Q1Analog Electronics
Question
3 marks
(c) Write at least four major differences between BJT and FET.
Answer
BJT is a current-controlled, bipolar device using minority carriers, while FET is a voltage-controlled, unipolar device using majority carriers, with differences in input impedance, noise, and fabrication.
- Control Mechanism: BJT is a current-controlled device (output collector current is controlled by base current ); FET is a voltage-controlled device (drain current controlled by gate-source voltage ).
- Carrier Type: BJT is bipolar — both majority and minority carriers participate in conduction. FET is unipolar — only majority carriers conduct.
- Input Impedance: FET has very high input impedance (M to G) because the gate is insulated (MOSFET) or reverse-biased (JFET). BJT has much lower input impedance (k range).
- Noise: FET generates less noise than BJT, making it preferred in low-noise amplifier applications.
- Thermal Stability: FET has a negative temperature coefficient of , so it is more thermally stable. BJT can suffer from thermal runaway.
- Fabrication: FETs are easier to fabricate in VLSI due to smaller size; BJTs are more complex to integrate.