5.5.2 Small-Signal AC Model

Chapter 5.5.2 Small-Signal AC Model

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

  • CHAPTER 5. TRANSISTOR CIRCUITS96DSGPPNDGSDGSDSGPNNN−ChannelP−ChannelFigure 5.14: Basic geometry and circuit symbols of JFETS.The JFET has two distinct modes of operation: the variable-resistance mode, and thepinch-off mode. In the variable-resistance mode the JFET behaves like a resistor whose valueis controlled by VGS. In the pinch-off mode, the channel has been heavily constricted withmost of the drain-source voltage drop occuring along the narrow and therefore high-resistancepart of the channel near the depletion regions.The characteristic curves of a typical JFET are shown in figure 5.16. At small valuesof VDS (in the range of a few tenths of a volt), the curves of constant VGS show a linearrelationship between VDS and ID. This is the variable-resistance region of the graph. AsVDS increases, each of the curves of constant VGS enters a region of nearly constant ID.This is the pinch-off region, where the JFET can be used as a linear voltage and currentamplifier. At VGS = 0 the current through the JFET reaches a maximum known as IDSS,the current from Drain to Source with the gate Shorted to the source. If VGS goes positivefor this N-channel JFET, the PN junction becomes conducting and the JFET becomes justa forward-biased diode.5.5.2Small-Signal AC ModelThe JFET characteristic curves can be described by an equation of the formID = ID(VGS,VDS),(5.50)where the function varies with the particular transistor.This expression yields the ACrelationship