jason said:

Hello All,

I am beginner in electronics

I wish to know why do we need to do DC analysis and ac analysis of a

circuit?

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).

As a rule , what component or node we must short to gnd for ac

equivalent circuit?

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

parts.

Then for DC analysis, what are the rule of thumb?

Please let me know in step by step. Or any online document you can

share

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.

http://www.linear.com/company/software.jsp
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.

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

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).