# Circuits Calculus Design!!

#### Nara Shikamaru

Joined Nov 24, 2007
92
Hey Everybody!!

i have always wonder how do you calculate complex circuits!!!....i mean, when youre in college they teach you how to solve by Nodes, kirchoff, thevenin and norton Resistive circuits and RLC circuits......but after that they never teach you how to calculate and design your own circuits....

my question is....how do you calculate the values of the components in a complex circuit???

for example, check this circuit of a voltaje regulator.....how do you calculate its values???

thanks!!

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#### hgmjr

Joined Jan 28, 2005
9,029
Before you can go forward with the design of the circuit, do you have any target performance specifications?

For example, what voltage and current do you want the linear regulator to provide?

hgmjr

#### mik3

Joined Feb 4, 2008
4,843
Hey Everybody!!

i have always wonder how do you calculate complex circuits!!!....i mean, when youre in college they teach you how to solve by Nodes, kirchoff, thevenin and norton Resistive circuits and RLC circuits......but after that they never teach you how to calculate and design your own circuits....

my question is....how do you calculate the values of the components in a complex circuit???

for example, check this circuit of a voltaje regulator.....how do you calculate its values???

thanks!!
The designer of the regulator tells you in the datasheet what capacitors and resistors to use for best results.
If you want to modify it u have to use your own mind and knowledge at maths, but its vary complicated cuz you have to take care of many factors which affect the overall function of the circuit.
I know its very complicated to design complicated electronic circuits entirely from the beginning. Its better to find subcircuits and combine them together to make your own circuit

#### Audioguru

Joined Dec 20, 2007
11,249
Ohm's Law is used to determine the values of some resistors. The manufacturer's formula for other resistors is on the datasheet and uses simple arithmatic, not calculus.
The datasheet discusses the recommended values of the capacitors.
No calculus. The circuit is simple, not complex.

#### Nara Shikamaru

Joined Nov 24, 2007
92
Ok....i didnt mean Calculus as Integrals and differentials......i meant.....how to calculate.....so let me get this straight......what youre saying is that with the datasheets of each device i get the notion of what resistor, capacitor or inductor must i use???....and from there i calculate the values??.....so why must you see in college Node analisys and thevenin and norton if theyre useless?......

so....heres another thing....check this circuit.....how did the designer got to those values??....just using datasheets???...

thanks for all the help!!

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#### Audioguru

Joined Dec 20, 2007
11,249
Opamps have a very low input bias current which is listed on the datasheet. So very high values can be used to bias the inputs.

The circuit uses the ratio of the resistors to do what it does. The ratio is simply a division of two numbers.

I guess you need to learn about the details of electronic circuits to make a design or to analyse a design. Then select the opamp and design/analyse the circuit to work properly with its parameters in its datasheet.

#### scubasteve_911

Joined Dec 27, 2007
1,203
Equivalent circuits and approximations are used a lot to hit a specific performance specification and limitations in terms of voltage/current available. Either circuits are modeled/calculated properly, or the designer uses empirical rules based upon experience or 'rules of thumb'.

Your question is very broad and the answer pretty much boils down to the core of engineering, which is a good theoretical background, reasoning, and most importantly, practical experience.

Steve

#### Papabravo

Joined Feb 24, 2006
13,931
The best advice I can give is divide and conquor. Read datasheets and application notes. Do experiments. Build up a circuit of interest, change values see what happens. Get a simulator, and do the same thing you would do with a real circuit.

#### hgmjr

Joined Jan 28, 2005
9,029
Ok....i didnt mean Calculus as Integrals and differentials......i meant.....how to calculate.....so let me get this straight......what youre saying is that with the datasheets of each device i get the notion of what resistor, capacitor or inductor must i use???....and from there i calculate the values??.....so why must you see in college Node analisys and thevenin and norton if theyre useless?......

so....heres another thing....check this circuit.....how did the designer got to those values??....just using datasheets???...

thanks for all the help!!
Actually, what you get from data sheets are the ranges of the various parameters associated with a particular device. It is up to the designer to tailor each device used in the design with the appropriate resistors, capacitors, and inductors to obtain the optimum performance from the device without exceeding the manufacturer's specified limits.

It is through the use of the various circuit analysis techniques (e.g. Ohm's Law, Thevenin's Method, Norton's Method, Kirchhoff's Voltage Law, Kirchhoff's Current Law, Nodal Analysis, and others) that the designer sets the various bias voltages, current limits, and gains in the circuit that best suits the exact performance being sought.

hgmjr

#### tronics

Joined Apr 16, 2008
14
i agree, i think once you get understand how the circiut functions, then it is just a case of choicingcomponents to fit the bill, and doing the maths, for example as someone said ealier, op amps use a r1/r2 type sum, so knowing that you can work out gains, or 10ib 11 ib for transsitors, they are rules of thumb you get to know when designing circiuts,it will come with experience, for example if im designg a simple transsitor amplifier, i start by looking at my input signal, deciding what i want my out put signal to be, then choicing a transistor which can tolerate those voltages, can collector current, then i choice the resistors to bias it correclty and set the quiesent point so i dont clip the output voltage, and the same for setting the gain,its just one of things you get used of over time and i dont theres any right or wrong, or hard and fast rule for doing, . hope ive helped a bit