Question
How does DC input impedance at the gate of a field-effect transistor (FET) compare with that of a bipolar transistor?
Answer Options
- A) They are both low impedance
- B) An FET has lower input impedance
- C) An FET has higher input impedance
- D) They are both high impedance
Correct Answer: C
Explanation
Bipolar Junction Transistors (BJTs) are current-controlled devices, meaning they require a significant base current to flow for operation. The base-emitter junction acts as a forward-biased diode, resulting in a relatively low DC input impedance at the base (typically hundreds to a few thousand ohms).
Field-Effect Transistors (FETs), such as MOSFETs and JFETs, are voltage-controlled devices. The control element (the gate) is either reverse-biased (JFET) or insulated from the channel (MOSFET) by a layer of oxide. This insulation or reverse bias results in an extremely low gate current. Therefore, an FET has significantly higher input impedance (often megohms or even teraohms) compared to the low input impedance of a BJT.
This topic was automatically created to facilitate community discussion about this exam question. Feel free to share study tips, memory tricks, or additional explanations!