JFET biasing

Chapter JFET biasing

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • the depletion region will completely obstruct the flow of charge carriers. This is calledpinchoff, and is illustrated at C.Again, think of the garden-hose analogy. More negative gate voltages, EG, corre-spond to stepping harder and harder on the hose. When pinchoff takes place, you’ve cutoff the water flow entirely, perhaps by bearing down with all your weight on one foot!Biasing beyond pinchoff is something like loading yourself up with heavy weights as youbalance on the hose, thereby shutting off the water flow with extra force.JFET biasingTwo biasing arrangements for an N-channel JFET are shown in Fig. 23-4. Thesehookups are similar to the way an NPN bipolar transistor is connected, except that thesource-gate (SG) junction is not forward-biased.At A, the gate is grounded through resistor R2. The source resistor, R1, limits thecurrent through the JFET. The drain current, ID, flows through R3, producing a voltageacross this resistor. The ac output signal passes through C2.At B, the gate is connected to a voltage that is negative with respect to groundthrough potentiometer R2. Adjusting this potentiometer results in a variable negativeEG between R2 and R3. Resistor R1 limits the current through the JFET. The drain cur-rent, ID, flows through R4, producing a voltage across it; the ac output signal passesthrough C2.In both of these circuits, the drain is positive relative to ground. For a P-channelJFET, reverse the polarities in Fig. 23-4. The connections are somewhat similar to theway a PNP bipolar transistor is used, except the SG junction isn’t forward-biased.JFET biasing41923-3At A, depletion region (solidarea) is not wide, and manycharge carriers (arrows) flow.At B, depletion region iswider, channel is narrower,and fewer carriers flow. At C,channel is completelyobstructed, and no carriersflow.