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RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 283 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. 1 , measures the distance from the Smith Chart centre to the nearest unstable point of the output load plane, corresponding to the output stability circle. � � ��|� �� | � | � �� ��� �� ∗ | �|� �� � �� | Eqn. 8.21 2 , measures the distance from the Smith Chart centre to the nearest unstable point of the source load plane, corresponding to the input stability circle. For unconditional stability, > 1. In addition, the larger the value of , the more stable the amplifier. The Rollet's stability factors K and B and the Geometric Stability Factors for both the input and output of an amplifier are available as measurements in Cadence AWR DE, so that an amplifier's stability can easily be determined at any frequency. It is important that the stability of an amplifier is determined at all possible frequencies, since if the device is unstable it will likely oscillate. If the designer only measures the performance of the amplifier in the operating band and it is oscillating at a frequency far removed from the operating frequency, the designer will be at a loss to explain the very poor amplifier performance, until a spectrum analyser is used to check for oscillations over a wide frequency range. Note that the possibility of oscillations of an amplifier determined by these stability factors are not the only ones that cause oscillations. The lack of power supply decoupling, poor circuit layout and ground-plane integrity and EM coupling between the output and input circuits and leads are common causes of oscillation. Sometimes these oscillations can be cured by using shielding and microwave absorbers [7] in strategic places. Figure 7.59 shows a good layout to avoid coupling between different parts of the hardware. If the amplifier is not unconditionally stable, then the source and load must be carefully controlled to ensure that the amplifier is stable under normal operating conditions. Design for Maximum Gain To design an amplifier for maximum gain, the input matching network and the output matching network are both designed to provide a conjugate impedance match at the operating frequencies, thus making G S = 1 and G L =1, as outlined above. Since the power gain depends on the impedance match at the input, one can plot constant gain circles on the Smith Chart, to show how the gain varies with the matching impedance. Designing an amplifier for maximum power gain may result in unstable operation. Often there is a compromise between stability and gain. Amplifier Noise Figure If an amplifier is used at the input of a receiver, then the amplifier should be designed for the lowest possible NF. In addition, a good linearity (High third order intercept (OIP 3 )) is required, so that unwanted signals do not degrade the reception of the wanted signal. The input matching conditions for the lowest NF are different from those for a high power gain, so that a reduced gain is obtained from the amplifier when one designs it for low noise performance. In addition, the match for best noise may make the amplifier only conditionally stable. This section gives a brief overview of NF, noise power and equivalent temperature. Pozar [5] gives a detailed description of these concepts. The NF (or F) of a circuit block or amplifier is defined as the ratio of the actual noise at the output of the circuit, over the RF Electronics: Design and Simulation 283 www.cadence.com/go/awr