7.8.1 Temperature Effects

Chapter 7.8.1 Temperature Effects

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 7.8 DC Bias Networks 223circuit to reduce the gain of the transistor, as shown in Figure 7.40(b). If Q1 is going to drive many current stages, then base current can affect the matching, so an ad-ditional transistor can be added to provide the base current without affecting Ibias, as shown in Figure 7.40(c). Another useful technique for an LNA design is to make the N×Q1 transistor function both as a mirror transistor and as the LNA driver transistor, as shown in Figure 7.40(d). In this case, resistors have to be added in the base to isolate the input from the low impedance of Q1. Provided that RB is big compared to the input impedance of the transistor N×Q1, little noise is injected here. With any of these mirrors, a voltage at the collector of N×Q1 must be main-tained above a minimum level, or else the transistor will go into saturation. Satura-tion will lead to bad matching and to nonlinearity.7.8.1  Temperature EffectsFor transistor current given by: »BETvvCSiI e (7.139)the temperature affects parameters such as IS and vBE. As well, the current gain β is affected by temperature. IS doubles every 10°C rise while the relationship for vBE and β with temperature is shown in (7.140) and (7.141). Figure 7.40  Various current mirrors. (a) Simple mirror. (b) Mirror with improved noise perfor-mance. (c) Mirror with improved current matching. (d) Mirror with transistor doing double duty as a current source and a driver.