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CHAPTER 5 - Amplifier Design 218 5.3.3.3.2 Available Gain Circles Available gain circles are a very useful graphical tool which allows us to design an amplifier with a specific gain when the unilateral assumption does not apply. In this case we follow a procedure which the dual of the one illustrated for operating gain circles. Available gain circles may be plotted on a Smith chart and indicate the loci of the terminations which we must present at the input of our transistor to achieve a specific gain. Figure 5.3-11 Available Gain Circles Picking a termination along an available gain circle corresponding to a specific gain value is only the first step of this procedure! Once a value for the input termination has been chosen, a matching network must be designed to transform our source impedance into such a termination. Once the input matching network is in place we must measure the S22 of the transistor yet again to obtain its new output impedance. We must then design a matching network to transform the load impedance into the complex conjugate of the transistor's own output impedance. Only then the gain that was specified by the circle on which we selected the input termination may be achieved! Notice how this technique may be applied to both stable and unstable devices. In the case of stable transistors, the circles will be fully contained within the Smith Chart whereas if the device is potentially unstable, part of the circles may lay outside the chart. This is illustrated in video tutorial 5.10. 0 1.0 1.0 -1.0 10.0 10.0 -10.0 5.0 5.0 -5.0 2.0 2.0 -2.0 3.0 3.0 -3.0 4.0 4.0 -4.0 0.2 0.2 -0.2 0.4 0.4 -0.4 0.6 0.6 -0.6 0.8 0.8 -0.8 Available Gain Circles Swp Max 6000MHz Swp Min 6000MHz p4 p3 p2 p1 GAC_MAX(1,4) Available Gain Circles p1: Freq = 6000 MHz G = 11.381 dB p2: Freq = 6000 MHz G = 10.381 dB p3: Freq = 6000 MHz G = 9.3815 dB p4: Freq = 6000 MHz G = 8.3815 dB Conquer Radio Frequency 218 www.cadence.com/go/awr

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