 ### Network # ac or dc

J

#### jason

Jan 1, 1970
0
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)

Actually we are calculating the Id in DC or ac?
In most of the time, we are interested in Id(dc) or Id(ac)?

I see there is book that put
Drain current = Id(dc) +id(ac)

So I just wondering if equation 1 will give us dc value or ac value?

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?

And also the input and output impedance, will there be difference for
ac or dc analysis?

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.

Thank you so much

rgds and thanks
Jason

A

#### Andrew Holme

Jan 1, 1970
0
jason said:
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)

Actually we are calculating the Id in DC or ac?
In most of the time, we are interested in Id(dc) or Id(ac)?

I see there is book that put
Drain current = Id(dc) +id(ac)

So I just wondering if equation 1 will give us dc value or ac value?

It gives you the sum:

1/2.W/L.unCox.(Vgs-Vth)^2 = Id(dc) +id(ac)
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?

You must use calculus to work out small-signal transconductance gm = d(Id) /
d(Vgs)
Also for transfer function, we are interested to find the Vout/Vin at
ac or dc value?

Frequency is a variable in the transfer function e.g. the Laplace s, or jw
(w = omega)
For DC, w=0
You can write a trasnsfer function, in terms of s, or jw, valid for both AC
and DC.
Often, but not always, we are only interested in AC
And also the input and output impedance, will there be difference for
ac or dc analysis?

For small-signal analysis, innput/output impedances may be calculated using
linear approximation i.e. taking the slope of the graph at that point and
assuming it approximates a striaght line.
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?

"Small signal analysis" means: small AC signal.

T

#### Terry

Jan 1, 1970
0
jason said:
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)

Actually we are calculating the Id in DC or ac?
In most of the time, we are interested in Id(dc) or Id(ac)?

I see there is book that put
Drain current = Id(dc) +id(ac)

So I just wondering if equation 1 will give us dc value or ac value?

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?

And also the input and output impedance, will there be difference for
ac or dc analysis?

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.

Thank you so much

rgds and thanks
Jason
Jason; having just taken a 'transistor' course at an advanced age and being
no expert it looks to me;

1) Upper case designations such Id are for DC.
2) Lower case, such as id are for AC which are the much slighter or 'Small
signal variations' through the device.
DC is the steady or unvarying quantity at one particular point of the
operating characteristic of the device.
3) Without referring to my text book;
Your Eqn. 1 looks to me like it is the one which determines a DC operating
point because it takes into account the threshold (Vgth)! i.e. (Vgs - Vgth)
.......

But willing to be corrected!

A

#### Active8

Jan 1, 1970
0
On Thu, 31 Mar 2005 12:44:58 -0330, Terry wrote:

Jason; having just taken a 'transistor' course at an advanced age and being
no expert it looks to me;

1) Upper case designations such Id are for DC.
2) Lower case, such as id are for AC which are the much slighter or 'Small
signal variations' through the device.
DC is the steady or unvarying quantity at one particular point of the
operating characteristic of the device.
3) Without referring to my text book;
Your Eqn. 1 looks to me like it is the one which determines a DC operating
point because it takes into account the threshold (Vgth)! i.e. (Vgs - Vgth)
......

But willing to be corrected!

Close. Lower case variables generally mean "instantaneous" values
i.e., wrt time. Upper case can be DC values (upper case subscript)
or rms and vector values (lower case subs). I the case of Id, the
convention is broken and if you look at a spec sheet, it's I_D. I
think people write Id so that the "d" is understood to be a
subscipt. You just have to take it all in context.

The OP was multi-posted to SED where I wrote a little table of
subsript conventions, so you might look there under the same subject
line.

J

#### jason

Jan 1, 1970
0
Dear Andrew Terry and Mike

It has been lucky to be able to see your posts with good explanation
Thanks a lot.
Terry you can refer to the same title under the group electronic
design, I think it is more comprehensive there Hope everyone learns this

rgds and thanks
jason