5.6 JFET Common Source Amplifier

Chapter 5.6 JFET Common Source Amplifier

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

  • CHAPTER 5. TRANSISTOR CIRCUITS97DNPPGSVDSVGSIDID++Figure 5.15: An N-channel JFET with DC bias voltages applied.iD =∂ID∂VGSvGS +∂ID∂VDSvDS,(5.51)where the AC currents and voltages are complex but the partial derivatives evaluate to realnumbers. In the pinch-off region the curves of constant VGS are essential flat (∂ID/∂VDS =0)and allow the equation to be written asiD = gmvGS,(5.52)where gm = ∂ID/∂VGS is the transconductance.5.6JFET Common Source AmplifierThe common source configuration for a FET is similar to the common emitter bipolar tran-sistor configuration, and is shown in figure 5.17. The common source amplifier can provideboth a voltage and current gain. Since the input resistance looking into the gate is extremelylarge the current gain available from the FET amplifier can be quite large, but the voltagegain is generally inferior to that available from a bipolar device. Thus FET amplifiers aremost useful with high output-impedance signal sources where a large current gain is the pri-mary requirement. The source by-pass capacitor provides a low impedance path to ground