P. Piot, PHYS 375 –Spring 2008Applications of Diodes and Transistors to Sources • Current & Voltage Sources based on transistor– FET-based– BJT-based• Signal Shaping– Rectification–ClampingP. Piot, PHYS 375 –Spring 2008IVIoIoV loadIIdeal Current source :→ Current provided by the source is independent of loadIdeal Voltagesource :VIVoVoV loadI→ Voltage at source terminals is independent of loadIdeal SourcesP. Piot, PHYS 375 –Spring 2008Real Current SourceVIIo→ The linear domain is the range where the component effectively operates as a current source“Linear region”↔equivalent representationcurrent source ↔ Ri>> V/I = Ze= load input impedance.ioRVII −=→oIconstI=≅⇒As long as I >> current in internal resistanceiRVEquivalent schematic:IoRiV loadIRi= “internal resitance”(Gi= 1/Ri= internal conductance)Assumption is : V∈linear regionThis is the equivalent of Thevenin Circuit for a voltage source it is called Norton equivalent circuit.P. Piot, PHYS 375 –Spring 2008Real Voltage SourceVIVoLinear region↔ equivalent representationVoltage source ↔ Ri<< ZeIRVVio−=→oVcstV=≅⇒As long as the voltage drop across Riis small compared to V()VIRi<<loadVIVoRiassumption : V∈linear regionEquivalent schematics (Thevenin circuit):P. Piot, PHYS 375 –Spring 2008➨According to the value of Ze/Riww have:-Voltage source (Ze>>Ri) -Current source (Ze<<Ri)In fact…≡“seen”by the loadVoRiRiIowithiooRVI == “short circuit current”(load replaced by a short-circuit)IRVVRVRVRVIIioiioio−=→−=−=since [Vo= open-circuit voltage ]loadloadIVIVCurrent versus Voltage Source --EquivalenceP. Piot, PHYS 375 –Spring 2008FET-based current source+VDDloadDSSDGSIIV =⇒=0IDSS= increases with VDSÙ output resistance not infiniteICurrent can be tunable by addition of a variable resistorDGSDGSGSDSSDIRVIVVIIoff→−=−≅21RIdand VGScan be obtained by solving this system of equationsGSDGSDCurrent correspondingto VGS=0from transistor characteristicsIdds: drain currentwith gate shorted tosourceP. Piot, PHYS 375 –Spring 2008A better FET-based current source (greater output impedance)+VDDloadT1T2T2and T1operate such that IDSS(T2) > IDSS(T1) Standard current sourceT1 ÎI = IDSS (T1 )Î VGS (T2) is such that ID(T2) = IDSS(T1)ÎVDS(T1) =VGS(T2)Iinfluence of load on VDS(T1) is strongly reducedÎI vary less with respect to load variationÙ Ouput impedance is larger compare to thescheme presented in previous slide.P. Piot, PHYS 375 –Spring 2008BJT as current sourcesloadRcVCCVBBREI•ECurrent sourceEBBRVVI7.0−≈→For any values of Rc…as long as transistor operates in active modeOperating domain :zEcccRIVR −≅maxfor Rc> Rcmax→ transistor in saturated regime ()CCCECCCCEVIRRVV<+−=<≈0()VVBB7.0>VBE~0.7 V implies active mode of operationSo IE=ICP. Piot, PHYS 375 –Spring 2008BJT-based current source (current mirror)Let’s assume the characteristics I(V) of the diode is identical to the transistor BE junctionRVIalD7.0−≅since VBE= VDIC= ID ICis the « mirror image » of ID…ValRIDICVDA I does not depend on the dashed circuitÙseen by A, the circuit behaves as a ideal current source (as long as the transistor operates in the active mode) In fact there is a small dependence of ICupon VCE ; this is refer to as Early effect (actually Early effectis the dependence of αTon VCE).VBE~0.7 V implies active mode of operationSo IE=ICBECP. Piot, PHYS 375 –Spring 2008Diode and DC power supplyTransform an AC signal in a DC signal.(e.g.: to supply DC voltage to an apparatus using the mains AC line) ■ Aim:The basis steps for an AC-to-DC converter arerectifierLow pass filterFeedback/regulationV>0V<0What you did in Lab #8P. Piot, PHYS 375 –Spring 2008Rectification110 V50HzRcVs7.0−≈mVVstRi=output impedance of transformerVm=secondary circuit voltageDouble RectificationD1D2D3D4RRcViVsVitVs, VVi4.1<~1.4VSingle RectificationP. Piot, PHYS 375 –Spring 2008RectificationWith low frequency filteringWith capacitorNo capacitorD1D2D3D4RVs50 ΩRc=10kΩVi200µFCapacitor charge through Rand discharge through RcÎ RC << RcCResidual ripplesP. Piot, PHYS 375 –Spring 2008ClampingShift the signal toward positive (or negative) voltage ↔ Eventually get a DC signal with a non vanishing average value■ Fonction : Example :VcVg(t)CVdDRgz With Vg -Vc< Vthreshold, diode reversed-biasedÎVc= constant (C cannot discharge!)Î Vd= Vg-VcVgRgCVcVd~ DC componentOperation : (assume a Si diode as we used in the Lab #8)z When Vg -Vc> Vthreshold, diode forward-biasedÎ C charge and Vcgoes toward Vg–VthresholdÎ Vd~ VthresholdVgRgCVcVd~ VthresholdIP. Piot, PHYS 375 –Spring 2008Clamping: exampleVcVg(t)CVdDRgzExample for a sinusoidal signal()0pour sin >⋅= ttVVmgω0pour 0<= tVc(C discharged)Î Transient where capacitor is being chargedt (s)C=1µFRg=1kΩf= 100hz Vm=5VVcVgVdCharging of capacitorVd≈0.7V=VthresholdP. Piot, PHYS 375 –Spring 2008Voltage multiplier■ Purpose : Produce a DC voltage from an AC input signal. The DC voltage is a multiple of input signal amplitude.Example : Voltage DoublerclampingSingle alternance rectifierand RC filter~VgRc>> RgRgVD1VRcVm=10V, f=50Hz, C=10µFRc=100kΩ.CCl()0for 2sin >⋅=ttfVVmgπtVD1,VRctransient steady-state* In steady state input current is small so high input impedance. mRVVc⋅≅→ 2* This is not a good voltage source since output current (in Rc) must remain small (~ high internal impedance)P. Piot, PHYS 375 –Spring 2008Summary■Transistor and diode find many application in power supply (either as current of voltage source)■Diode are used in scheme aimed at generating DC signal from an input AC signal■There is another class of applications of transistor: amplifier1947: The 1sttransistorToday: ATHLON 64 dual core:233000000 transistors1971: intel processor 4004: 2250