1Vacuum Tube- Evacuated tubes used in PhotoElectricEffect and early X-ray work circa 1890.- Vacuum tube invented circa 1903 forradio Marconi group. Diode action!- Triode allowed modulation of anodecurrent by small change in grid voltage.anodecathodeA - - - - - Vacuumdiode2Solid State Transistors-Lilienfeld, Heil (Germany)-Shockley and Pearson(Bell Labs Patent) circa 1930.- Current modulation and gainICE=βiBE (β∼100)emittercollectorn np npbasegatebipolarfield effect3BiPolar Transistor Operation-+-+Quiescent State:Open (diode action) No current flow.Conducting State:Closed (Forward Bias)Internal Electric Fields E are cancelled.IE = IC + IBIB~ 0 smallα = IC/ IE ~ 1.β = IC/ IB = α/(1−α)β ∼hFE transistor gainN NP+ +emitter collectorbaseVBE+VCC- - -++++++E EICIEIBsignalE EHoles injected into base lower barrier potential. VCE>2V to allow electrons to flow across barrier . (They can be captured by injected holes!)4Cross Section of an NPNBipolar Devicep- substraten+pn+Metal contactCE BFigure 3.135 Transistor Curves•IC the current through the collector is almost constantwhen the transistor is operated in its linear range. IC = β IBhttp://www.st-andrews.ac.uk/~jcgl/Scots_Guide/info/comp/active/BiPolar/bpcur.html6 Transistor Curves•The base current IB will begin to flow freely when VBE>0.7V .•We say the transistor has turned on at this point.http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/info/comp/active/BiPolar/bpcur.html7Amplification and ZinAmplification1)!! IE= Ib+ IC!!!!!!!!!!!!!!!!!!!!!!!"IE="IC!2)!!#= IC/ Ib~ IE/ Ib!!!!(Ib~ 0 = few!µA)2)!!VE=!Vb+ 0.7V !!!!!!!!!!!!!!!!!!"Vb="VE="IERE!!!2)!VCC= VC+ VE+ 0.7V !!!!!!!!!"VE= $"VC="ICRC!!!!!!!!!!!!!!!!!!!!!!!! AV="VC"Vb=$"ICRC"IERE= $"ICRC"ICRE= $RCRE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! AV= $RC/ REInput!Impedance!!(Im pedance!looking!in!to!base)ZIN=!Vb/ Ib%!VE/ Ib!=! IERE/ Ib= IE/ Ib( )RE=#RE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!ZIN=#RE!!!!IEICIbδVbVERC+VccREVC0.7V8Temperature Dependence IC= IC 0 eqVgkT(1!T /T0)+qVBE 0kT(T /T0)" eqVBEkTThe base-emitter voltage of the silicon pn junction in a transistor iswritten as a function of temperature T, reference temp T0, bandgapgap voltage Vg, collector current IC and current IC0 at T0 . α is adevice constant as:This temperature dependence will make any transistor amplifier gaindrift up or down. Differential pairs of transistors are commonly usedto cancel common mode drifts.IC~ IC 0!eqVBEkT9Differential Pair• In order to cancel transistor temperature drifts and some common mode noise we sometimes see a differential pair used.• Temperature dependent leakage currents in the two transistors tend to cancel when Q1 and Q2 are located near each other. • When the input is grounded Q1 and Q2 cancel each others signal so the output is clamped to zero also.VinVout10Frequency ResponseflofhiAV midband rangeflo~12! RINCINfhi~12! ROUTCS low pass CIN=1µFCS =1nFROUT= =10Ωhigh passRIN=10KΩ• We can think of the amplifier as a combination of low and high pass filters in the circuit below.• flo = mainly due to the input capacitor CIN and input impedance RIN=ZIN.• fhi = The stray capacitances CS is internal to the every circuit and represents small capacitances due to connection leads and internal fabrication. ROUT=ZOUT11Transistor Switch•A transistor can be used as a robust switch to turn currents on and off.•A small base current can control a large voltage supply current. •When the switch is closed the transistor is driven in to saturation and acts as a short circuit. •Transistors are useful switches when driving loads from high impedance sources (small current) such as microcomputers etc. •Proper choice of RB important to transistor life. +VCCRBRCr VOUT=Vcc!0!"#!!!transistor!OFF / OPEN !!!!!!transistor!ON / CLOSED!(IC= 0!and !Vout = Vcc)Vout ! IC!rtransistor~0"#$= 0Biasing!ResistorsIB= VCC/ RBIC=!! IB=!VCC/ RB( )VC= ICRC=!VCC/ RB( )RCVC/ VCC=!RC/ RB( )!"!1!!!!(Saturation!)!RB#!!RC12Common Emitter Amplifier•Emitter is grounded  “ Common Emitter ”•NPN Transistor with positive +Vcc forward biases the base-emitter junction.•The transistor acts as a “Constant Current Source” when forward biased correctly.•The resistance across the base-emitter junction is about rE = 25mv / IE•Rule of Thumb  IC ~ IE ~ 1ma•Usually the input is “AC coupled” by inputing Vin through capacitor C. Only the AC component of a signal is passes!•GAIN = Vout/Vin = -RC / RE and Vout is180o inverted.•Input inpedance rIN = (1/R1 + 1/R2 + 1/β RE)-1 if R1 and R2>>RErIN ~ β RE•rOUT ~ (1/RC + 1/RE )-1 ~ RC looking into the output.•VE ~ Vin indicating small voltage drop across base-emitter junction.•Vout = Vcc-IC RC = VC•Vout maximum ~ Vcc (power supply voltage)13Common Collector- Emitter Follower• A emitter follower is a power booster circuit with unity gain.• It is sometimes called a repeater or buffer amplifier.• A mall base current can be boosted to a large base current whilepreserving the signal shape, but significantly increasing the output power.•The output is taken from the emitter. •The emitter follower can be used to match a high impedance input toLow impedance output. Output!1)!VE= VB! 0.6V !!!!!!!!!VE! VB!!!!!!VOUT= VINInput!Impedance!(Im pedance ! to ! ground! seen!looking!in!to!base)1)!VE/ RE~ VB/ RE=! IE= IB+ IC= (1 +")IB!!!!!!!!!!VB/ R =!(1 +")IB!!2)!!ZIN=!VB/ IB= (1 +")RE~"RE!!!!!!!!ZIN="REOutput!Impedance(!Effective!series!impedance!at !transistor!output)!!ZOUT=! ZSOURCE/"ZS14Cascading Amplifiers•To achieve higher gain or input impedance we can cascade amplifiers output-to-input.+VccRCRCREREVinVoutVin+VccRCRCREDarlington PairVinVoutAv = β2 (RC/RE)Rin = β2 REAv = 1Rin =β2 RERout = Rsources/β215FeedbackVo' =Vs AVVs = Vi+βVo'Vo' = AV(Vi+βVo') -> Vo’(1- βAV ) = Vi AVA'V = Vo'/Vi = AV/(1-βAV)•An amplifier may become more stable if a fraction of the output signal is fed back in to the input. (β = feedback fraction)• Positive feedback will enhance oscillatory behavior.• Negative feedback will tend to cancel unwanted oscillatory behavior.AVVsVi V'oβVo' β>0 positive feedback oscillator β<0 negative feedback stable amplifier16Basic Transistor AmpIE=IB + ICVCC=VC+VE+.7V δVC+δVE=0+VccRCRER1R2GNDVinVoutnpnVE+