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ac or dc

Discussion in 'Electronic Basics' started by jason, Mar 31, 2005.

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  1. jason

    jason Guest

    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?

    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
     
  2. Andrew Holme

    Andrew Holme Guest

    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.
     
  3. Terry

    Terry Guest

    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!
     
  4. Active8

    Active8 Guest

    On Thu, 31 Mar 2005 12:44:58 -0330, Terry wrote:

    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.
     
  5. jason

    jason Guest

    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
     
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