ac and dc

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? 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 share

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 thanks

Jason

Reply to
jason
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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).

or

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 ....

Reply to
Andrew Holme

On 9 Mar 2005 08:17:54 -0800 in sci.electronics.basics, "jason" wrote msg :

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 one:

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--
Al Brennan
Reply to
Kitchen Man

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 parts.

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.

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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).

--
John Popelish
Reply to
John Popelish

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 circuit? 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 circuit? If they are the same, why capacitors are remain at small signal circuit? 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 Jason

Reply to
jason

mirror

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.

the

Yes.

equivalent

Yes.

small

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.

Reply to
Andrew Holme

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

Jason

Reply to
jasonclass

On 9 Mar 2005 22:08:31 -0800 in sci.electronics.basics, "jason" wrote msg :

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.

--
Al Brennan
Reply to
Kitchen Man

them

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.

Reply to
Andrew Holme

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

Jason

Reply to
jason

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