AWR eBooks

RF Electronics Chapter 10: Operational Amplifiers Page 356 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. noise models. The Analog Devices AD797.cir can be directly imported as a netlist into Cadence AWR DE. At 1 kHz, that amplifier has an input noise voltage of 0.9 nV/Hz and an input current noise of 2.0 pA/Hz. Since this is a voltage feedback amplifier, this input current noise value applies to both the +ve and the –ve input terminals. Table 10.3 shows that for the non-inverting amplifier, the dominant noise contribution is from the microphone source resistance Rs, and the second largest is due to the input current noise at the +ve input terminal. That current, flows through Rs in parallel with Rt and the resulting noise is thus directly related to Rs. All the other noise contributions are more than 10 times smaller than the noise power due to Rs. That is close to the best that can be achieved. For the inverting amplifier, the noise caused by the 1 kΩ resistor Rg is the largest noise contributor. That resistor value is the lowest that can be used to load the microphone output. The input current noise at the -ve input terminal is the second largest noise contribution and this is mainly due to the value of Rf, which is limited by the gain required. The third largest noise contribution is due to Rs. The noise produced by Rs is thus not the dominant noise contribution. A non-inverting amplifier configuration is thus always preferred for low noise designs as it has a significantly higher signal to noise ratio and much lower noise figure than an inverting amplifier configuration. These calculations are for a microphone amplifier at audio frequencies, however the same principles apply for low noise operational amplifiers used in say MRI applications. Figure 10.19 shows the Spice simulation of the amplifier. The 1.00 dB noise figure in table 3 for the non-inverting amplifier agrees closely with the 1.009 dB noise figure obtained for the simulation in figure 10.19. From table 10.2, using an ADA8099, OPA847 or LMH6620, OpAmp will achieve a much higher gain, lower bandwidth and similar noise performance to the MMIC and transistor amplifier design in figures 8.7, 8.8, 8.13, 8.19 and 8.20, so that low noise OpAmps can be used for receiver front end designs provided the required frequency is in the operational frequency range of the OpAmp. Figure 10.19. Gain and noise figure of amplifiers of fig. 10.17 and 10.18. RF Electronics: Design and Simulation 356 www.cadence.com/go/awr

• Cover