Electronic Devices and Circuit Theory Chapter 4 - DC Biasing PDF

Electronic Devices and Circuit Theory Boylestad DC Biasing - BJTs Chapter 4  Ch.4 Summary Biasing Biasing: Applying DC voltages to a transistor in order to turn it on so that it can amplify AC signal...

Electronic Devices and Circuit Theory Boylestad DC Biasing - BJTs Chapter 4  Ch.4 Summary Biasing Biasing: Applying DC voltages to a transistor in order to turn it on so that it can amplify AC signals. Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Operating Point The DC input establishes an operating or quiescent point g called the Q-point. ① Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary The Three Operating Regions Active or Linear Region Operation Base–Emitter junction is forward biased Base–Collector junction is reverse biased Cutoff Region Operation Base–Emitter junction is reverse biased Base–Collector junction is reverse biased Saturation Region Operation Base–Emitter junction is forward biased Base–Collector junction is forward biased Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary DC Biasing Circuits Fixed-bias circuit Emitter-stabilized bias circuit Collector-emitter loop Voltage divider bias circuit DC bias with voltage feedback Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary FB = -VE , ILBEB , VeeV-IR Fixed Bias fixed DC VBE doesn't vary #Lisin ? significantly during use. (Q) QuEL on -DC RB and VCC (DC voltage #chos source) are constant, so IB 1- doesn't vary significantly. Single VCC keeps the base emitter junction forward biased and the collector base junction reverse biased. The operating point can &IQ) Point Constant be fixed anywhere in the &Base Enter forward active region of the characteristics by simply changing the value of RB S Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  - Fixed Bias Circuit >putput Apply K.V.L. to the base circuit (Input) - VCC-IBRB-VBE=0 IBRB=VCC-VBE IB=(VCC-VBE)/RB Apply K.V.L. to the collector circuit (Output) VCC-ICRC-VCE=0 VCE=VCC-ICRC IC =  IE + ICBO ICRC=VCC-VCE IC=VCC-VCE/RC Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary The Base-Emitter Loop * ~ From Kirchhoff’s voltage law: - +V – I R – V = 0 CC B B BE ↓ - Ve + RBIB + VBE =o Solving for base current: ↳ VCC − VBE - can beefrent IB = with error Wil, a nightly ging RB 7inPat & So the EB) S gr -. Slithy Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Collector-Emitter Loop · From Kirchhoff’s voltage law: -VatRcIctVac o u = Apply K.V.L. to the collector circuit (Output) VCC-ICRC-VCE=0 VCE=VCC-ICRC ICRC=VCC-VCE Va-Vae IC=(VCC-VCE)/RC Jic- RC Collector current: minating IC =  IE + ICBO-temptere on d ICIE Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary * Lisin Circuit Q (a) Forward calclute Both Saturation values and I - * Dose the Fo is must be Glcis max= Voc Vo is Zero *Q: , # When the transistor is operating in saturation, current through the transistor is at its maximum possible value. # V ICsat = CC R C - Max VCE  0 V - Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  #E # Ch.4 Summary #Exam is the Load Line Analysis (9) function ↑Names for multible choose output #Ec = o The load line end points are: (fixed) ↑ Vac = o to see the mat *Lisin Quiz ICsat & di connect to R max IC = VCC / RC I need ittofinsdections & 1 - - VCE = 0 V it will be given ? VCEcutoff - VCE = VCC IC = 0 mA - · # The Q-point is the operating point where the value of RB sets the # - value of IB that controls the values of VCE and IC. it > - make fixed to Set the I Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary The Effect of VCC on the Q-Point Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary G The - Effect of R C on the Q-Point # No have an efect on Q-Point (c) effect & Effect * [Rc) given and whose &Q: 2 values Q: How is the Ac higher Aus : the IC will change by how much ? IBQ effect the Q-Point - , T Ei Jean Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary The Effect of IB on the Q-Point VCC − VBE IB = RB ↳ IB changes by changing RB for a certain circuit. linked to the Ep) increase Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Emitter-Stabilized Bias Circuit becuse the tepture the diffrent is adding (R) to make it more stable (it's not stable Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit with regards to changing ambient temperatures. D- Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Base-Emitter Loop From Kirchhoff’s voltage law: Trace it + VCC − I B RB − VBE − I E RE = 0 Since IE = ( + 1)IB: VCC − I B RB − (β + 1 )I B RE − VBE = 0 Solving for IB: - anothe VCC − VBE IB = RB + (β + 1)RE 3 - VIE make it more stable Counters changes in current due to temperature variations. Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Collector-Emitter Loop From Kirchhoff’s voltage law: I E RE + VCE + IC RC − VCC = 0 Since IE  IC: VCE = VCC – I C(RC + RE ) Also: VE = I E RE VC = VCE + VE = VCC − IC RC VB = VCC – I R RB = VBE + VE Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Improved Biased Stability Stability refers to a condition in which the currents and voltages remain fairly constant over a wide range of temperatures and transistor Beta () values. Adding RE to the emitter improves the stability of a transistor. Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Saturation Level The endpoints can be determined from the load line. VCE = 0 V VCEcutoff: VCE = VCC ICsat: VCC IC = 0 mA IC = RC + RE Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Voltage Divider Bias This is a very stable bias circuit. Optimal Shape The currents and voltages are nearly independent of any variations in . * the for (Rs) more four used Stable - - Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Exact Analysis Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary - Approximate Analysis ↑ Where IB = 10R2: VE IE = RE VE = VB − VBE VCE = VCC − IC RC − IE RE From Kirchhoff’s voltage law: IE  IC VCE = V CC−IC (RC + RE ) Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Voltage Divider Bias Analysis Transistor Saturation Level V CC I Csat = ICmax = RC + RE Load Line Analysis Cutoff: Saturation: V VCE = VCC I = CC C R +R C E IC = 0 mA VCE = 0 V Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary DC Bias With Voltage Feedback Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. In this bias circuit the Q-point is only slightly dependent on the transistor beta, . Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc Boylestad Upper Saddle River, New Jersey 07458 All Rights Reserved  Ch.4 Summary Base-Emitter Loop From Kirchhoff’s voltage law: VCC – IC RC –I B RB –VBE –I E RE = 0 Where IB