BJT’sForward CharacteristicsIc- Vbe CharacteristicsBiasingThe Transistor As An Amplifier – DC ConditionsThe Base Current and the Input Resistance at the BaseTransistor as An Amplifier - Small Signal ApproximationSmall-Signal Model Small-Signal Analysis Directly on CircuitDiffusion CapacitanceFrequency Dependence of the Common-Emitter Current GainTransconductanceThe Early Effect (James Early form Bell Labs)Tolerances in Bias CircuitsMonte Carlo AnalysisElectronic Devices and Circuits – 11/5/00Monte Carlo Analysis – Using PspiceBJT’spnCo llecto r CiCBiBnBaseEiEActiv e Base Reg io nEmitterEmitter-Base Ju n ctio nCo llecto r-Base Ju n ctio nF i g u r e 5 . 1 ( a ) - S i m p l i f i e d c r o s s - s e c t i o n o f a n n p n t r a n s i s t o r w i t h c u r r e n t s t h a t o c c u r d u r i n g" n o r m a l " o p e r a t i o np+p+p+p+ppA ctiv e Tran sisto r R eg io niBiCiEppppF i g u r e 5 . 1 ( b ) - C r o s s - s e c t i o n o f a n i n t e g r a t e d n p n b i p o l a r j u n c t i o n t r a n s i s t o r( b )Ba s e ( B)Co l l e c t o r ( C)Em i t t e r ( E)iCiEiBnCo llecto rpBasenEmittervBCvB EiCiBiE( a ) F i g u r e 5 . 2 - ( a ) I d e a l i z e d n p n t r a n s i s t o r s t r u c t u r e f o r a g e n e r a l b i a s c o n d i t i o n( b ) C i r c u i t s y m b o l f o r t h e n p n t r a n s i s t o rForward CharacteristicsnpnC o llecto rB aseE m itteriEiBiCvB EiFCBEFiFnpnC o llecto rB aseE m itteriEiBiCvB EiFCBEFiFnpnC o llecto rB aseE m itteriEiBiCvB EiFCBEFiFF i g u r e 5 . 3 - n p n t r a n s i s t o r w i t h vB E a p p l i e d a n d vB C = 0 .Reverse CharacteristicsnpnC o llecto rB aseE m itteriEiBiRiCvB CCEBiRRF i g u r e 5 . 4 - T r a n s i s t o r w i t h vB C a p p l i e d a n d vB E = 0 .T a b l e 5 . 1C o m m o n - E m i t t e r a n d C o m m o n - B a s eC u r r e n t G a i n C o m p a r i s o nF o r R0 . 1 0 . 1 10 . 5 10 . 9 90 . 9 5 1 90 . 9 9 9 90 . 9 9 8 4 9 9 FF1  F or RR1  RENGR 311 – Electronic Devices and Circuits October 26, 2000Transistor Model: Current AmplifierA Summary For Clarification (assume npn for the following general rules/properties – for pnp reverse polarities)Rules / Properties1 – The collector must be positive than the emitter.2 – The base-emitter and base-collector circuits behave like diodes. Normally the base-emitter diode is conducting and the base-collector diode is reverse-biased3 – When 1 and 2 are obeyed Ic is proportional to Ib (Ic = beta . Ib)Both Ib and Ic follow to the emitter.Note: the collector current is not due to forward conduction of the base-collector diode; that diode is reverse-biased. Just think of it as “transistor action.”Property 3 gives the transistor its usefulness: a small current flowing into the base controls a much larger current flowing into the collector.Note the effect of property 2. This means you can’t go sticking a voltage across the base-emitter terminals, because an enormous current will flow if the base is more positive than the emitter by more than about 0.6 to 0.8 volt. This rule also implies that an operating transistor has Vb = ~ Ve + 0.6 (Vb = Ve + Vbe) (for an npn).Let me emphasize again that you should not try to think of the collector current as diode conduction. It isn’t, because the collector-base diode normally has voltages applied across it in a reverse direction. Furthermore, collector current varies very little with collector voltage (it behaves like a not-too-great current source), unlike forward diode conduction, where the current rises very rapidly with applied voltage.Current flow The forward bias on the base-emitter junction will cause current flow across this junction. Current will consist of two components: electrons injected from the emitter into the base, and holes from the base into the emitter. The electrons injected from the emitter into the base are minority carriers in the p-type base region. Because the base is usually very thin the excess minority carriers (electron) concentration in the base will have an almost straight-line profile. The electrons will reach the boundary of the collector-base depletion region. Because the collector is more positive than the base these electrons will be swept across the CB junction regioninto the collector. They are then “collected” to constitute the collector current. By convention the direction of icwill be opposite to that of the electron flow; thus ic will flow into the collector terminal.Ic – Vce Characteristic for an npn TransistorIc- Vbe CharacteristicsBiasing For common emitter amplifierENGR 311 - BJTs – Exercises - October 29, 2001Examples SolutionExample 1 - Beta = 100, vBE = 0.7V at iC = 1mA. Design circuit so that a current of 2mA flows through the collector and a voltage of +5V appears at the collector.Example 2 - In the circuit below vC = -0.7V. If Beta = 50, find IE, IB, IC and VC.Example 3 – In the circuit below, Vb = 1V, VE = 1.7V. What are alfa and beta for this transistor? What voltage VC do you expect at the collector.Example 4 - Beta = 100 – Determine all node voltages and branch currents.Example 5Determine the voltages at all nodes and current through all branches.Assume beta 1 and beta2 = 200. Assume Q1 is in the active mode.ENGR 311 - Graphical Representation of Transistor Characteristics - October 31, 2001Conceptual circuit for measuring the iC-vCE characteristics of the BJT. (b) The iC-vCE characteristics of a practical BJT.The iC-vCB characteristics for an npn transistor in the active modeDetermine the voltages at all nodes and the currents at all branches in the circuit below.SolutionThe Transistor As An Amplifier – DC ConditionsThe Collector Current and The TransconductanceThe Base Current and the Input Resistance at the Base(a) Conceptual circuit to illustrate the operation …