Chapter 4 bjt
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The document discusses bipolar junction transistors (BJTs). It describes the basic construction of an NPN and PNP transistor including the emitter, base, and collector regions. It explains that the base-emitter junction must be forward biased and the base-collector junction must be reverse biased for the transistor to operate. The document also discusses BJT biasing circuits, operating regions including cutoff, saturation, and active modes, and uses of BJTs as switches and amplifiers.
Chapter 4 bjt
- 1. Chapter 4 Bipolar Junction Transistors (BJTs)
- 2. Contents • Bipolar Junction transistors • Construction • Circuit diagrams • BJT biasing circuits • Q-point • BJT as a switch • BJT as an Amplifier
- 3. • Transistor is a combination of two words i.e. transfer and resistor. It is because a transistor is basically a resistor that amplifies electrical impulses as they are transferred through it from its input to output terminal. Transistor
- 4. • A transistor has three doped regions. • The bottom region is called the emitter • The middle region is the base • And the top region is the collector. • In an actual transistor, the base region is much thinner as compared to the collector and emitter regions. • The transistor Shown in figure (b) is an npn device because there is a p region between two n regions. Recall that the majority carriers are free electrons in n- type material and holes in p-type material. • Transistors shown in figure (c) is an pnp. A pnp transistor has an n region between two p regions. To avoid confusion between the npn and the pnp transistors, our early discussions will focus on the npn transistor. Architecture of a Transistor
- 5. • There are two types of BJTs, the npn and pnp. • The two junctions are termed the base-emitter junction and the base-collector junction • The term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structure • In order for the transistor to operate properly, the two junctions must have the correct dc bias voltages – the base-emitter (BE) junction is forward biased(>=0.7V for Si, >=0.3V for Ge) – the base-collector (BC) junction is reverse biased Architecture of a BJTs
- 6. • An integrated circuit (IC) consists of transistors, resistors, diodes and capacitors combined together in one wafer-thin chip of silicon. This one wafer-thin chip is called a microchip. The microchip is only a few millimeters square with a thickness of 0.5 mm. IC?
- 7. Emitter It is the most heavily doped part of the transistor. Its major function is to supply the majority charge carriers to base. Base It is the smallest part of the transistor with 10.6mm area and it is lightly doped. Collector It is physically the largest part of the transistor. Its major function is to collect the charge carriers. Parts of a Transistor
- 8. Diode equivalent of a Transistor
- 9. The minus signs represent free electrons. The heavily doped emitter has the following job: to emit or inject its free electrons into the base. The lightly doped base also has a well-defined purpose: to pass emitter-injected electrons on to the collector. The collector is so named because it collects or gathers most of the electrons from the base. The left source VBB of forward-biases the emitter diode, and the right source VCC reverse-biases the collector Diode. VBB forward-biases the emitter diode, forcing the free electrons in the emitter to enter the base. The thin and lightly doped base gives almost all these electrons enough time to diffuse into the collector. These electrons flow through the collector, through RC, and into the positive terminal of the VCC voltage source. Basic Circuit of a BJT
- 11. • Recall Kirchhoff’s current law. It says that the sum of all currents into a point or junction equals the sum of all currents out of the point or junction. When applied to a transistor, Kirchhoff’s current law gives us this important relationship: IE = IC + IB IC >> IB IE = IC • alpha (DC) IC = DCIE • beta (DC) IC = DCIB – DC typically has a value between 20 and 200. DC Analysis of BJTs (Transistor Currents)
- 13. Examples
- 14. DC Analysis of a Transistor As the base-emitter junction is forward bias so, VBE ≈0.7 V Since the emitter is at ground (0V), by Kirchoff’s Voltage law VRB=VBB – VBE Also by OHM’S law VRB = IBRB Substituting for VRB yields IBRB = VBB – VBE Solving for IB IB = VBB – VBE RB The voltage at the collector with respect to the grounded emitter is VCE = VCC – VRC Since the drop across RC VRC=ICRC VCE = VCC – ICRC Where IC = DCIB The voltage across the reverse-biased collector-base junction is VCB = VCE - VBE DC Analysis of BJTs
- 15. • DC voltages for the biased transistor: • Collector voltage VC = VCC - ICRC • Base voltage VB = VE + VBE – for silicon transistors, VBE = 0.7 V – for germanium transistors, VBE = 0.3 V DC Analysis of BJTs
- 16. Example
- 17. • The base current, IB, is established by the base bias. • The point at which the base current curve intersects the dc load line is the quiescent or Q-point for the circuit. Q-Point
- 18. • The voltage divider biasing is widely used • Input resistance is: RIN DCRE • The base voltage is approximately: VB VCCR2/(R1+R2) DC Analysis of BJTs
- 19. • When used as an electronic switch, a transistor normally is operated alternately in cutoff and saturation. – A transistor is in cutoff when the base-emitter junction is not forward-biased. VCE is approximately equal to VCC – When the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated. BJTs as a Switch
- 20. BJTs as a Switch
- 21. BJT Operating Regions Operation Region IB or VCE Char. BC and BE Junctions Mode Cutoff IB = Very small Reverse & Reverse Open Switch Saturation VCE = Small Forward & Forward Closed Switch Active Linear VCE = Moderate Reverse & Forward Linear Amplifier Break- down VCE = Large Beyond Limits Overload
- 22. •Common emitter mode •Linear Active Region •Significant current Gain Example: •Let Gain, = 100 •Assume to be in active region -> VBE=0.7V •Find if it’s in active region BJT as Amplifier