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RF Electronics Chapter 10: Operational Amplifiers Page 347 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The output stage of figure 10.4 is a current feedback OpAmp that has an output capability to drive 50 Ω loads. The gain of 3.9 for that amplifier is chosen to be close to 12 dB, being half the total gain required. Having a 56 Ω series resistor is used to prevent damage to the amplifier if the output is accidentally shorted. That also provides a good match when driving a 50 Ω load. A 1 kΩ resistor to ground is used to provide a DC path to ground for any output connection. The output amplifier gain of 3.9, will ensure that any clipping will occur in this amplifier, rather than the differential or the Sallen Key amplifier stages. The 10 kHz high pass filter used to remove the mains voltages, consists of a first order high pass filter at the high input impedance amplifiers A1 and A2, together with a second order Sallen-Key high Pass filter for A4, to result in a third order Butterworth high pass frequency response. The required resistor values are 804 Ω and 3.17 kΩ, together with 10 nF capacitors. Those resistor values are more suitable for voltage feedback amplifiers. However, the impedance levels can be changed so that the feedback resistor, matches the required value for the current feedback amplifiers to be used. The AD8058 is a relatively low cost Dual OpAmp, one package can be used for both the differential amplifier and the Sallen-Key high Pass filter. The decision on whether to use voltage or current feedback amplifiers is thus mainly determined by cost and PCB area used, as well as the limitations outlined above and in [1-3]. The gain for the differential amplifier A3 is selected to produce the required Gain = 1 for the whole circuit, by changing resistors Rd for the differential amplifier in figure 10.4. Figure 10.8 shows the resulting frequency responses, for Rp = 15, 30, 60 and 120 Ω. The frequency response of an ideal 3 rd order Butterworth high pass filter with a 10 kHz cut off frequency. The Voltage Sensor circuits match that response well below 200 kHz. Extending the frequency range down to 50Hz, shows that the whole circuit has 138 dB attenuation at 50 Hz, so that a 240V rms mains frequency input signal causes 30 µV rms at the Voltage Sensor output. Figure 10.8. Frequency response of the whole circuit of figure 10.4. RF Electronics: Design and Simulation 347 www.cadence.com/go/awr