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CHAPTER 5 - Amplifier Design 210 5.3.2 Stage 2 – Stabilisation As we saw in section 5.1.4, feedback may take place between input and output of our transistor. If the right conditions are met, positive feedback may occur in such a way as to turn our transistor into an oscillator! This may lead to transistor damage or even burnout as well as system failure. Usually for any given transistor at any specific bias level, input and output terminations, which belong to identifiable sets, are the cause of potential instabilities. These sets may be easily identified if a full two-port set of S-parameters is available for the transistor 37 . It is in the nature of transistors however to be potentially unstable in certain frequency bands and completely stable in others. This is to say that there might exist regions of the frequency spectrum where, no matter what terminations we connect at the input and/or output of the transistor, oscillations would not occur. We may use a very simple measurement called μ-factor, derived from the S-parameters of our device, to determine the frequency bands over which the transistor may be potentially unstable. If this factor is greater than one then the transistor in unconditionally stable i.e. there exist no input or output terminations which may cause oscillations. If the μ-factor is less than one there may exist certain terminations which will cause oscillations. Figure 5.3-4 shows a μ-factor simulation for a BFP405 BJT. Figure 5.3-4 Transistor stability, μ-factor Figure 5.3-4 shows that if we were to design a transistor at 1.9 GHz for instance we might run the risk of using terminations which would cause oscillations. However, yet again, by using mathematical formulae based on S-parameters we may derive two very useful graphical aids which may be plotted on a Smith chart: Input and Output stability circles. 37 If we have a non-linear model for the device, instead of an S-parameter set for the device, we can always use a simulator to derive the S-parameters from the model. Potentially Unstable Region Conquer Radio Frequency 210 www.cadence.com/go/awr