jason wrote:
Hello All,
I have been studying electronics for some time but my understanding
on
ac and dc analysis is still not too clear.
I wish you all can put some time to explain a little about the
following;
Say for a mosfet(or cmos inverter) which has a DC biasing and ac
signal
applied to it).
When we are using equation
Id = 1/2.W/L.unCox.(Vgs-Vth)^2 ---------eq(1)
Drain current as Mosfet just begins to conduct in active region as
opposed to ohmic region.
Actually we are calculating the Id in DC or ac?
Actual current
In most of the time, we are interested in Id(dc) or Id(ac)?
The Mosfet needs to operate in proper region you desire--you must
calculate where that needs to be.
I see there is book that put
Drain current = Id(dc) +id(ac)
Here drain current is total current
So I just wondering if equation 1 will give us dc value or ac value?
Total current where Mosfet just begins to conduct.
Or the equation (1) is true for both ac and dc where we need to use
it
at a ac small signal circuit or dc small signal circuit?
Also for transfer function, we are interested to find the Vout/Vin
at
ac or dc value?
You're using the Mosfet as an signal amplifier--what is the gain at
bias point? Vout/Vin
And also the input and output impedance, will there be difference for
ac or dc analysis?
Anyone who has any document to explain about this, kindly share with
me.
In books, it seems like when it use small signal circuit to analyse
id,
gmvgs and so on. All are written in small letter. Are they all ac
value
to be taken into consideration?
Kindly shed some lights on these topic.
Please help
Thank you so much
rgds and thanks
Jason
In general terms--the circuit you design forces the MOSFET to operate
in a particular region. When that is fixed, small signals applied
produce outputs that correspond to small changes around your set point.
Many times small signal models are used to make the circuit less
complicated for small signal evaluation (i.e.-the ac signal)--since the
other components just set the operation point (bias) and once that's
done can be overlooked for small signal analysis--generally.
sci.electronic also might be of help
Good Luck
.
