1. ANALOGUE ELECTRONICS I
EKT 204
Basic FET Amplifier

2. Graphical Analysis, Load Lines Small-signal Parameters
MOSFET AMPLIFIER
Graphical Analysis, Load Lines
Small-signal Parameters

3. Graphical Analysis, Load Lines
IDQ
VDSQ
Common-source transistor characteristic

4. gm0 is the gm value when VGS = 0
MOSFET Parameters
Transconductance, gm
gm0 is the gm value when VGS = 0
gm also known as gFS : forward transconductance

5. Small-signal Equivalent Circuit
VDD RD vi vO
VGSQ
vDS
vGS + - iD RD vi vo
vgs id
NMOS common source circuit with time-varying signal source in series with gate DC source
AC equivalent circuit

6. Small-signal Equivalent Circuit
vO = vDS = VDD – iDRD
= VDD – (IDQ + id)RD
= (VDD – IDRD) - idRD
Small-signal parts:
vo = vds = -idRD
id = gmvgs
vi = vgs RD vi vo
vgs id
AC equivalent circuit

7. Small-signal Equivalent CircuitD G S
vgs
vds
ids + - ro
vgs
gmvgs + -
ids
Common source NMOS with small signal parameters
Simplified small signal equivalent circuit for NMOS
Small-signal output resistance
where

8. DC Analysis – Saturation RegioniD
VDD vO vi R2 RD R1 CC vG ID
VDD R2 RD R1
VDS
VGS + - VG
Voltage-divider circuit for MOSFET
DC equivalent circuit

9. DC Analysis – Saturation RegionID
VDD R2 RD R1
VDS
VGS + - VG
DC equivalent circuit

10. Example VTN = 1 V K = 0.1 mA/V2 VDD 5 V 20 k 30 k RD R1 iD vO CC vGvi R2 RD R1 CC vG
30 k
20 k
5 V
VTN = 1 V
K = 0.1 mA/V2

11. DC equivalent circuit ID
VDD R2 RD R1
VDS
VGS + - VG
The transistor is biased in the saturation region because VDS > VDS(sat) ;
since VDS(sat) = VGS – VTN = 2 – 1 = 1 V

12. Common-Source Amplifier Common-Drain Amplifier Common-Gate Amplifier
MOSFET AMPLIFIER
Common-Source Amplifier
Common-Drain Amplifier
Common-Gate Amplifier

13. COMMON-SOURCE AMPLIFIER - Basic Configuration
Common-source circuit with voltage divider biasing & coupling capacitor

14. Small-signal equivalent circuit
input
resistance
input
voltage
output
voltage
small-signal
voltage gain

15. DC load line & transition point
VDSQ
IDQ
VDD
vDS iD
Q-point
Transition point
DC load line & transition point

16. COMMON-SOURCE AMPLIFIER - with Source Resistorvo RL RS RD R1 R2 C1 C2 vi
+VDD
2 k
12V
3 k
RSi
300 k
200 k
CALCULATION EXAMPLE
Given:
VTN = 2V, K = 1 mA/V2,
 = 0
Determine:
i- Q-point values (ID , VDS)
ii- small-signal voltage gain
Stabilize the Q-point against variation of transistor parameters

17. Q-point values dc equivalent circuit VDD=12V RD=3 k R1=300 k VG
RS=2 k
RD=3 k
R1=300 k
R2=200 k VG
dc equivalent circuit

18. Q-point values (Cont)
For VGS = 0.54 V, MOSFET  cutoff ‘coz VGS<VTN .
Therefore, VGS = 2.96V
So, the Q point values:

19. Small-signal analysisVi Vo
Vgs
gmVgs
R1||R2
RD||RL + _ RS
RSi

20. COMMON-SOURCE AMPLIFIER - with Bypass Capacitorvo RL RS RD R1 R2 C1 C2 C3 vi
+VDD
RSi
Minimize the lost in small-signal voltage gain while maintaining the Q-point stability

21. Small signal equivalent circuitVi Vo
Vgs
gmVgs
R1||R2
RD||RL + _ ro Ri
RSi

22. COMMON-DRAIN AMPLIFIERvo RL RS R1 R2 C1 C2 vi
+VDD
RSi

23. Small signal equivalent circuit_ Vi Vo
Vgs
gmVgs
R1||R2 + ro
RSi
RS||RL
Vin G S D
input
impedance
Output
impedance
input
voltage
output
voltage
small-signal
voltage gain

24. COMMON-GATE AMPLIFIERvo RL IQ RD C1 C2 vi V+
RSi C3 RG
V -

25. Small-signal equivalent circuitRi
gmVgs Vi Vo
Vgs + _ RL
RSi RD Ii G D S
input
impedance
input
voltage
output
voltage
input
current
small-signal
voltage gain

26. JFET AMPLIFIER JFET Parameters DC Analysis Common-Source Amplifier
Common-Drain Amplifier

27. JFET Parameters Transconductance, gm Small-signal output resistor, ro
Magnitude only
i.e remove the –ve sign
Vp= VGS(off)

28. JFET Amplifier: DC Analysis

29. COMMON-SOURCE CIRCUITvo vi
VDD R1 RD R2 RL RS C1 C2 CS

30. Small-signal equivalent circuitVo
Vgs ro
RD||RL
gmVgs Vi + -
R1||R2
output
voltage
input
voltage
small-signal
voltage gain

31. COMMON-DRAIN CIRCUIT vo vi V+ RG RL RS C1 C2 V-

32. Small-signal equivalent circuitVo
Vgs ro RS
gmVgs Vi RG RL - +
input
voltage
output
voltage
small-signal
voltage gain

33. Small-signal equivalent circuit
Another form of small-signal equivalent circuit
small-signal
voltage gain