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# Difference between large signal and smal singal behaviour

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#### santom

##### Full Member level 2
hi all

Thank u very much for the relevant answers for my previous question

Can you people please help me in finding out the difference between large signal and smal singal behaviour of MOS Transistors..

Thank u very much in advance...[/b]

Re: Difference between large signal and smal singal behaviou

the main idea is that in the case of small signal behaviour the part of the transfer function we would be working on can be assumed linear which is not the case for large signal.... so the variation of parameters of MOS transistor with voltage and current levels need not be considered for small signal...

### santom

Points: 2
Thank u very much for the reply Mr.Anand.
But can u be more specific if you don mind writing about it...

Re: Difference between large signal and smal singal behaviou

tell me in which terms you want the answer to be.... i mean the configuration and which appplication or circuit.....

### santom

Points: 2
Anand..
am just starting to read botht the anlayses..

So i want to know the differences bet those two b4 i go further into this topic...like what are the parameters we would analyse in each of the sections respectively..

what are the advantages and disadvantages of each..
kindly throw some short info about those as i was unlucky enough to get a good website which teaches me this perfectly.

Thank u very much in advance..

Re: Difference between large signal and smal singal behaviou

k k i understand,.... the main parameters that i have come across in terms of MOS are Cgd,Cgs,Cox,Cbs,Cbd,r0,gm etc.... for example consider the Id vs Vds curve of a MOS... we bias the MOS at a particular point( i.e. Vgs value)... this point is usually taken in the saturation region if the circuit we are tryin is a amplifier... now consider a swing in Vgs then there will be a corresponding swing in Id and Vds....

here Id is given by Id=1/2kn(Vgs-Vt)² and for small variations of Vgs you can assume that Id directly proportional to Vgs (the limit upto which you call it small variation is decided by you based on the amount of violation to the assumption can be tolerated)

Now if the variation is large then Id is directly proportional to Vgs² and hence the amplification is non linear....
hope it is clear.... if you need anymore help.. post it....

### santom

Points: 2
YA ..

I understood to some extent Anand..

If am i right...

So as far as these two analyses are concerned,non linear systems can be analysed through large signal analysis and the linear ones by small signals

The limits for that region(linear/non linear) are decided by us by means of the values of the parameters under investigation.

So what exactly we are getting out of it...and also its pros and cons anand..

Thanks a ton for helping me and also for the future helps...

Re: Difference between large signal and smal singal behaviou

i think you got one of the points wrong... for linear systems both small and large signal analysis proceed the same way... for nonlinear it is not the case i.e. it maybe linear for small variations of something....

well small signal analysis ignores a lot of things which is not the case in large signal analysis i.e. many parameters that have been neglected in small signal analysis cannot be neglected in large signal.... formulas derived by small signal analysis may not be applicable to a point near it though it is the case for large signal too but the formulas found using large signal analysis are valid in the part under consideration and hence from that formula small signal analysis in the part under consideration can be done...

### santom

Points: 2
Thanks Anand..

Then why do we need small signal analysis on a whole if we can able to derive all the parameters' expressions from large signal analysis in case of a circuit..

Re: Difference between large signal and smal singal behaviou

well when you are going to use your circuit over a small predetermined region then what is the use in undergoing all the trouble of playing with more complex equations and more parameters.....

### santom

Points: 2
Exactly Anand..!!

A great salute to ur excellent snapshot answer..!!

So,it is obvious that we can use the respective analysis according to ur requirements..

Re: Difference between large signal and smal singal behaviou

ya man.... that is the catch... especially MOS is involved in a lot of misbehaviour as a circuit component... the amount of capacitance and noise makes things complex.... so large signal analysis is a big trouble...

one more info... maybe it would be helpful to you.... the problem with large signal analysis is that many parameters are interdependent and hence when a new value of something is found it would lead to change of someother parameter and you have to keep on recalculating things until the change of various parameters becomes negligible....
actually this is the basis of solving things using graphs because you can draw a line using two points based on your design parameter and the intersection of the line with the characteristics curve gives our required value of parameters.....

### santom

Points: 2
Ok sure Anand..!
Thats really a great help..
I owe u with much respect..
Hope u will clear out my future doubts too..
Good Luck

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