how to analyze MOSFET CS amp small signal model

Thread Starter

leese

Joined Dec 11, 2017
3
i want to find the voltage gain of this circuit.

so i tried to set input signal = 0 and assume the virtual Voltage source, Vx.

because i thought that find Vx/ix (Resistance at out) first, and than multiply -'gm' , the result is A.

1. is this way correct?
2 are there any circuits which cannot use this method(assume virtual V source and set input to GND)?
3. if 2. is Yes, what kind of circuit cannot use this method?

Im really sorry about my bad english but im waiting your answers. thankyou
 

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Jony130

Joined Feb 17, 2009
5,022
And if you want to find the voltage gain. All you need is to add the Vx voltage source at the input terminal and find the corresponding Vout voltage.
 

Thread Starter

leese

Joined Dec 11, 2017
3
i connected Rd to source terminal because i learned Vdd (and all DC voltage sources) goes GND.

and i saw similar example small signal models, Rd to GND, thus Rd and ro(Drain-Source resistor) are parallel and have same GND at MOS's source.

what is wrong about this circuit? Rd to GND or connecting Rd and Rs at same node?
 

Thread Starter

leese

Joined Dec 11, 2017
3
yes, you are right. i tried to draw new small signal model and i found my mistakes. thank you for useful answers.
 
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