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ELE1110C Tutorial 11Luk Chun Pong29-11-2006Outline:JFET and MOSFETBJT, JFET and MOSFET
BJTJFETMOSFETBaseCollectorEmitterDrainDrainBaseSourceSourceGateGate
BaseCollectorEmitter
GateDrainSource
SourceDrainGateBodyBodyBJTJFETMOSJETFunctionVoltageControlledCurrent sourceVoltage controlled Resistor, Voltage controlled current sourceVoltage controlled Resistor, Voltage controlled current sourceOperationVBE controls IC
VGS controls ID for Large VDSVGS controls ID for Large VDSAdvantageHigh Gain (gm)
Extremely High Input Impedance
Extremely High Input Impedance
TypePNP, NPNn-channel, p-channeln-channel, p-channel
BJT, JFET and MOSFETPart 1: JFETJunction Field Effect TransistorExtremely large input impedanceVery small gate currentAdvantageSmaller gain (gm) than bipolar transistorDisadvantageMore difficult to analyzeFor BJT, VBE ~ 0.6V (~0.5 0.8)For JFET, VGS vary over a wide rangeJFETs are operate at depletion-mode they're "on" unless a gate voltage is applied
Two types of JFET
N-channel JFETP-channel JFETDifferenceCase 1: VDS is small,Operation of JFET (n-channel)VG=0PNVS=0VDVT increase depletion => reduce IDIncrease VDS => increase IDAnd so ID remains constant -> current sourceVG= -2VVS=0VD = 5VPN-channelMore reverse biased:-2 5 = -7VLarge VDS saturation region (constant ID, voltage controlled current source)
Larger depletion region
Use VGS to control the current ID (Voltage controlled current sourceVDS increases, ID remains constantLinear region,VDS increases, ID increases(Voltage controlled current source)
IDSSVPSaturation current IDSS = I (drain-source saturation)Depends on FETVGS = 0, ID = MAXID vs VGS (saturation region)
Problem Set 8
Part 2: MOSFETMetal Oxide Semiconductor Field Effect TransistorTwo modes: enhancement mode and depletion mode 4 terminal devices: source, drain, gate, and bodyDifferent between Enhancement and depletion modeFor n-channel
1. Enhancement:increase VGS, increase the current
2. Depletion: increase the VGS, decrease the current
pnnmetalinsulatorgatesourcedrain- - - - - -- - - - - - -ve charge, conducting channelbodyStructure of MOSFET (N-channel)NMOS and PMOS
Operation of MOSFET (Enhancement mode)1. Cut-off regionVGS < VTH (Threshold Voltage), No conduction
2. Triode or linear region VGS > VTH and VDS < VGS VTH
3. Saturation regionVGS > VTH and VDS > VGS VTH
VDSIDVGS VTH = VDSConstant ID for wide range of VDS, but increase VGS increase IDID increase as VDS increases or VGS increasesNMOS inverter and problems
large static power dissipation
Large Time constantSlow response!C(Complementary)MOS Inverter
NMOSPMOSAdvantage:- Zero Resistance , small RC, fast switching- No static power consumption
Disadvantage:Both MOSFETs are partially conducting during state changing
Charging up the stray capacitor of next stage consumes energy
Total energy dissipation per second =
Problem set 8 Q2