1. ANALOGUE ELECTRONICS CIRCUITS 1
EKT 204
Introduction to FET Amplifier:
FET (Review)

2. FIELD EFFECT TRASISTOR (FET)
ADVANTAGES OF FET
TYPES OF FET & ITS OPERATION

3. FET Advantages Voltage-controlled amplifier: input impedance very high
Low noise output: useful as preamplifiers when noise must be very low because of high gain in following stages
Better linearity: distortion minimized
Low inter-electrode capacitance: at high frequency, inter-electrode capacitance can make amplifier work poorly. FET desirable in RF stages (high frequency)

4. Types of FET FET JFET MOSFET MESFET Enhancement mode Depletion mode
n channel
Enhancement mode
p channel
Depletion mode

5. Junction FET (JFET)
ohmic contact
Structure
n-channel
p-channel
Symbol

6. Metal-Oxide-Semiconductor MOS (MOSFET)
DEPLETION p n p
dielectric
ENHANCEMENT
metal p n p
n-channel
p-channel

7. JFET Operation depletion region VDD  VDD VGG
Gate-source is reversed-biased
 zero current at gate
IDS flow through the channel and the value is determined by the width of depletion region and the width of the channel

8. electron inversion layer
MOSFET Operation
electron inversion layer G G S D S D SS SS
No voltage applied to gate
Current is zero
+ve voltage applied to gate
Electron inversion layer is created
Current is generated between source and drain

9. FET BIASING JFET BIAS CIRCUITS MOSFET BIAS CIRCUITS Self-bias
Voltage-divider bias
MOSFET BIAS CIRCUITS
Drain-feedback bias

10. Equivalence biasing of JFET & BJT
<==>
<==>
<==>

11. JFET Bias Circuits - Self-Bias
+VDD RD
IG = 0 RG RS

12. JFET Bias Circuits - Voltage-divider Bias
+VDD R1 RD ID VG R2 RS

13. MOSFET Bias Circuits - Voltage-divider Bias
+VDD R1 RD R2

14. MOSFET Bias Circuits - Drain-Feedback Bias
+VDD RD RG
IG = 0

15. SELF-BIASED JFET VOLTAGE-DIVIDER BIAS JFET
LOAD LINE
SELF-BIASED JFET
VOLTAGE-DIVIDER BIAS JFET

16. LOAD LINE - SELF-BIASED JFET
+VDD 9V RD
2.2K RS
680 RG
10M
Example
Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.

17. IDSS=4mA; so ID=IDSS=4mA VGS=-IDRS=-(4m)(680)=-2.72V
For ID=0,
VGS=-IDRS=(0)(680)=0V
From the curve,
IDSS=4mA; so ID=IDSS=4mA
VGS=-IDRS=-(4m)(680)=-2.72V
ID (mA)
Q point is the
intersection between
the transfer
characteristic
curve and the
load line
4 IDSS
ID=2.25mA
VGS=-1.5V Q
2.25
-VGS (V) -6
VGS(off)
-2.72
-1.5

18. LOAD LINE - VOLTAGE-DIVIDER BIAS JFET
+VDD 8V
Example
Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure. R1
2.2M RD
680 R2
2.2M RS
3.3K

19. For ID=0, For VGS=0, ID (mA) Q point is the intersection between
-VGS (V)
12 IDSS -3
VGS(off)
1.8
1.2
-1.8 Q 4
VGS (V)
Q point is the
intersection between
the transfer
characteristic
curve and the
load line
ID=1.8mA
VGS=-1.8V

20. EXERCISES (Load Line JFET)
1. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.
+VDD 6V RD
820 RS
330 RG
10M
ID (mA)
-VGS (V)
IDSS = 5mA
VGS(off)=-3.5

21. EXERCISES (Cont.)
2. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.
+VDD
12V RD
1.8K R1
3.3M R2
2.2M RS
3.3K
ID (mA)
IDSS = 5mA
-VGS (V)
VGS(off)=-4V

22. FET CHARACTERISTICS
JFET
MOSFET

23. JFET CHARACTERISTICS DRAIN CHARACTERISTIC VP=|VGS (off)|
Drain characteristics curve or sometimes known as current-voltage characteristics.
VP : pinch-off voltage is the level of VDS where IDS becomes almost constant.
IDSS : Value of drain-source current that is flowing in the JFET when VGS reaches pinch-off.
Ohmic region/triode region : where the channel resistance is virtually constant i.e. ID and VDS follow the principle of Ohm’s law.
Pinch-off region/saturation region: JFET acts as voltage-controlled constant-current source.
Breakdown region: VDS value is too large - like avalanche in PN diode
VP=|VGS (off)|

24. JFET CHARACTERISTICS TRANSFER CHARACTERISTIC
Also known as drain current vs gate-to-source voltage characteristics.

25. JFET DATA SHEET For MMBF5459 VGS (off) = -8.0V (max)
IDSS = 9.0 mA (typ.)

26. MOSFET CHARACTERISTICS
TRANSFER CHARACTERISTIC
(Depletion MOSFET)
Depletion mode : means that a channel exists even at zero gate voltage. Negative gate voltage must be applied to the n channel depletion MOSFET to turn the device off.

27. MOSFET CHARACTERISTICS
TRANSFER CHARACTERISTIC
(Enhancement MOSFET)
VTN : threshold voltage of the n-channel MOSFET. Define as the gate voltage required to turn on the transistor.
For n-channel : VTN is positive value ‘coz positive gate voltage is required to create the inversion charge. If VG < VTN , the current in device is zero.
K in formula can be calculated by substituting data sheet values ID(on) for ID and VGS at which ID(on) is specified for VGS

28. E-MOSFET DATA SHEET ID(on) = 75 mA (minimum) at
VTN = 0.8 V and VGS = 4.5V
VGS(th) = VTN