What is the purpose of this "VD=VS=GND" MOSFET in the circuit

[bhl777]

Hi All, when I read some circuit, I often saw one block like the picture below. Can anyone teach me what is the purpose of this "VD=VS=GND" MOS? Thank you!

 

[ashugtiwari]

as far as i understand the circuit, i will try to explain things:
1. by Vd = Vs = GND means, drian voltage = source voltage = 0V i.e. GND, as seen from circuit, its pretty straight forward.
2. now replacing mosfet with its equivalent circuit, the significant thing is gate to channel capacitance i.e. a mosfet based variable capacitance is prominant at that circuit point.
3. the equivalent circuit for mosfet, between drain and source is transresistance and stray capacitance, where both will decrease when applied i/p voltage gate is higher, so in case if ground has high amount of surges or induced high freq emf, the lower stray capacitance and transresistance will keep it in GND.
I cant see any other significance of mosfet but i still try to analyse it.

hope that helps.

 

[AdvaRes]

Hi
Its a MOSCAP: The transistor is used as a capaciatance

 

[bhl777]

Thank you!

Hi
Its a MOSCAP: The transistor is used as a capaciatance