small signal model, large signal model?

anyone can explain the small signal model and large signal model?
1) for Hspice, there are 49 levels model. are they large or small signal model? can we classify them as that?
2) how to choose which model should be used when design a circuit?
3) we use hspice to design both of digital and analog IC.can we use the same model to simulate them? what 's the model difference for digital and analog IC design?
4) small signal model is only used to design op amp? not for digital circuits?
5) how about other model? AC model, high frequency model.... what is the difference? when are they going to be used?

HSPICE is for simulating the transistor irrespective of its applications. It characterizes the MOSFET (n and p).
Large signal analysis : is what happens at all kinds of input values.
Small signal analysis : is what happens when all transistors are in saturation region
Obviously, digital is never continuously in saturation. so no use of small signal analysis.
small signal models are a way to easily understand the operation of the circuit.
I hope it clears...
srivats

49levels should be BSIM3 model. It should include both small-signal and large-signal model.

A good model should involve both of them.

AC model, high frequency model might be same as small-signal model.

triquent said:

anyone can explain the small signal model and large signal model?
1) for Hspice, there are 49 levels model. are they large or small signal model? can we classify them as that?
2) how to choose which model should be used when design a circuit?
3) we use hspice to design both of digital and analog IC.can we use the same model to simulate them? what 's the model difference for digital and analog IC design?
4) small signal model is only used to design op amp? not for digital circuits?
5) how about other model? AC model, high frequency model.... what is the difference? when are they going to be used?

Click to expand...

Hspice 49 model is large signal model, but simulator can create small signal model from lvl 49 model at AC sim. Small signal is when model is linear, diode become resistor, capacitor and other linear. Same true for mos and bjt.

anyone can explain the small signal model and large signal model?
-> Network analysis supports only linear elements. i.e. superposition theorem and
krichoff's laws are only applicable to linear elements. If you take transitor its not
a linear device if you take it from saturation to cutoff. i.e. its tranfer characteristics
is nonlinear. So we can not apply our network theorms to transistor circuits. But
still there is a way around if you consider small area of transitor transfer curve,
we can approximate it with a straight line. If we restrict transistor operation in this
region we can apply the network theorm to analyze transistor circuit. We model
transistor for this "small" region and call that equivalent network model as small
signal model.
But in some circuits we make use of full transistor transfer function. Example is
power amplifiers where we take transistor from almost cutoff to saturation. In such
a case we need to form a nonlinear model for transitor known as Lasrge signal
model For BJT Ic=Is exp(Vbe/Vt) is the Lasge signal model.

Small signal models are linear models, they depends on biasing of device.
Lasge signal models are generally nonlinear.


1) for Hspice, there are 49 levels model. are they large or small signal model? can we classify them as that?
-> I dont know.

2) how to choose which model should be used when design a circuit?
-> You should first decide what you want to analyze. Small signal AC analysis
use small signal ac model, In applications where ur are operation ur transistor
over a large portion of its characteristics you shoud go for Large signal model.
If ur interested in high frequency response go for small signal high frequency
transistor model.

3) we use hspice to design both of digital and analog IC.can we use the same model to simulate them? what 's the model difference for digital and analog IC design?
-> For digital we need to use large signal model because here we take transistor
from cutoff to saturation. Here in digital circuits we are only interested in switching
capability of transitors so here we can use special switch models. We select
special models to reduce computation time without loosing accurracy in our results


4) small signal model is only used to design op amp? not for digital circuits?
-> Yes! Some times large signal model is used to design op amp o/p stage.

5) how about other model? AC model, high frequency model.... what is the difference? when are they going to be used?
-> AC model is where you neglect biasing. High frequency model is small signal
model containing junction capacitances.
Hope this helps

-nand_gates

nand_gates said:

anyone can explain the small signal model and large signal model?

Small signal models are linear models, they depends on biasing of device.
Lasge signal models are generally nonlinear.

Click to expand...

Hi, I still get a little confused on this issue: "small signal models: linear; large signal models: nonlinear". May I say that the harmonics are all generated from large signal models?

BTW, what's the hspice level 49 model and BSIM model? one for BJT, one for MOSFET?

Thanks.

1) the Hspice level 49 is the BSIM3 modelling for the MOSFET alone!!!!!!! it is the complete large signal model only!!!!...hspice will derive the small-signal model from the large signal model parameters in the model file and bias condition (which u set up).

2) you dont use the 100's of equations when doing design...the complicated models such the BSIM3( level 49) are only useful to predict how well your on-paper design worked.

3) You must of course use the same model...what do u mean by this? what else will u use? what do u mean by difference in the model when u use the same model(49 or Bsim3)??

4) yes

5) ohhhhh