--- title: "BJT bipolar, MOSFET unipolar; BJT diff amp" subject: "Basic Electronics Engineering" chapter: 3 key: "semester_2/basic_electronics_engineering:bjt-bipolar-mosfet-unipolar-diff-amp" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/bjt-bipolar-mosfet-unipolar-diff-amp site: BCT Engineering --- # BJT bipolar, MOSFET unipolar; BJT diff amp Basic Electronics Engineering · Chapter 3: Transistor - Asked in: 2073 Bhadra, 2+2 marks; 2068 Bhadra, 2+2 marks (asked 2 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/bjt-bipolar-mosfet-unipolar-diff-amp - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-3 ## Question Why BJT is a bipolar and MOSFET is an unipolar device? And draw the circuit diagram of differential amplifier using BJT. ## Short answer BJTs are bipolar because both electrons and holes participate in conduction. MOSFETs are unipolar because current involves only one type of carrier (electrons in n-channel, holes in p-channel). A differential amplifier uses two matched BJTs with common emitter resistor or current source to −VEE, collectors to RC to VCC, inputs at bases, outputs at collectors. ## Answer #### Why BJT is bipolar In a BJT, current is carried by **both types of charge carriers**, electrons and holes. In an NPN transistor, the emitter injects electrons (majority carriers in the emitter) into the p-type base, where they are minority carriers. A small base current of holes replaces the holes lost to recombination in the base. Because both majority and minority carriers take part in conduction, the device is called **bipolar**. #### Why MOSFET is unipolar In a MOSFET, current flows through a channel carried by **only one type of carrier**, the majority carriers: electrons in an n-channel device or holes in a p-channel device. There is no minority carrier injection, and the gate is insulated, so no gate current flows. Conduction is controlled by the gate voltage, which creates or modifies the channel. Therefore it is a **unipolar** device. #### Differential amplifier using BJT ```text +VCC +-------+-------+ | | [RC] [RC] | | vo1 o----+ +----o vo2 | | C| |C v1 o----B| Q1 Q2 |B----o v2 E| |E | | +-------+-------+ | [RE] (or constant current source) | -VEE ``` Two matched transistors Q1 and Q2 share the same emitter resistor or current source. The inputs v1 and v2 are applied at the bases, and the outputs vo1 and vo2 are taken from the collectors. The output depends on the difference v1 − v2 (differential gain Ad); a signal common to both inputs is rejected (low common-mode gain Ac), which gives a high CMRR = Ad/Ac. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a BJT called a bipolar device? A. It has two PN junctions B. Both majority and minority carriers (electrons and holes) take part in conduction C. It needs two supply voltages D. It has two terminals for current Answer: B. Emitter electrons are injected into the p-base where they are minority carriers, and base-region recombination involves holes, so both carrier types contribute. (Key point: Why BJT is bipolar) 2. Why is a MOSFET called unipolar? A. It has only one terminal B. Current in the channel is carried by only one type of majority carrier C. It uses only one junction D. It works with one polarity of VGS only Answer: B. Only electrons (n-channel) or holes (p-channel) carry the channel current, with no minority carrier injection. (Key point: Why MOSFET is unipolar) 3. What does the BJT differential amplifier amplify, and what does it reject? A. Amplifies the sum of the inputs, rejects the difference B. Amplifies the difference v1 - v2, rejects the common input signal C. Amplifies both equally D. Amplifies only dc Answer: B. The output depends on v1 - v2 (gain Ad) while a common signal gives only a small output (Ac), so CMRR = Ad/Ac is high. (Key point: Differential amplifier operation) 4. In the differential amplifier circuit, what do Q1 and Q2 share? A. The same collector resistor only B. The same base C. A common emitter current source or resistor D. The same output terminal Answer: C. Two matched transistors have their emitters joined to a common constant-current source or RE, with inputs at the bases and outputs at the collectors. 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