Electronic PRINCIPLES

Transcription

1 MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION

2 Chapter 13 JFETs

3 Topics Covered in Chapter 13 Basic ideas Drain curves Transconductance curve Biasing in the ohmic region Biasing in the active region Transconductance

4 Topics Covered in Chapter 13 (Continued) JFET amplifiers JFET analog switch Other JFET applications Reading data sheets JFET testing

5 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

6 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

7 Drain family of curves 10 0 V 8 6 I D in ma V DS in Volts V GS -1 V -2 V -3 V -4 V

8 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)

9 I DSS 10 0 V 8 6 I D in ma V P = 4 volts Constant current region V DS in Volts V GS -1 V -2 V -3 V -4 V V GS(off)

10 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

11 10 V P 0 V 8 Ohmic region 6 I D in ma 4-1 V 2-2 V -3 V 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

12 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

13 Transconductance curve 10 I D = I DSS 1- ( ) 2 V GS V GS(off) V GS in volts I D in ma

14 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.

15 Transconductance curve Max. slope g m0 ( V g m = g m0 1- GS ) V GS(off) 6 Larger slope I D in ma Smaller slope V GS in volts 0

16 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

17 Q point in the ohmic region 10 0 V I D(sat) 8 6 I D in ma Q I D R DS V DS in Volts V DD V GS -1 V -2 V -3 V -4 V

18 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

19 Self-bias

20 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

21 Q point in the active region 10 0 V I D(sat) 8 6 I D in ma Q V DS in Volts V DD V GS -1 V -2 V -3 V -4 V

22 Two-supply bias

23 Current-source bias

24 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

25 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

26 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

27 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

28 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

29 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)

30 Multiplexer v in1 v in2 v in3 v out R D V 1 V 2 V 3

31 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.

32 Cascode amplifier Low input capacitance allows the circuit to amplify high frequencies

33 Current source Provides a fixed load current even though resistance changes

34 Current limiting A JFET in series with the load keeps current limited to a safe value

35 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

36 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

37 Automatic gain control

38 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

39 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