dc or ac

on

signal

You bias the FET and find the biasing voltages/currents in order to find the small signal model (which applies to the AC). Generally you have DC biasing voltages well above what you expect for an AC signal you want to use. This allows the AC not to affect biasing to a certain degree. You generally want to do something to the AC only, i.e. amplify or buffer.

That is the DC biasing equation. It may work for AC, but since Vgs will be changing, it wouldn't be worth much.

Id DC determines the small signal model used to find Vout or Iout (AC). For example, transconductance in small signal is determined by Id (DC) but is used to determine id (AC)

DC, id is determined by the small signal model.

it

Small signal models usually replace the transitor with a current source and two resistors.

at

AC

Yes

These are lecture notes to a course at UCSD I took. The BJT and Mosfet show how small signal models are derived. It may help.

id,

value

They are small signal AC values.

It gives you the sum:

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 For DC, w=0 (w = omega) 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, input/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.

Hi Geo, Andrew and Mike,

Thanks so much for the time to explain. It is very helpful advice and guide.

Tried the calculus solving gm = d(Id) / d(Vgs) Find that Dc part which is a constant will vanish after the differentiation So left with the ac terms ->d id(ac)/dvgs (Note that dVgs is actually dvgs) AM I right Andrew?

Then for Mike, you have make it really a simplified way to understand the notations. One last thing to check that is , what does IOW stand for ?

Thanks a lot Geo , Andrew and Mike

rgds Jason

I've seen capitalization used incorrectly in places, but here's the way I learned to use it:

UC = upper case LC = lower case

variable subscript qty represented example UC UC steady dc component I_C, V_CE UC UCUC DC power supply V_CC LC UC total instantaneous i_B, v_BE IOW, referred to 0V

LC LC instantaneous AC component i_b, v_ce IOW referred to DC component I_B or V_CE - the average value of the sine

UC LC RMS value of AC signal I_b, I_c, V_be

note that the last notation is used in sinusoidal stady-state analysis, where we use vector values.

Also note that in F(s) = L f(t), F(s) suggests a steady state or rms value and after solving and transforming back to the time domain, you get f(t) which is used to solve instantaneous values. You can see that in V(s) = L v(t).

You shouldn't have snipped out the relevant equation:

Id = 1/2.W/L.unCox.(Vgs-Vth)^2 ---------eq(1)

and you didn't show your answer.

Yeah, constants drop out, but here's the prob. You're using the model of the MOSFET in saturation. I think you want it in the quadratic region: 2 W V_DS Id = u.Cox - [ (V_GS - V_T)V_DS - ---- ] L 2

The other prob is that dI/dV should really be the partial diff of I_DS wrt V_GS with V_DS held constant. IOW:

D = round d

D I_D |

----- | D V_GS | V_DS

Then you'll get:

W gm = u.Cox - V_DS L

In saturation, i.e., eq(1):

W gm = u.Cox - (V_GS - V_T) L

See how ugly it looks when I do V_GS rather than just Vgs.

Try the calc for an exercise.

"In other words." The Dialog news reader underlines it and gives the definition in a tool tip pop up. I'm trying out the Pan reader and I don't think it does that, but you can always pop abbrevs and acronyms into google and see what happens. Sometimes adding "jargon" or other key words helps.

You're welcome. Thanks for helping to reinforce my knowledge.

Yes.

Using Mike's notation strictly, the total instantaneous variables would be: i_D, v_GS; instantaneous AC components: i_d, v_gs; steady DC components: I_D, V_GS

So: v_GS = V_GS + v_gs d(v_GS) = d(v_gs)

-- IOW = "in other words"

I read in sci.electronics.design that Andrew Holme wrote (in ) about 'dc or ac', on Fri, 1 Apr 2005:

Note the date. 'IOW' means 'Isle of Wight'.

-- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?'

LOL. Good observation, but I posted and jason queried on the 31st. Andrew posted on the 1st and I hopefully straightened out the whole subject (the gm thing) on the 1st.