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Junction field-effect transistors (JFET)
43 questions By Tony R. Kuphaldt
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Question 34 of 43
Calculate the amount of gate-to-source voltage necessary to regulate a JFET’s drain current at a value of 2.5 mA, given the following transistor parameters:
\(V_{GS(off)}=3 \ vol\)
\(I_{DSS}=17 \ mA\)
Also, write the equation used for obtaining the value of VGS.
Reveal answerVGS = 1.85 volts
$$V_{GS} = V_{GS(off)} (1 - \sqrt{\frac{I_D}{I_{DSS}}})$$
Notes:Once students have found the standard drain current formula (transconductance equation), the rest is algebraic manipulation.
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Question 35 of 43
Cutoff voltage is not the same thing as pinch-off voltage, although the names seem quite similar. Give a concise definition for each of these field-effect transistor parameters, referencing them to a graph of characteristic curves if possible.
Reveal answerCutoff voltage is that gate-to-source voltage (VGS) resulting in zero drain current. Pinch-off voltage is that drain-to-source voltage (VDS) value where further increases do not yield significantly greater drain current values.
Follow-up question: which of these two parameters is more appropriate to consider when using an FET as a switch, and which is more appropriate when using an FET as a linear amplifier?
Notes:This contrast and comparison is probably best shown on a graph of characteristic curves, but I leave this to the student to research!
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Question 36 of 43
The typical amount of current through a JFET gate terminal is far less than the typical amount of current through a BJT base terminal, for similar controlled currents (drain or collector, respectively). Explain what it is about the construction and/or use of the JFET that limits the input current to almost nothing during normal operation.
Reveal answerThe gate-source PN junction is reverse-biased.
Notes:If need be, refer back to a “cut-away” diagram of a JFET to help your students understand why the input impedance of a JFET is what it is.
