss is obtained at 915 MHz frequency at a circuit input power of 0 dbm. This is resulted from our designed matching circuit and shows its desirable performance. In **Figure 10**, rectifier circuit output voltage and current are indicated with the

For frequencies of 580, 650, 760 and 915 MHz, the outputs of voltage multiplier circuit are shown in **Figure 11**. As it can be seen, we obtain a 8.8 V DC voltage for

**102**

**Figure 11.**

**Figure 10.**

*Rectifier circuit output voltage and current.*

*Voltage multiplier circuit output.*

**Figure 13.** *Charger circuit.* indicated in **Figure 15**. So knowing that power equals to current times voltage, we obtain 532*μ* W output power for 915 MHz signal. It should be noted that voltage drop in 915 MHz (8.8 V to 3.6 V) is because that by connecting battery to charger circuit, battery charging process starts and in this process power must be constant, so by increasing the current drawn by the battery, output voltage drops.
