# MOSFET transistor ac analysis vs dc analysis differences in currents

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#### Jumala

##### Newbie level 3
Hi,

I am doing an ac analysis of a nmos transistor, where I have applied 3 volts to gate and tied source and bulk to gnd.

If I insert a ac magnitude of 1 volt to the drain of the transistor and plot the current, why is it different when applying a dc voltage of 1 volt to the input? Can I calculate or measure the resistance of the transistor via ac analysis?

Thanks for the responds.

Hi,
Your post seems to be confusing. What is the exact input port? Is it gate or drain? And why you are comparing AC and DC analysis, of course they are used for different purposes. You should attach the schematic, and clearly show what are you trying to do. Then we may help.

• Jumala

### Jumala

Points: 2
Sorry if I did not explain clear enough, the input for the transistor where I am applying the ac magnitude is the drain (on the left side of the transistor). The gate is fixed to 3 volts, and bulk & source tied to gnd.

I am confused why the current (Ids) is different when I apply ac magnitude of 1 volt and dc voltage of 1 volt to the input (drain). For ac it seems to be in order of 5,759mA and dc 3.751mA (plotted from the source terminal)

I am using the transistor for an RF-application and want to know the resistance. You are calculating the ac resistance. There are lots of transitor parameters that will change between DC and AC situations. One basic difference will be due to the complex part of the transition impedance of the transistor. For example if the impedance is in the form of Z=R + jwX, since the latter part is frequency dependent, you will see different behaviors between DC and AC situations. You should calculate the resistance at your frequency of interest if you are planning to use it in an RF application. If you will use it in a frequency band for example 1.5-1.6 GHz, you may sweep the frequency of AC input.

• Jumala

Points: 2