# MOSFET Amplifier by trigyy

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```									Analogue Electronics I
EAB 2014

Jan 2009

Lecture 9 - MOSFETs
Amplifier

1
Topics To Cover
•   Small-Signal Analysis
•   Common-Source Amplifier
•   Common-Drain Amplifier
•   Common-Gate Amplifier
•   Refer to Chapter 8 Boylestad

2
Small-Signal Analysis
Transconductance:
• The relationship of VGS (input) to ID (output)
• Transconductance is denoted by gm and given as:

ΔI D                  2I DSS    ⎡ VGS ⎤
gm =                  gm =           ⎢1 − V ⎥
ΔVGS                   VP       ⎣     P ⎦

3
Small-Signal Analysis
2I DSS                         ⎡ V ⎤
When VGS = 0V         g m0 =                     ∴ g m = g m0 ⎢1 − GS ⎥
VP                            ⎣ VP ⎦

V GS            ID
And                 1−      =
VP          I DSS

⎛    V     ⎞               ID
g m = g m0 ⎜ 1 − GS
⎜          ⎟ = g m0
⎟
⎝     VP   ⎠              I DSS
b                c
g = 2K n V GSQ @ TN
V
But most of the time,ma simplified formula for DC bias is used:

4
Small-Signal Analysis
Input Impedance: Zi = impedance at the input node
Output Impedance:
Z o = total impedance at output node
1    ΔVDS
= rd =     =
yos   ΔI D     VGS = cons tan t

yos = admittance equivalent circuit parameter listed on FET
specification sheets (open circuit)

Voltage Gain:           AV = Vo
Vi
5
Equivalent Circuit
MOSFET                  Small Signal Circuit

Notes: For p-MOSFET, the current flows upwards

6
Common-Source (CS) Amplifier

Fig. 1. Example of Common-Source (CS) amplifier connection

7
Conversion to Small-Signal Model

Fig. 2. Small-signal common-source (CS) small-signal equivalent circuit
8
Common-Drain Amplifier

Fig. 3. Example of common-drain (CD) amplifier connection

9
Conversion to Small-Signal Model
Small-signal equivalent circuit
of NMOS source-follower

Fig. 4. Simplified small-signal model where all ground
are put to a common node                                        10
Common-Gate Amplifier

Fig. 5. Example of common-gate (CG) amplifier connection

11
Conversion to Small-Signal Model

Fig. 6. Small-signal model for a CG amplifier   12
Conversion to Small-Signal Model

gmVgs
Ii       RSi       Vgs              RD   ID   RL

Fig. 7. Small-signal model for a CG amplifier with
Norton equivalent current source                              13