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How does the DC input impedance at a field-effect transistor gate compare with a bipolar transistor?

The FET has higher input impedance than a bipolar transistor.

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  1. The FET has lower input impedance
  2. The FET has higher input impedance ✔
  3. Both have low input impedance

Why: A FET gate is insulated or reverse biased, so it draws almost no current. A bipolar transistor needs base current to work, so its input looks much lower.

Huh, didn't know that: A depletion-mode FET still conducts between source and drain with no gate voltage applied (E6A09).

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A MOSFET said BJTs are a bad choice because they need current to work. The BJT replied, that accusation is baseless, it's just MOSFET gatekeeping.
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References

Answer: The FET has higher input impedance. A FET gate is insulated or reverse biased, so it draws almost no current. A bipolar transistor needs base current to work, so its input looks much lower.

  1. NCVEC 2024-2028 Amateur Extra Question Pool, E6A05 (4th errata, Feb. 4, 2026) www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
    C. An FET has higher input impedance
    Checked 2026-10-11.
  2. NCVEC 2024-2028 Amateur Extra Question Pool, E6A09 (4th errata, Feb. 4, 2026) www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
    A. An FET that exhibits a current flow between source and drain when no gate voltage is applied
    Checked 2026-10-11.

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