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ac and dc

Discussion in 'Electronic Basics' started by jason, Mar 9, 2005.

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  1. jason

    jason Guest

    Hello All,

    I am beginner in electronics
    I wish to know why do we need to do DC analysis and ac analysis of a
    As a rule , what component or node we must short to gnd for ac
    equivalent circuit?
    Then for DC analysis, what are the rule of thumb?
    Please let me know in step by step. Or any online document you can

    Also if input is voltage source or current source which I do not know
    if it is ac or dc type, what should I do with them when I draw an ac or

    dc equivalent circuit?

    Kindly help

  2. The common names for the two kinds of analysis are transient and AC.
    Transient analysis follows the response of each component through
    time, as all the surrounding conditions change. It makes no
    assumptions about linearity, but simply follows the models given for
    each component. AC analysis assumes a single set of operating
    conditions and assumes that every part acts in a linear fashion, and
    solves for the gain and phase shift of the circuit over a range of
    frequencies. Transient analysis works in the time domain (where one
    instant follows another, and sine waves are a foreign language) and
    AC analysis works in the frequency domain (where all signals are waves
    that exist for all time).
    None. But before you do AC analysis, you have to figure out what bias
    point all the nonlinear parts (diodes, transistors, etc.) are
    operating at so you can pick the linear value you will use for the AC
    analysis. This bias point calculation assumes all capacitors are open
    circuit, and all inductances are shorts, so that time does not have to
    get involved in this calculation, since these are the time dependent
    DC analysis (and the bias point calculation that precedes AC analysis)
    requires detailed models of all the components.
    I have a shelf of books on the details of how each kind of analysis is
    done. However, you can download a free circuit simulator that does
    both types and gives you graphical and numerical results.
    LTspice/SwitcherCAD III.
    It also exports schematic files in ASCII form, so you can post them in
    places like this, for help with design questions. They also have a
    Yahoo forum you can join that deals with operation of the simulator,
    itself. But before you can understand how it works, you need to do
    quite a bit of study of the analysis methods, themselves.
    Draw a squiggle (one cycle of AC ~ ) after the voltage label on the
    voltage or current source.
    That is the one with all the capacitors open (removed) and inductances
    shorted (replaced by their winding resistances).
  3. Andrew Holme

    Andrew Holme Guest

    DC analysis is about bias point and quiescant (quiet) state i.e. what
    the voltages and currents are when there is no signal.

    AC analysis is about gain and frequency response to signals.
    Anything that is at a constant potential e.g. because it is decoupled
    to ground by a capacitor; or it's a power supply rail ...
    Well, for bipolar transistors, you can often get away with:

    1. Ie = Ic+Ib
    2. Vbe = 0.7V
    3. Ic = beta * Ib

    If you set the base voltage with a potential divider, the quiescant
    emitter voltage will be fixed about 0.7V below that. You can then set
    the currents with a resistor from the emitter to ground. It's a big
    subject, but that should give you a flavour. There are many books
    about this (e.g. The Art of Electronics by Horowitz and Hill).
    Well, you can usually guess from the context. If it's a signal, it's
    probably AC. If it's a capacitively-coupled input, you would omit it
    from your DC analysis ....
  4. Kitchen Man

    Kitchen Man Guest

    The math is different. For DC (steady state), you can make do with
    algebra. AC and transient requires calculus and imaginary numbers.

    You may find answers to this and your other questions on sites like this
  5. jason

    jason Guest

    Thank you everyone for spending time and effort to help.
    I really appreciate it.
    I would like some confirmation from you for the following;
    1) For ac analysis, current sources from coponent like a current mirror
    or current sink must be shorted? Am I right?

    2) For ac analysis, voltage source such as battery must be shorted.
    While anything that is represented by a circle with ~ in it (~) is the
    alternating voltage which must be kept for ac analysis. Am I right?

    Kindly let me know if the above statement is right

    Then is ac equivalent circuit another name for small signal equivalent
    Are they the same ?

    In small signal circuit, I do see capacitors still being kept in the
    circuit. So does it mean small signal circuit is not ac equivalent
    If they are the same, why capacitors are remain at small signal
    Another thing is that I read that capacitor is considered shorted at
    high frequency. So how will this affect any general ac signal or small
    signal model?

    Kindly enligthen.

    Thank you

    rgds and thanks
  6. Andrew Holme

    Andrew Holme Guest

    No. These constant current sources/sinks bias the transistors
    *without* shunting (i.e. loading) the signal. They present a
    high-impedance to AC signals so, in your AC analysis, you replace them
    with an *open* circuit i.e. remove them.
    It is not black and white / open or closed curcuit. There are degrees.
    The reactance of a capacitor depends on frequency. Big capacitors are
    almost a short circuit to high frequencies. It depends on the circuit
    designer's intention. If he just wants to couple a signal and block
    DC, he selects an appropriately large capacitor. Other times, he
    chooses an in-between value that does not approximate an open or a
    closed curcuit. These in-between values are there for various reasons:
    impedance matching, phase shifting, filtering .... and, yes, you need
    to leave them in your AC equivalent circuit. What constitutes
    "in-between" depends on the reactance of the capacitor compared to the
    impedance levels in the surrounding circuitry. It's relative.
  7. Guest

    Hi Thanks a lot for the suggestion
    But may I know what does the archieves function do for us?

  8. Kitchen Man

    Kitchen Man Guest

    Jason, my best advice is to invest in some formal training, or at the
    very least get some good textbooks and make sure you have the math
    skills to follow the analyses. Circuit analysis is not trivial. I came
    out of the USAF with an excellent background as a RADAR technician, and
    when I started on my Electronics Engineering degree, the circuit
    analysis classes kicked my butt.
  9. Andrew Holme

    Andrew Holme Guest

    Imagine someone connects a voltage source to the constant current
    source; now imagine the voltage changes by a small amount dv. The
    current does not change - because it's constant - so:

    di/dv = 0

    r = dv/di = infinity

    So, to AC, the constant current source "looks" like a high resistance.
    It's never infinite in real life because no current source is pefect.
  10. jason

    jason Guest

    Thanks a lot Joseph, Andrew and Kitchen
    It is really great to see such an excellent answer in this forum. I
    believe some of them cant never be found on books. Actually I am
    following some text, else I wont have so many questions as the more I
    read , the more I would ask and the more I will know . Of course thanks
    to all who have guided me here.
    Cheers and thank you all

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