Question 3-5A6

Field-effect transistors (FETs) are voltage-controlled devices. Their input, the gate, is either insulated from the channel (as in MOSFETs) or reverse-biased (as in JFETs). This design means very little to no current flows into the gate, presenting an extremely high resistance path – hence, high input impedance. Bipolar junction transistors (BJTs), on the other hand, are current-controlled devices. Their input, the base, forms a forward-biased PN junction with the emitter. For the transistor to operate, current must flow into the base. A forward-biased diode junction inherently presents a relatively low resistance path, resulting in low input impedance. Therefore, option A accurately describes this fundamental difference in their input characteristics. Option B is incorrect because input impedance is an inherent characteristic of the device type, not solely dependent on supply voltage. Options C and D are incorrect as they contradict the basic operating principles of these transistors.