5.7 JFET Common Drain Amplifier

Chapter 5.7 JFET Common Drain Amplifier

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 5. TRANSISTOR CIRCUITS99With the approximation iS = iD we can write the drain voltage asvD =−vSRDRS.(5.55)The voltage gain is thusvDvG=−A =−gmRD1+ gmRS.(5.56)If gmRS1, this reduces tovDvG=−A =−RDRS.(5.57)5.7JFET Common Drain AmplifierThe common drain FET amplifier is similar to the common collector configuration of thebipolar transistor. A general common drain JFET amplifier, self-biased, is shown in fig-ure 5.18. This configuration, which is sometimes known as a source follower, is characterizedby a voltage gain of less than unity, and features a large current gain as a result of having avery large input impedance and a small output impedance.RsRGvGvsVDDFigure 5.18: JFET common drain amplifier.Using AC circuit analysisiD =gmvGSand(5.58)vSRS=gm(vG− vS).(5.59)The voltage gain between the gate and source isvSvG=gmRS1+ gmRS.(5.60)If gmRS1, vS = vG and we have a voltage follower.