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CHAPTER 5 - Amplifier Design 206 5.2 Two-port S-Parameter characterisation of a transistor So we have seen how input and output impedance of a transistor may be modelled and also looked at the physical mechanisms behind feedback but how can we quantify these for a physical device? This is when S-parameters come in very handy! S-parameters are particularly useful when characterising active devices since, unlike impedance and admittance parameters, they do not require short and open circuit terminations to be measured (section 3.7). However, as we have seen in previous sections, S-parameters are linear and are only valid when the small-signal assumption applies. This is a very important point, NEVER forget it! There is no such thing as non-linear S-parameters! Also, bear in mind that S-parameters are bias dependent and hence need to be re-measured if the bias voltages of the transistors are changed. Now, although our transistor is a three terminal device, for most applications, one of its terminals will be common to input and output and hence a two-port S-parameter measurements may be carried out. This is shown in Figure 5.2-1 for our BJT in common-emitter configuration. Figure 5.2-1 2-port S-parameters for a BJT We have seen how the reflection coefficient for a one port network allows us to work out the impedance of the network as seen from the measurements point (section 3.5). For a two port network (section 3.8) we get a similar deal but in this case we have both an input and output impedance. The input impedance may be derived from the input reflection coefficient, S11, whereas the output impedance may be derived from the output reflection coefficient, S22. The S21 parameter, which is always less than 1 for a passive network, helps us work the gain of the transistor. Remember however that S-parameters are voltage-type quantities (section 3.7) and that we are interested in power gain (section 5.1.5). In order to get the basic power gain of the transistor therefore, we need to square the modulus of S21. | | S 12 S 21 S 11 S 22 C E B Conquer Radio Frequency 206 www.cadence.com/go/awr

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