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RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 299 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. It is also possible to make the amplifier unconditionally stable by placing a 50 Ω, 5.35 dB attenuator between the amplifier output and the load. That results in a slightly higher gain of 20.42 dB at 100 MHz and a lower NF than the amplifier output of figure 8.28b, as shown in the orange (PiAttn) curves of figure 8.30. Finally, the NF of the amplifier can be reduced by tuning the input impedance, Rin and Xin. Changing the input impedance will change both the NF and the gain of the amplifier. A low NF together with a good gain occurs when Zin =121 + j0 Ω, as shown in the green (ZinOpt) curves of figure 8.30. Changing the input impedance required the attenuation to be changed to 5.4 dB for unconditional stability. The corresponding stability circle plots look very similar to that of figure 29b. They can be obtained from the accompanying Ch8_Fig28-33 AWR DE project files. The 121 + j0 Ω input impedance is close to the impedance of the centre of the green 25 MHz NF circles in figure 8.12. Figure 8.31 shows the corresponding circuit and that includes an attenuator. Comparing the green curves in figure 8.30 for the amplifier of figure 8.31, with the red curves for the amplifier of figure 8.7 (Z in = Z out = 50 Ω), shows that by optimising the input impedance and placing a 5.4 dB attenuator before the output, the amplifier's performance is improved. The amplifier of figure 8.31 has a gain of 22.15 dB and a NF of 0.475 dB at 100 MHz and is unconditionally stable. Figure 8.31 Tuning input impedance to obtain optimum NF. Figure 8.32 Stability Factors: a) Amplifier of figure 8.28a. b) Amplifier of figure 8.28b Figures 8.32 and 8.33 show the stability factors of the above amplifiers. Figure 8.32a shows the stability factors of the amplifier of figure 8.28a with the 75 Ω input impedance. That amplifier is only conditionally stable, with both K and Mu2 being <0. Figure 8.32a is very similar to the stability factors of this amplifier with a 50 Ω input impedance as shown in figure 8.10. The amplifier of figure 8.28b with the 46 Ω additional load resistance, and the amplifiers of figure 8.33 with an attenuator before the output are all unconditionally stable. RF Electronics: Design and Simulation 299 www.cadence.com/go/awr