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RF Electronics Chapter 10: Operational Amplifiers Page 343 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. such as those used for xDSL drivers are current feedback amplifiers. For a fair comparison, the fastest commercial grade operational amplifiers from the same manufacturer, capable of operating from 4V to 6V supplies, are used in table 10.1. Some OpAmps require a minimum gain for them to be stable [4, 5]. Table 10.1. Current and Voltage Feedback performance comparison. Current Feedback Voltage Feedback Device LMH 6702 THS4211 3dB Bandwidth (Gain = 2) 1.7 GHz 325 MHz Slew rate 3100 V/s 970 V/s Noise Voltage 1.83 nV/Hz 7 nV/Hz (1 MHz) Noise Current 18.5- 3.0+ pA/Hz 4 pA/Hz (10 MHz) Distortion (HD2) -79 dBc (20 MHz, 2Vpp) -68 dBc (30 MHz, 2Vpp) Distortion (HD3) -88 dBc (20 MHz, 2Vpp) -80 dBc (30 MHz, 2Vpp) Input Offset Voltage 1 mV 3 mV Input Bias Current 8 A 7 A Input Offset Current 0.3 A Load Resistor (test circuit) 100 499 Feedback Resistor (test circuit) 237 392 Feedback Resistor range Narrow Very wide Supply Voltage 4 V to 6 V 2.5 V to 7.5 V Supply Current 12.5 mA 19 mA Output Current 80 mA 170 mA Output Voltage 5Vs 3.5V 5V Input Impedance 1.4 MΩ 6 GΩ Input Capacitance 1.6 pF 0.3 pF Example 10.1: High Impedance 0.01-20 MHz Amplifier As part of an On-Line Impedance Analyser [6], a Voltage Sensor is required, to measure Power Line Communication (PLC) signals, with frequencies from 10 kHz to 20 MHz, which are injected onto 240V, 50 Hz, mains voltage power lines. The PLC signals are much smaller than the mains voltage and the mains voltage must be filtered out, so that the PLC signals can be accurately measured. The input impedance of the circuit should be greater than 1 MΩ at all frequencies. The design of this Voltage Sensor shows how either current or voltage feedback amplifiers, best satisfy the requirements for different parts of the circuit. The PLC signals on the power line are about 1V, so that more than 60 dB mains voltage attenuation is required for a >12 dB signal to mains voltage ratio, to enable accurate signal detection. Filtering out the mains voltage and passing the PLC signals, requires a high pass filter with a 10 kHz cut off frequency. The whole circuit is to operate from 5V supplies. OpAmps with a 12V total (6V) supply voltage capability are used for reliability. AD8058 Dual OpAmps are used for the voltage feedback amplifiers and AD8017 Dual OpAmps are used for the current feedback amplifiers. Their .cir Spice models can readily be imported into Cadence AWR DE, to permit simulation. RF Electronics: Design and Simulation 343 www.cadence.com/go/awr

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