Small Signal Circuit Design

[ee01akk]

how to draw small signal

Can anyone tell me why we use small signal equivalent circuits?
Does anyone know any particularly useful websites that explain how to draw small signal circuits for common source, common gate and common drain amplifiers?
Thanks.

 

[rayen]

how to draw small signal model for mos

equivalent circuit to small signal is one very important method to analysing the anolog circuit!there're some useful models for the circuit apparatus.so it's very important way to analysing the circuit by hand although we now have many EDA tools for computer to simulate and analyse.

 

[v_naren]

drawing small signal model

it works like this.....

1) We first set the VDS and ID or VCE and IC at a fixed value to ensure that our devices(mos or bjt) will first of all act as a linear VCVS or CCCS respectively....hope u understand this first point correctly.

2) Next once the "Q" points have been set correctly, now we "send in" or "inject" a very "small signal".....I mean we impose alternating voltages or currents onto our device. These are btw the RF or biomedical or some signals...that we want to boost up in power(either thru Vgain(CB) or Igain(CC) or thru both Vgain and Igain(CE)).

3) So now if u see the ID vs VDS for different VGS and IC vs VCE for different IB u "must" observe that the curves are somewhat like IC=mVCE+c and ID=mVDS+c for bjt and mosfet for a small range or ID/IC and VDS/VCE around the bias point(provided u biased the mos/bjt in such a region in the ID/IC vs VDS/VCE graph)

4) so essentially u can now analyze the entire circuit first by writing down a complete ID=f(VGS,VDS) form.

now we take partial derivatives on both sides

=> d(ID) = (df/dVGS) d(VGS) + (df/dVDS) d(VDS) + higher derivatives....these higher derivatives are zero or near zero by our statement in point 3

so hence these 3 signals d(ID) and d(VGS) and d(VDS) are the "small signals"....they are just "pertubations or deviatons which are injected or caused by us"

PLEASE UNDERSTAND THIS CLEARLY...I DONT KNOW IF I HAVE CONVEYED THE FULL IDEA SO FAR....BUT I AM GOING ON....

5) now these sort of "math" can be carried out purely on paper alone for any electronic circuit....but it is a bit tedious.....and not a general method which can be used very easily...so we understand these equations by drawing what are called "small signal circuits"...or rather circuit in which the voltages and currents are the dID or dIC and dVGS or dIB and dVDS or VCE....and so are all the other V and I also at other points....go on below..

6) so we "represent" the "differential equation" in terms of circuit connections using L,C,R and controlled sources......this representation is called the "small signal circuit representation".........it is not a physical circuit....but just a way in which we humans can understand and more easily perform analysis...or rather atleast have a general first step for analysing the circuits wrt small signals.....PLEASE KEEP IN MIND THAT WE ARE DEALING WITH A DIFFERENTIAL EQUATION...SO ALL OUR CURVES....ANYTHING FOR THAT MATTER ARE ALL LINEAR around Q point or bias point...AND HENCE WE CAN DRAW SMALL SIGNAL CIRCUIT ...IN TERMS OF LINEAR COMPONENTS LIKE L,C,R AND CONTROLLED SOURCES....

IT IS A VERY VERY IMPORTANT QUESTION THAT HAS BEEN RAISED....I HAVE ALSO MENTIONED IN A PREVIOUS POST

PLEASE REFER TO CHAPTER 1 OF THE BOOK

Electronic Principles: Physics, Models and Circuits
by paul e. gray and campbell l. searle

in fact this book is really the best book for starting electronic circuit design mainly for analog....it is quite a dense book like grey and meyer for analog ic design.....i would say the former is the grey and meyer equivalent for discrete circuits or something....anyway....

hope I have been able to direct you in the right directions at least?
best of luck


p.s:- just found out...go here hxxp://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-002Circuits-and-ElectronicsFall2000/3C831A49-1BB2-4403-B201-27BE302EA6B5/0/6002L7.pdf

 

[amarnath]

method of drawing small signal equivalent circuit

the best way to learn about electronics is to solve as many problems as possible...there is a simple method of drawing ac. equivalent of an amplifier..

what u do is make all dc voltages as short circuits and all dc current sources as open circuits.then mark the ground of the circuit and just replace the transistor with its model.noww just transfer the elements connected in the amplifier and connect them to the model u have used for the amplifier...then use basic circuit analuysis to solve for voltage or current gain....schaums outline for electronic circuits by jimmy j cathey is a very good book...just take that and solve as many problems as possible..

any questions can be asked..i think the best way to learn is to solve as many problems as possible..so i suggest u either follow millman and halkias which is an excellent text if u want to know the physics of semiconductors and also u can follow sedra and smith which delas with the concept and lesser device physics....many solved problems are given in sedra and smith expecially..so practise them to attain a mastery of the subject...

 

[linxf2003]

how to draw small signal equivalent circuit

Small signal model is useful to find its AC or high frequency characteristics. I think every analog book will deal with this problem. It is not difficult, I think and it is a very useful skill.

 

[Edward_2288]

how to draw a small signal model

the small signal model only deals with the linearity.

 

[chevron_guy]

id vs vds small signal

edward is right,
the small signal model of transistors assume the input signal is small enough such that the characteristics of the transistor does not change.

for example, when the input voltage of a transistor is small enough, the transconductance gm remind constant, resulting in a linear behavour of the device.
Hence the small signal model only works when the input is small enough and the device operate linearly

my 2 cents