171C H A P T E R 7LNA Design7.1 Introduction and Basic AmplifiersThe LNA is the first block in most receiver front ends. Its job is to amplify the signal while introducing a minimum amount of noise to the signal.Gain can be provided by a single transistor. Since a transistor has three termi-nals, one terminal should be ac grounded, one is the input, and one is the output. There are three possibilities, as shown in Figure 7.1, each shown with a bipolar and a MOS transistor. Each one of the basic amplifiers has many common uses and each is particularly suited to some tasks and not to others. The common-emitter/source amplifier is most often used as a driver for an LNA. The common-collector/drain, with high input impedance and low output impedance, makes an excellent buffer between stages or before the output driver. The common-base/gate is often used as a cascode in combination with the common-emitter/source to form an LNA stage with gain to high frequency as will be shown. The loads shown in the diagrams can be made either with resistors for broadband operation or with tuned resonators for narrowband operation. In this chapter, the LNA with resistors will be discussed first, followed by a discussion of the narrowband LNA. As well, refinements such as feedback can be added to the amplifiers to augment their performance. 7.1.1 Common-Emitter/Source Amplifier (Driver)To start the analysis of the common-emitter amplifier, we replace the transistor with its small-signal model, as shown in Figure 7.2(a). ZL represents some arbitrary load that the amplifier is driving. For the common-source amplifier using a MOS transis-tor, the small-signal model would not have an input resistor rp, and Cp, Cm, vp, and rb would be replaced by Cgs, Cgd, vgs, and rg, respectively, as shown in Figure 7.2(b). Since at our frequency of interest the impedance of Cp is considerably less than the impedance of rp, the two circuits will have similar results.At low frequency, the voltage gain of the amplifier can be given by: vooLm LibevrZAg Zvrrr== -» -+ (7.1)where re is the small signal base-emitter diode resistance as seen from the emitter. Note that rp » βre and gm » 1/re. For low frequencies, the parasitic capacitances have been ignored and rb has been assumed to be low compared to rp.