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RF Electronics Chapter 10: Operational Amplifiers Page 345 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. in a higher bandwidth and less intermodulation due to mains voltages but requires more subsequent amplification, resulting in more noise at the output. To investigate this the frequency response of the amplifier shown in figure 10.5 is plotted for Rs = 1 MΩ and Rp being 15 kΩ, 30 kΩ, 60 kΩ or 120 kΩ. Initially Rg in figure 10.5 is removed, so that the amplifier gain is determined by Rs and Rp only. When Rs = 1 MΩ and Rp = 120 kΩ the attenuation is calculated to be 19.4 dB and the 2 pF OpAmp input capacitance causes an upper corner frequency of 743 kHz. Rp = 60 kΩ causes an attenuation of 24.9 dB and a corner frequency of 1.41 MHz, Rp = 30 kΩ causes an attenuation of 30.7 dB and a corner frequency of 2.73 MHz, while Rp = 15 kΩ causes an attenuation of 36.6 dB and an upper corner frequency of 5.4 MHz. Figure 10.6 shows the corresponding frequency responses using a full Spice simulation of the circuit of figure 10.5. The simulated gains at 100 kHz are about 0.1 dB more than the calculated values and the corner frequencies are close to the calculated values shown above. The AD8058 voltage feedback OpAmp can be changed to an AD8017 current feedback OpAmp, to illustrate the difference between voltage and current OpAmps. For the current feedback OpAmp, Rs = 1 MΩ and Rp = 60 kΩ. Figure 10.6 shows that for this application, the current feedback amplifier has 6 dB less gain than the expected gain, which is shown by the gain of the voltage feedback amplifier. For the AD8017, the +input resistance is 50 kΩ, which is in parallel with Rp and will thus cause a significant gain reduction. Figure 10.5. High impedance input amplifier. Figure 10.6. High input impedance operational amplifier frequency response. RF Electronics: Design and Simulation 345 www.cadence.com/go/awr