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RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 295 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. (GAC_MAX) circles results in the highest amplifier gain. The circles show a constant NF contour or a constant gain contour. The spacing for the NF circles is 0.1 dB and the gain circles is 1 dB. Figure 8.24 shows that matching the input of the amplifier to 75 Ω results in a normalised impedance of 1.5 (=75/50) on the Smith Chart. That gives a NF of 0.1 dB (Inner circle) below the optimum value and a 2 dB (second circle) below the optimum gain value. From figure 8.22 is can be seen that the location of the centres () of the NF circles is in a stable input region of the stability circle at 1 GHz, so that the optimum NF can be obtained at that frequency, by suitably matching the input. Changing the input and output impedances of the amplifier of figure 8.19, to 75 Ω, and retuning R1 and R2 to obtain a low NF as well as a good gain, results in R1=209 Ω and R2=2670 Ω. The blue curve in figure 8.20 shows that an improvement of 0.2 dB in the NF is obtained. The stability circles for the amplifier with 75 Ω input and output impedances is very similar to those shown in figure 8.23, for the amplifier with 50 Ω impedances. Changing the output impedance to 50 Ω, does not change the input stability circles, but makes the amplifier output slightly more stable and reduces the gain as shown in the green curves of figure 8.20. Changing the output impedance does not change the NF. Figure 8.24 shows that a match for optimum noise performance will not result in the optimum gain. The centres () of the constant gain circles of figure 8.24 lie in the unstable input region of the input stability circle at 1 GHz on figure 8.22, so that the optimum gain cannot be obtained, but 1 dB less gain will result in a stable amplifier. Normally the first stage of a low noise amplifier is matched for the lowest NF and the second stage is matched for the highest power gain, since equations 8.26 and 8.27 show that the NF of the first stage is critical, but the NF of later amplifier stages have much less effect. Improving Amplifier Noise Figure and Stability The techniques described in this section can be used to improve amplifier noise figure and stability either singly or in combination. Depending on the amplifier some will be more effective than others. Bandwidth Limiting using Reflectionless Filters Figure 8.25. Reflectionless (L) and Diplexer (R) high pass filters with Fc = 300 MHz. RF Electronics: Design and Simulation 295 www.cadence.com/go/awr

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