Chapter Transconductance

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

  • curves is called a family of characteristic curves for the device. The graph of Fig. 23-6shows a family of characteristic curves for a hypothetical N-channel JFET. Engineersmake use of these graphs when deciding on the best JFET type for an electronic circuit.Also of importance is the curve of ID vs EG, one example of which is shown in Fig. 23-5.422 The field-effect transistor23-6A family ofcharacteristic curvesfor a hypotheticalN-channel JFET.TransconductanceRecall the discussion of dynamic current amplification from the last chapter. This isa measure of how well a bipolar transistor amplifies a signal. The JFET analog of this iscalled dynamic mutual conductance or transconductance.Refer again to Fig. 23-5. Suppose that EG is a certain value, with a corresponding IDresulting. If the gate voltage changes by a small amount dEG then the drain current willalso change by a certain increment dID. The transconductance is the ratio dID/dEG.Geometrically, this translates to the slope of a line tangent to the curve of Fig. 23-5.The value of dID/dEG is obviously not the same everywhere along the curve. Whenthe JFET is biased beyond pinchoff, in the region marked Y in the figure, the slope ofthe curve is zero. There is no drain current, even if the gate voltage changes. Only whenthe channel is conducting will there be a change in ID when there is a change in EG. Theregion where the transconductance, dID/dEG, is the greatest is the region marked X,where the slope of the curve is steepest. This is where the most gain can be obtainedfrom the JFET.The MOSFETThe acronym MOSFET (pronounced “moss-fet”) stands for metal-oxide-semiconductorfield-effect transistor. A simplified cross-sectional drawing of an N-channel MOSFET,along with the schematic symbol, is shown in Fig. 23-7. The P-channel device is shown in