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CHAPTER 6 Small-Signal RF Amplifier Desi... > Design Using S Parameters - Pg. 141

Design Using S Parameters 141 6. Once Y in is known, set B S equal to the negative of the imaginary part of Y in , or: B S = -B in 7. Calculate the gain of the stage using Equation 6-12. From this point forward, it is only necessary to provide input and output networks that will present the calculated Y S and Y L to the transistor. Example 6-3 illustrates the procedure. D E S I G N U S I N G S PA RA M ET E R S As we discussed in Chapter 5, transistors can also be com- pletely characterized by their scattering or S parameters. With these parameters, it is possible to calculate potential instabilities (tendency toward oscillation), maximum available gain, input and output impedances, and transducer gain. It is also possi- ble to calculate optimum source and load impedances either for simultaneous conjugate matching or simply to help you choose specific source and load impedances for a specified transducer gain. EXAMPLE 6-3 Consider a transistor with the following Y parameters at 200 MHz: y i = 2.25 + j7.2 y o = 0.4 + j1.9 y f = 40 - j20 y r = 0.05 - j0.7 Set B L equal to -b o of the transistor, B L = -j1.9 mmhos The load admittance is now defined. Y L = 4.24 - j1.9 mmhos Calculate the input admittance of the transistor using Equation 6-13 and Y L . Y in = y i - y r y f y o + Y L