MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION
Chapter 13 JFETs
Topics Covered in Chapter 13 Basic ideas Drain curves Transconductance curve Biasing in the ohmic region Biasing in the active region Transconductance
Topics Covered in Chapter 13 (Continued) JFET amplifiers JFET analog switch Other JFET applications Reading data sheets JFET testing
Junction field effect transistor (JFET) n Drain G D Gate p p V DD S D V GG Source G Voltage controlled the gate voltage controls the drain current S
JFET Unipolar device (one polarity of charge carrier no minority carriers) High input impedance Source and drain are interchangeable in most low-frequency applications Two diodes: gate-source and gate-drain For normal operation the gate-source diode is reverse-biased
Drain family of curves 10 0 V 8 6 I D in ma 4 2 0 5 10 15 20 V DS in Volts V GS -1 V -2 V -3 V -4 V
Drain curves With V GS = 0 the drain current is maximum at I DSS V P = the pinchoff voltage When V DS = V P the depletion layers almost touch With V DS > V P the JFET acts as a current source V GS(off) = -V P (Turns the JFET off)
I DSS 10 0 V 8 6 I D in ma 4 2 0 V P = 4 volts Constant current region 5 10 15 20 V DS in Volts V GS -1 V -2 V -3 V -4 V V GS(off)
Ohmic region V P 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. R DS = V P /I DSS
10 V P 0 V 8 Ohmic region 6 I D in ma 4-1 V 2-2 V -3 V 0 5 10 15 20-4 V V DS in Volts V GS When a JFET operates in the ohmic region, it has an ohmic resistance equal to V P / I DSS
The transconductance curve A graph of drain current versus gatesource voltage I D increases more rapidly as V GS approaches zero The normalized curve shows that I D equals one-quarter of maximum when V GS equals half of cutoff
Transconductance curve 10 I D = I DSS 1- ( ) 2 V GS V GS(off) -4-3 -2-1 V GS in volts 8 6 4 2 0 I D in ma
Transconductance Tells how effective the gate voltage is in controlling the drain current. g m = i d /v gs Common units for JFETs are the micromho (μmho) or the more modern microsiemen (μs). g m is the slope of the transconductance curve. g m0 is the maximum value and occurs at V GS = 0.
Transconductance curve Max. slope g m0 ( V g m = g m0 1- GS ) V GS(off) 6 Larger 8 4 2 slope I D in ma -4-3 -2-1 Smaller slope V GS in volts 0
Gate bias is suitable for the ohmic region. +V DD +V DD R D I D(sat) = V DD R D R D R DS R G -V GG Use 0 volts for V GS and I D(sat) << I DSS. Equivalent circuit
Q point in the ohmic region 10 0 V I D(sat) 8 6 I D in ma 4 2 0 Q I D R DS 5 10 15 20 V DS in Volts V DD V GS -1 V -2 V -3 V -4 V
Biasing in the active region Self-bias is used only with small-signal amplifiers because the Q point is unstable VDB can setup a stable Q point Two-source bias can swamp out variations in V GS and set up a stable Q point When supply voltages are low, currentsource bias can produce a stable Q point
Self-bias
Voltage-divider bias +V DD R 1 R D I D(sat) = V DD R D + R S R 2 V S = V G -V GS R S I DQ = V G -V GS R S
Q point in the active region 10 0 V I D(sat) 8 6 I D in ma 4 2 0 Q 5 10 15 20 V DS in Volts V DD V GS -1 V -2 V -3 V -4 V
Two-supply bias
Current-source bias
JFET amplifiers A common-source amplifier has a voltage gain of g m r d and the output is inverted An important JFET application is the source follower which has a high input resistance
Common-source amplifier +V DD r d = R D R L R 1 R D A V = g m r d R L v out R 2 v in R S
Source follower +V DD R 1 R D r s = R S R L A V = g m r s 1 + g m r s R 2 v in R S R L v out
JFET analog switch Transmits or blocks a small ac signal The JFET is biased into hard saturation or cutoff Shunt and series switches are used Series type switches have higher on-off ratios
Shunt analog switch v in v out V GS R D v in < 100 mv R D >> R DS Series analog switch v in v out R D Better on-off ratio than the shunt switch V GS
Other JFET applications Multiplexers (ohmic) Chopper amplifiers (ohmic) Buffer amplifiers (active) Voltage controlled resistors (ohmic) AGC circuits (ohmic) Cascode amplifiers (active) Current sources (active) Current limiters (ohmic and active)
Multiplexer v in1 v in2 v in3 v out R D V 1 V 2 V 3
Voltage-controlled resistance Operates in the ohmic region with V GS values between 0 and cutoff. Works well for ac signals of 200 mv PP or less. Small-signal resistance: r ds = V DS /I D As V GS becomes more negative, r ds increases. Both series and shunt operation can be used.
Cascode amplifier Low input capacitance allows the circuit to amplify high frequencies
Current source Provides a fixed load current even though resistance changes
Current limiting A JFET in series with the load keeps current limited to a safe value
Chopper amplifier Used to convert input dc to a square wave The peak value of this square wave equals V DC Chopper amps use conventional ac amplifiers Amplified output can be peak-detected to recover the amplified dc signal
Buffer amplifier Isolates the preceding stage from the following stage Has high input impedance and low output impedance The source follower is an excellent buffer
Automatic gain control
Reading data sheets Review maximum ratings first Some ratings (i.e. V GS(OFF) ) may be omitted The large spread in JFET parameters justifies using ideal approximations for analysis and troubleshooting
JFET testing JFETs can be tested using an ohmmeter or DMM on the diode test range JFET current limits must not be exceeded Curve tracers and circuits can be used to display dynamic characteristics