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RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 297 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. For the BGB707 MMIC at 3 V and 6 mA, figure 8.9 shows that the input stability circles dominate the amplifier's stability. A reflectionless filter or diplexer at the output affects those input stability circles. Applying a reflectionless filter at the input does not have much effect of the stability. Figure 8.26 shows the stability circles of the BGB707 MMIC amplifier of figure 8.7, with the reflectionless filter of figure 8.25 at the amplifier's output. Comparing this with the performance of the amplifier alone, as shown in figure 8.10, shows that the amplifier stability is improved. Figure 8.27 shows the resulting gain and NF of the amplifier, with and without the reflectionless filter or diplexer at the output. Above 500 MHz, the filters have little effect on the amplifier performance. The diplexer uses more components but gives a higher out of band attenuation and a better out of band noise figure. Reflectionless filters or diplexers can thus be used to stabilise low noise amplifiers and prevent them being overloaded by out-of-band signals, without affecting the NF and gain of the amplifier in the desired frequency band. Resistors at Output Provided an amplifier has a reasonable stability, like the amplifiers in the three examples in this chapter, then the amplifier's stability can be improved by adding a resistive attenuator at the output. For an amplifier with a 50 Ω output impedance, a 250 Ω load corresponds to a 5:1 VSWR. Placing a 3 dB (50 Ω) attenuator in-between the amplifier and the 50 Ω load reduces the VSWR that the amplifier sees, to < 2:1. Similarly, a bigger mismatch can be handled by a bigger attenuation. Adding an attenuator to the output of an amplifier normally has little effect on the NF. Provided the amplifier's gain is much larger than the attenuation inserted, the loss of gain can be made up with little change in NF by adding additional amplifier stages. Generally, if one adds resistors at the input, then the NF of an amplifier will increase, however the stability can be improved. So adding resistors at the amplifier output is preferred. There are three possible ways to add resistances to the output: 1) In series with the output, thus increasing the impedance that the amplifier sees. 2) As a shunt across the output, thus decreasing the impedance that the amplifier sees. 3) As a T or Pi Pad, thus keeping the amplifier's load impedance at its designed value. It is also possible to use either a combination of 1) and 2) or using 3) to have a different load and amplifier impedances. For the first stability improvement, a resistor is placed in series or parallel with the amplifier output and the impedance is tuned, to implement 1) and 2) above. To illustrate this, this technique is used on the BGB707 MMIC of the first example in this chapter. Figure 8.9 shows that this amplifier is conditionally stable and that an input VSWR of 1.2 results in instability. Figure 8.13 shows that having a 75 Ω input and output impedance results in a higher gain and lower noise, however for frequencies below 40 MHz, the Smith chart centre is inside the unstable regions of the input stability circles. Changing the output impedance to 50 Ω as shown in figure 8.28a, results a slightly improved stability with the centre of the Smith Chart to be just inside the stable regions of the stability circles as shown in figure 8.29a. In addition, that reduces the gain from the 27.52 dB at 100 MHz for the blue curve in figure 8.13, to 25.77 dB for the blue curve in figure 8.30. Placing a 46 Ω resistor in parallel with the 50 Ω amplifier output load, as shown in figure 8.28b, makes the amplifier unconditionally stable, but reduces the gain RF Electronics: Design and Simulation 297 www.cadence.com/go/awr

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