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MALVINO

SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. JFETs. Chapter 13. Junction field effect transistor (JFET). D. Drain. G. n. Gate. p. p. S. V DD. D. V GG. G. Source. S. JFET. Unipolar device (one polarity of charge carrier)

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MALVINO

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  1. SIXTH EDITION MALVINO Electronic PRINCIPLES

  2. JFETs Chapter 13

  3. Junction field effect transistor (JFET) D Drain G n Gate p p S VDD D VGG G Source S

  4. JFET • Unipolar device (one polarity of charge carrier) • Voltage controlled (gate voltage controls drain current) • High input impedance • No minority carrier storage • Source and drain are interchangeable in most low-frequency applications

  5. Drain family of curves 10 0 V 8 VGS 6 -1 V ID in mA 4 -2 V 2 -3 V -4 V 0 5 10 20 15 VDS in Volts

  6. Drain curves • With VGS = 0 the drain current is maximum at IDSS • VP = the pinchoff voltage • When VDS = VP the depletion layers almost touch • With VDS > VP the JFET acts as a current source • VGS(off) = -VP

  7. 10 IDSS 0 V 8 VP = 4 volts VGS Constant current region 6 -1 V ID in mA 4 -2 V 2 -3 V -4 V 0 5 10 20 15 VDS in Volts VGS(off)

  8. Ohmic region • VP separates the active region from the ohmic region. • The ohmic region is the almost vertical part of the drain curve. • In this region, a JFET acts as a resistor. • RDS = VP/IDSS

  9. 10 0 V 8 Ohmic region VGS 6 -1 V ID in mA 4 -2 V 2 -3 V -4 V 0 5 10 20 15 VDS in Volts

  10. 2 ( ) VGS 1- ID = IDSS VGS(off) Transconductance curve 10 8 6 ID in mA 4 2 0 -2 -1 -3 -4 VGS in volts

  11. +VDD RD VDD ID(sat) = RD RDS Equivalent circuit Gate bias is suitable for the ohmic region. +VDD RD Use 0 volts for VGS and ID(sat) << IDSS. RG -VGG

  12. Q Q point in the ohmic region 10 0 V 8 VGS ID(sat) 6 -1 V ID in mA 4 -2 V 2 -3 V -4 V 0 5 10 20 15 VDS in Volts VDD ID RDS

  13. Voltage-divider bias +VDD RD R1 R2 VDD ID(sat) = VG - VGS RD + RS RS IDQ = RS Gate bias is not suitable for the active region. VS = VG - VGS

  14. Q Q point in the active region 10 0 V 8 VGS ID(sat) 6 -1 V ID in mA 4 -2 V 2 -3 V -4 V 0 5 10 20 15 VDS in Volts VDD

  15. Transconductance • Tells how effective the gate voltage is in controlling the drain current. • gm = id/vgs • Common units for JFETs are the micromho (mmho) or the more modern microsiemen (mS). • gm is the slope of the transconductance curve. • gm0 is the maximum value and occurs at VGS = 0.

  16. ( ) VGS 1- VGS(off) Transconductance curve Max. slope gm0 8 gm = gm0 6 Larger slope 4 ID in mA 2 0 -2 -1 -3 -4 Smaller slope VGS in volts

  17. rd = RD RL A = gmrd vout Common-source amplifier +VDD RD R1 RL R2 vin RS

  18. rs = RS RL gmrs A = 1 + gmrs vout Source follower +VDD RD R1 R2 vin RS RL

  19. Shunt analog switch vout vin VGS vout vin RD VGS RD vin < 100 mV RD >> RDS Series analog switch Better on-off ratio than the shunt switch

  20. vout Multiplexer vin1 vin2 vin3 RD V1 V2 V3

  21. Voltage-controlled resistance • Operates in the ohmic region with VGS values between 0 and cutoff. • Works well for ac signals of 200 mVPP or less. • Small-signal resistance: rds = VDS/ID • As VGS becomes more negative, rds increases. • Both series and shunt operation can be used.

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