# ac or dc

• posted

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?

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?

Thank you so much

rgds and thanks Jason

• posted

It gives you the sum:

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

You must use calculus to work out small-signal transconductance gm = d(Id) / d(Vgs)

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

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.

"Small signal analysis" means: small AC signal.

• posted

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!

• posted

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

• posted

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.

```--
Best Regards,
Mike```

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.