MOSFET Small Signal Operation and Modelling

 MOSFET Small-Signal Model

 

• Since gate is insulated from channel by gate-oxide input resistance of transistor is infinite.
• Small-signal parameters are controlled by the Q-point.

            

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Body Effect in Four-terminal MOSFETs:

      

• Drain current depends on threshold voltage, which in turn depends on VSB.
• Back-gate trans-conductance is given as:

 

• Where, 0 < η < 3 is called the back-gate trans-conductance parameter.
• The bulk terminal is a reverse-biased diode. Hence, no conductance from the bulk terminal to other terminals.

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 Small-Signal Model for PMOS Transistor:

           

• Positive signal voltage vgg reduces source-gate voltage of the PMOS transistor, causing decrease in total
• current exiting the drain terminal, equivalent to an increase in the signal current entering the drain.The NMOS and PMOS small-signal models are the same!
• For a PMOS transistor : 
  

 

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