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RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 286 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. input biasing impedance. Changing L1 to 10 uH results in a flat NF frequency response below 100 MHz. However increasing inductance L1 may result in a lower impedance at high frequencies due to the inductor having a lower self-resonance. That would then increase the NF at higher frequencies. Figure 8.8 shows that the gain increases with supply current, but that for a 3 V supply, the NF is best at 6 mA supply current. The manufacturer's measured data used for figure 8.8 does not include noise figure measurements above 10 mA, so the noise figure for 20 mA cannot be plotted. The stability of the amplifier can now be determined. Figure 8.9 shows the input and output stability circles for a 3 V supply and 6 mA current, corresponding to the best NF in figure 8.8. The frequency increment is 10 steps per decade (10spd), to show the individual circles. The side of the circles marked with dots is unstable, so for a perfect 50 Ω source and load termination (the centre of the Smith Chart), the amplifier is stable. Figure 8.9 includes a green circle with a VSWR = 1.2. The top part of that circle is inside the unstable region of the input stability circle for frequencies between 57 MHz to 437 MHz. This can be verified by running the accompanying Ch8-Fig7-13.emp AWRDE project file and changing the BGB 707 Stability Circles 50 Ohm measurements shown in figure 8.9, by changing both the SCIR1 and SCIR2 measurements from sweep frequency > Use for x-axis to select with tuner. Then do the simulation with Simulate > Tune and change the frequency pointer using the Tuner. A VSWR of 1.2 is only a small mismatch, so that the amplifier is not very stable. Figure 8.9. Stability circles for the BGB707 MMIC at 3 V and 6 mA. Figure 8.10 shows the stability factors for this BGB707 MMIC. For unconditional stability K>1, B>0, MI1>1 and MU2>1. To provide smooth plots, the frequency increment is 50 steps per decade (50spd). It can be seen that none of the stability conditions are satisfied and that great care must be taken in the input and output matching to keep the amplifier stable. Despite this, the amplifier has a wide bandwidth a low NF and a low cost. Figure 8.11 shows the input and output impedances, plotted on a Smith Chart. They can also be produced as a table, by simply duplicating the graph as a Tabular Graph. Alternately, the data on the graph can be exported by selecting Graph Export Trace Data and then specifying the file name for the data to be saved. The resulting data can then be used if needed to design matching networks as described in Chapter 9. RF Electronics: Design and Simulation 286 www.cadence.com/go/awr