A 2N5457 general-purpose JFET has the following specifications : [latex]I_{DSS(min)} = 1 mA[/latex] , [latex]I_{DSS(max)} = 5 mA[/latex] , [latex]V_{GS(off)(min)} = -0.5 V ,[/latex] [latex]V_{GS(off)(max)} = - 6 V.[/latex] Select a self-bias resistor for this JFET.

Typical of small-signal JFETs, the range of [latex]I_{DSS}[/latex] and [latex]V_{GS(off)}[/latex] is very large. To select the best resistor, check the extremes of the specified values [latex]V_{GS(off)}[/latex] and [latex]I_{DSS}[/latex] [latex]R_{S} = \frac{\left|V_{GS(off)(min)} \right| }{I_{DSS(min)} } = \frac{0.5 V}{1.0 mA} = 500 \Omega [/latex] [latex]R_{S} = \frac{\left|V_{GS(off)(max)} \right| }{I_{DSS(max)} } = \frac{6 V}{5.0 mA} = 1.2 k\Omega[/latex] A good choice is 820 Ω , a standard value between these extremes. To see what this looks like on the transconductance curve, sketch the curves with this resistor and plot the maximum and minimum Q-points. This is done in Figure 4–14 . Despite the extreme variation between the minimum and maximum specification, the 820 Ω resistor represents a good choice for either.

Question 4.5: A 2N5457 general-purpose JFET has the following specificatio...

Analog Fundamentals: A Systems Approach

A 2N5457 general-purpose JFET has the following specifications : $I_{DSS(min)} = 1 mA$ , $I_{DSS(max)} = 5 mA$ , $V_{GS(off)(min)} = -0.5 V ,$ $V_{GS(off)(max)} = – 6 V.$ Select a self-bias resistor for this JFET.

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Typical of small-signal JFETs, the range of $I_{DSS}$ and $V_{GS(off)}$ is very large. To select the best resistor, check the extremes of the specified values $V_{GS(off)}$ and $I_{DSS}$

$R_{S} = \frac{\left|V_{GS(off)(min)} \right| }{I_{DSS(min)} } = \frac{0.5 V}{1.0 mA} = 500 \Omega$

$R_{S} = \frac{\left|V_{GS(off)(max)} \right| }{I_{DSS(max)} } = \frac{6 V}{5.0 mA} = 1.2 k\Omega$

A good choice is 820 Ω , a standard value between these extremes. To see what this looks like on the transconductance curve, sketch the curves with this resistor and plot the maximum and minimum Q-points. This is done in Figure 4–14 . Despite the extreme variation between the minimum and maximum specification, the 820 Ω resistor represents a good choice for either.