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CHAPTER 18 TRANSISTORS AND THYRISTORS 1 BIPOLAR JUNCTION TRANSISTORS (BJTs) • BJT is constructed with three doped semiconductor regions separated by two pn junction • There are three regions : – Emitter – Base – Collector • There are two type of BJT: – npn – pnp 2 Basic construction of BJT 3 Transistor Biasing • The BE junction is forward-biased • The BC junction is reverse-biased 4 Transistor Operation 5 Transistor Currents 6 Alpha and Bata • The collector current is equal to the emitter current times has a value between 0.950 and 0.995 • The collector current is equal to the base current multiplied by has a value between 20 and 200 7 Transistor Voltages VC VCC I C RC VB VE VBE 8 VOLTAGE-DIVIDER BIAS • Use only a single dc source to provide forward-reverse bias to the transistor • Resistor R1 and R2 form a voltage divider that provides the base bias voltage 9 Input Resistance at the Base VB RIN IB VB VE I E RE I E βDC I B RIN βDC I B RE βDC RE IB 10 Base Voltage R2 RIN VB R1R2 RIN RIN R2 VCC R2 VCC VB R1R2 VE VB 07 V 11 THE BIPOLAR JUNCTION TRANSISTOR AS AN AMPLIFIER 12 THE BIPOLAR JUNCTION TRANSISTOR AS AN AMPLIFIER • When both junction are forward-biased, the transistor is in the saturation region of its operation • When VCE exceeds 0.7 V, the base-collector junction becomes reverse-biased and the transistor goes into the active or linear region • When IB=0 the transistor is I in the cutoff region. CEO – Collector leakage current, – B-E and B-C are reverse-biased 13 Load Line Operation 14 Quiescent or Q-Point 15 Quiescent or Q-Point 16 Signal (ac) Operation of an Amplifier • The circuit which produces an output signal with the same waveform as input signal but with a greater amplitude is called amplification RC Av RE 17 Signal Operation on the Load Line 18 THE BJT AS A SWITCH Conditions in cutoff VCE ( cutoff ) VCC Conditions in saturation I C ( sat) VCC RC I B (min) IC sat DC 19 BJT PARAMETERS AND RATINGS IF the temperature goes up, DC goes up, and vice versa. 20 BJT PARAMETERS AND RATINGS • Maximum Transistor Ratings IC PD max VCE PD(max) = maximum power dissipation 21 THE JUNCTION FIELD-EFFECT TRANSISTOR (JFET) • The JFET is a type of FET that operates with a reverse-biased junction to control current in the channel 22 Categories FET JUNCTION Depletion P channel N channel METAL-OXIDE semiconductor Depletion P channel N channel Enhancement P channel N channel 23 JFET Basic Operation • The JFET is always operated with the gate-to-source pn junction reverse-biased 24 JFET Symbols 25 JFET CHARACTERISTICS IDSS (Drain to Source current with gate shorted) is maximum drain current occurring for VGS =0 V, and the value of VDS which ID becomes constant is the pinch-off voltage 26 Cutoff Voltage • The value of VGS that makes ID approximately zero is the cutoff voltage VGS(off) 27 VGS controls ID. •JFET must be operated between VGS =0 and VGS(off) 28 Comparison of Pinch-Off and Cutoff • Vp is the value of VDS at which the drain current becomes constant and is always measured at VGS =0 V • VGS(off) and Vp are always equal in magnitude but opposite in sign 29 THE METAL-OXIDE SEMICONDUCTOR FET(MOSFET) • Depletion MOSFET (D-MOSFET) – D-MOSFET can be operated either the depletion mode or the enhancement mode 30 Depletion Mode (D-MOSFET) 31 Enhancement Mode (D-MOSFET) 32 D-MOSFET Symbols 33 Enhancement MOSFET(E-MOSFET) 34 E-MOSFET Symbols 35 FET BIASING • Self-Biasing a JFET VGS VG VS 0V I D RS VGS I D RS nchannel VGS I D RS pchannel VD VDD I D RD VDS VD VS VDS VDD I D RD RS 36 D-MOSFET Bias VGS 0V ,I D I DSS VDS VDD I DSS RD 37 • Example 38 E-MOSFET Bias Drain-feedback bias negligible gate current ; VGS VDS Voltage-Divider bias R2 VDD VGS R1R2 VDS VDD I D RD (a) Drain-feedback bias (b) Voltage-Divider bias 39 The Other Types of MOSFET LDMOSFET TMOS VMOS Dual-gate n-channel40 UNIJUNCTION TRANSISTORS (UJTs) 41 UJT operation • In normal UJT operation, base 2 (B2) and the emitter are biased positive with respect to base 1 (B1), and total resistance between B1 and B2 is RBB • The ratio RB1/RBB is designated η and is defined as the intrinsic standoff ratio • It take an emitter voltage of VB + η *VBB to turn the UJT on, called the peak voltage • When the emitter voltage decreases, it reaches a value called the valley voltage, and the UJT turns off 42 UJT operation 43 UJT Applications 44 THYRISTORS • Thyristors are devices constructed of four layers of semiconductive material such as the silicon-controlled rectifier (SCR), the diac, and the triac • When turned on (triggered), they become low-resistance current paths and remain, although the trigger is removed, until the current is reduced to a certain level or until they are turned off 45 Silicon-Controlled Rectifier (SCRs) • The SCR has three terminals 46 SCR application Phase control circuit 47 Triacs • The triac is also known as a bidirectional triode thyristor 48 Diacs • The diac us a bidirectional device that does not have a gate • It conducts current in either direction when a sufficient voltage, called the breakover potential 49 Transistor packages 50 FIGURE 4-26 Metal cases for general-purpose/small-signal transistors. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e 51 Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FIGURE 4-27 Typical multiple-transistor packages. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e 52 Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FIGURE 4-28 Typical power transistors. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e 53 Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FIGURE 4-29 Examples of RF transistors. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e 54 Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.