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.