P. Piot, PHYS 375 –Spring 2008Electronics Switches & Controls• Conventional controls– Potentiometer– switches• Transistor as switches and controls– FET-based– BJT-basedP. Piot, PHYS 375 –Spring 2008Variable potentiometers-Variable resistance sometimeneeded (see Labs)- potentiometer provides sucha variable resistance. -Make sure the circuit worksfor any transitor from thisbatch.P. Piot, PHYS 375 –Spring 2008Electronics Switches-Switches are elements with two states « Open » and « Close »-Simplest switches are mechanical switchesP. Piot, PHYS 375 –Spring 2008PNPICIEIB ■BJTNPNICIEIB Æ IE= IB+ICSDGJFET P-channelGDJFET N-channelSRefresher on transistorsÆ IG= IS+ID■FET-Transistors can be used as switches, to control resistance, etc…P. Piot, PHYS 375 –Spring 2008Sat. mode :VVBE8.0≈VVCE2.0≈BcIIβ≠ ~0.2VBCE~0.8VSaturated modeOpen mode :0≅BICCCEVV≅0≈CIBCEOpen modehFEIBBEC~0.7VIB≅Active modeActive mode :BcIIβ≈ VVBE7.0≈CCCEVVV<<3.0~BCEVCC= external voltage source supplytin C et E. VCEcannot be larger than Vcc!Operating mode a a NPN BJTP. Piot, PHYS 375 –Spring 2008Active mode :BcIIβ≈ VVBE7.0−≈Open mode:0≅BI)0(3.0~<<<−CCCEVVVCCCEVV≅0≈CISat. Mode:VVBE8.0−≈VVCE2.0−≈BcIIβ≠ BEC~0.7VhFEIBIBBCE≅Active mode~0.2VBCE~0.8VSaturated modeBCEOpen modeOperating mode a a NPN BJTP. Piot, PHYS 375 –Spring 2008+VCCRcRBVBBt0.7Vt>0 :VBE> ~0.8V,so that RcIc~VCC →VCE ~few 100mV~0.8V ~0.2V <<VCCVCCRCRB“Closed switch”CCCCCCRRVRVIC≅−=2.0t<0 :VBE< 0.7V → Open mode+VCCRCRB“Open switch”0=CRIBJT as a switchP. Piot, PHYS 375 –Spring 2008BJT as a switch ICVCEVCCOpen switchcccRVBBEcccBRVRVIswitchlosed8.0min)C(min−≅≅βClosed switchRcRBVBBt0.7V+VCCP. Piot, PHYS 375 –Spring 2008BJT as a switch: turning on/off a light Ω=−=−=⇒+= kiVVRiRVVBBEINBBBBEIN22102.06.053-BJT with 5 V voltage source,- Design a system to switch a 20 mA, 5V lamp on and off,- The gain factor for a given batch oftransitor varies between 100<β<500,-Make sure the circuit works for anytransitor from this batch. mAiiiiiBcBcB2.0)max()min(1)max(1=⇒=⇒=ββP. Piot, PHYS 375 –Spring 2008+VCCVBBvBRERCVOUT•EB•ICEBcRvi ≈→Neglecting variation of VBE:assumptions :zTransistor operates in active mode when vB= 0z Amplitude of vBsignal small enough to have active mode operationFinally :sSCcccoutvVIRVV+=−=with : CccSIRVV⋅−=andbEcccsvRRiRv −=−=Signal vBis amplified by a factor EcvRRA −=cCCEBEBEiIIRVVI +=≈−≈→(IB<<IC)z In 1stapproximation :BJT as an amplifierbBBBvVV+≡P. Piot, PHYS 375 –Spring 2008BJT as an amplifier: light detection z Previous circuit can be used to amplify low intensity signal, e.g. such as produced in photodiode (typically 1 µA/1µW of incident light)zCircuit is modified as follows:+VCCRDRERCVOUT•EB•ICEDDEBEDDEBEBCRIRRVIRRVVI7.0−≈−≈−=CCCCOUTIRVV−=DECDOUTDECDECCCECDDCCOUTIRRRVIRRRRRVRRIRVV−=−+=−−=7.0)7.0(From ground-E-B-ground loop:Output voltage is given by:Finally output voltage is:IDP. Piot, PHYS 375 –Spring 2008Two-states electronics■ When Vin=0 transistor is open, Vout= 20 V■ As Vinincreases the transistor turns on (active mode ) and current begin to flow →Voutis loweredtoward ground.■ Eventually VBE~ 0.8 V (saturated mode), andwhen Vin~ 1 V→ Ic=βIb~ 20 mA so VOUTis close to 0■This circuit has only two output states (withexception of 0.8 V <Vin<1 V) ■Two-states electronics important components of digital electronicsP. Piot, PHYS 375 –Spring 2008FET as control()221offoffGSGSGSGSDSSDVVkVVII −=−≅for :satDSDSVV >for :satDSDSVV<Pinch-off regime()DSDSGSGSDVVVVkIoff⋅−−≅22Linear (Ohmic) regime2offGSDSSVIk =PGSsatDSVVV+=ID(mA)VDS(V)24680481216VGS=-1VVGS=0VGS(V)-2 -1.5 -1 -0.5VGS=-1VVGS=0DSSIVGSofftransistor Reverse biasedVPsatDSDSVV >P. Piot, PHYS 375 –Spring 2008•When VGS ≤ VGSoff, the channel isdepleted Î transistor is blocked•When VGSoff < VGS< 0, and VDS > VGS+VP , (with VP~ |VGSoff|), Î ID saturate and its value isquadractically dependent upon VGS.•VP is the value of VDS for which ID saturates and VGS is zero. •The pich-off regime occurs for VDS= VGS+VP (VGS is non zero)•For 0 < VDS<< VGS+VP , ID isproportional to VDS (linear regime) andthe slope is smaller as VGS tends towardVGSoff.FET as controlPinch-offohmicbreakdownP. Piot, PHYS 375 –Spring 2008For VGS > VGSoffand VDS <VGS +VP : ()−+⋅1≅2DSPGSDSVVVkRexample:vinvoutVconRinDSDSoutvRRRv+=→= variable attenuation controlled by VconChoosingonDSRR >>, voutvaries between ~0 et vinProblem: Problem: RDSdepends on VDS → nonlinear responseFET as resistance controlP. Piot, PHYS 375 –Spring 2008()PcomDSVVkR+≈→1vinvoutVconRR1R122comDSGSVVV +=→()0≈GIBetter linearity than circuit in previous slide()−+⋅1≅2DSPGSDSVVVkRFET as resistance control: a better solutionP. Piot, PHYS 375 –Spring 2008FET as resistance control: variable frequency filterCRDS1≈ωvinvoutVconR1R1()PcomDSVVkR+≈→1z variably controlled resistance in an low (or high) pass filter zDynamical adjustment of cut off frequencyP. Piot, PHYS 375 –Spring 2008FET switch based on MOFSET01010≈⇒Ω>OUTDSvRz When gate at groundz When gate at +15 VinDSOUTvkRkvΩ+Ω=4747zBased on a voltage dividerinOUTDSvvR≈⇒Ω≈100P. Piot, PHYS 375 –Spring 2008FET switch based on MOFSET: example of applicationoutputinputSelection of cut-off