7.8 DC Bias Networks

Chapter 7.8 DC Bias Networks

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

  • 222 LNA DesignFigure 7.39  Redrawn transformer-coupled LNA.is increased by a factor of n, the current is decreased by a factor of n. However, in this circuit, the transistor feeds the primary current into the secondary adding it to the secondary current, but also allowing a lower impedance to be driven. The net result is that the gain S21 is approximately equal to n. Thus with a turns ratio of 4:1, the amplifier can achieve a gain of 12 dB.The advantage of this circuit is that the gain is determined largely by the trans-former turns ratio, thus minimizing the dependence on transistor parameters. The transformer has high linearity and low noise; thus, the amplifier also has high lin-earity and low noise. Recently, this type of amplifier and variations of it have also been realized in CMOS with similar benefits of enhanced linearity and low noise with low power supply voltages [8, 9]. 7.8  DC Bias NetworksA number of circuits have already been discussed in this text, and it is probably ap-propriate to say at least a few words about biasing at this point. Bias networks are used in all types of circuits and are not unique to LNAs. The most common form of biasing in RF circuits is the current mirror. This basic stage is used everywhere and it acts like a current source. Normally, it takes a current as an input and this current is usually generated, along with all other references on the chip, by a circuit called a bandgap reference generator. A band-gap reference generator is a temperature independent bias-generating circuit. The bandgap reference generator balances the VBE dependence on temperature, with the temperature dependence of vT to result in a voltage or current nearly independent of temperature. Design details for the bandgap reference generator will be discussed further in Section 7.8.2 [10]. Perhaps the most basic current mirror is shown in Figure 7.40(a). In this mir-ror, the bandgap reference generator produces current Ibias and forces this current through Q1. Scaling the second transistor allows the current to be multiplied up and used to bias working transistors. One major drawback to this circuit is that it can inject a lot of noise at the output due primarily to the high gm of the transistor N×Q1 (larger than Q1 by a factor of N), which acts like an amplifier for noise. A capacitor can be used to clean up the noise and degeneration can be put into the