RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 287
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 8.10. Stability factors for the BGB707 MMIC at 3 V and 6 mA.
Figure 8.11. Zin and Zout for the BGB707 MMIC at 3 V and 6 mA.
The Gain and NF will vary with the input source impedance. That variation can be plotted
on the Smith Chart as shown in figure 8.12. In that figure five NF Circles, being contours
of constant NF are plotted as the green circles. The difference between each of the
contours is 0.1 dB. That plot shows that the NF at 25.12 MHz can be improved by 0.4 dB
by matching the input for optimum NF rather than 50 Ω. The first circle (p1) has a NF
0.1 dB above the minimum NF. Contour p4 nearly passes through the Smith Chart origin
(50 Ω) and is 0.4 dB above the minimum NF. Figure 8.12 also plots the input and output
stability circles and they show that the input impedance location for minimum noise is
inside the unstable region of the input stability circle. To prevent oscillations with the
minimum NF, the source used in figure 8.7 must be selected to correspond to the
minimum NF and the load impedance must be selected for stable operation and prevent
oscillations. Figure 8.12 shows the Gain, NF and stability at 25.12 MHz. To prevent
oscillations, the same evaluation needs to be done over a wide range of frequencies.
RF Electronics: Design and Simulation
287 www.cadence.com/go/awr
