MOSFET Amplifier by trigyy

VIEWS: 78 PAGES: 14

More Info
									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
Question & Answer




                    14

								
To top