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I'm learning JFET self biasing. what I've understood so far is the resistor R_s is used to create a bias voltage as shown. since no gate current flows that means no current in R_G so the potential of the gate is Zero because it is like connecting the gate with the zero potential (ground), then what is the purpose of the resistor R_G? they could have directly shorted the gate with ground, why this extra resistor?

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  • \$\begingroup\$ Why do you say no current in \$R_G\$? \$\endgroup\$
    – Hearth
    Jan 10, 2021 at 16:17
  • \$\begingroup\$ @Hearth JFET is a voltage controlled device so is there any current in gate? I am not sure, I am learning \$\endgroup\$
    – Sayan
    Jan 10, 2021 at 16:18
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    \$\begingroup\$ See the answer below. There's no (significant, DC) current into the gate, but there is still current in that resistor. \$\endgroup\$
    – Hearth
    Jan 10, 2021 at 16:24

1 Answer 1

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There will be current in that resistor, and there also will be no (DC) current into the gate. Here, take a look:

enter image description here

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  • \$\begingroup\$ great! this is what I was thinking, but i was not sure. thank you. \$\endgroup\$
    – Sayan
    Jan 10, 2021 at 16:25
  • \$\begingroup\$ so the resistor's purpose is to vary the voltage drop in accordance with signal so that different GS voltage produces, right? \$\endgroup\$
    – Sayan
    Jan 10, 2021 at 16:29
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    \$\begingroup\$ @Sayan Not exactly; if the signal is a voltage it doesn't need that resistor. But the resistor keeps the gate pulled to 0 volts if the signal is disconnected; leaving a FET's gate floating is poor form. \$\endgroup\$
    – Hearth
    Jan 10, 2021 at 16:30
  • \$\begingroup\$ yes now understood. \$\endgroup\$
    – Sayan
    Jan 10, 2021 at 16:38

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