7.3.7 Noise in the Common-Collector/Drain Amplifer

Chapter 7.3.7 Noise in the Common-Collector/Drain Amplifer

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

  • 204 LNA Design7.3.7  Noise in the Common-Collector/Drain AmplifierSince this type of amplifier is not often used as an LNA stage, but more commonly as a buffer, we will deal with its noise only briefly. The amplifier with noise sources is shown in Figure 7.27. Noise due to rb is directly in series with noise due to RS. If noise at the output were due only to RS, the noise figure would be 0 dB. The noise due to collector shot noise is reduced due to negative feedback caused by RE. For example, the current added causes the vbe to decrease (as the ve in-creases), and ie is decreased, counteracting the added current. Note that noise due to RE sees the same effect (negative feedback reduction of noise).The base shot noise current is injected into the base where an input voltage is developed across RS: vnbs » inbsRS » vo. The exact relationship between the output noise voltage vo_nbs due to base shot noise current inbs is: -=+++πππ_(1)(1)Em So nbsnbsEmSR Zg RviRg ZRr (7.78)Assuming that RE is large, gmRS >> 1, gmZp >> 1, and then: -=»»+ππππ_(1)(1)Em SEm So nbsnbsnbsS nbsEmE mR Zg RR Z g RviiR iRg ZR g Z (7.79)The relationship between the collector shot noise incs and the output noise volt-age vo_ncs can be shown to be: +=+++πππ_()(1)ESo ncsncsEmSR RZviRg ZZR (7.80)Assuming that RE is large, and RS >> Zp, then: +»»+ππ_ncs1So ncse ncsmRZvir ig Z (7.81)Therefore, the collector shot noise current sees re, a low value, and output volt-age is low. Thus, the common-collector adds little noise to the signal except through rb. Figure 7.27  A common-collector amplifier with noise illustrated.