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Chapter 12 High Frequency Transistor Cir... > 12.7 High Frequency Equivalent Circu...

12.7 HIGH FREQUENCY EQUIVALENT CIRCUIT OF MOSFET

Before analysing the HF response of a MOSFET, studying its small signal LF equivalent circuit provides a better insight. MOSFET Gate is connected through a high quality layer of Silicon Dioxide, with Gate to Source path acting as an insulator with a resistance of 1014 to 1015 Ω. Hence, Gate current is not considered. The only current to be considered is ID, Drain to Source current, which is a function of VD and VG at quiescent operating point.

Considering small incremental changes in Drain voltage VD and Gate voltage VG, small signal component change in ID can be represented by the linear relation

 

ID = gm · VG + gD · VD,

 

where gm is transconductance representing the control of input Gate voltage over Drain current and gD is the output conductance representing the control of output voltage over Drain current. In the discussion of small signal response, the capacitances inherent in the operation of the device and the parasitic capacitances have no influence. Small signal low frequency equivalent circuit of a MOSFET is shown in Fig. 12.17.


  

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