# what is smal & large signal models ?

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#### p.sivakumar

##### Member level 1 large signal modeling

Hi

This is siva,i was new for analog design,so i have one basic doubt that is ......

What is smalsignal model ?

What is large signal model ?

what is actual difference between the smal signal model and large signal model ?and what is the necessity of that models ?

Whare we can use smal signal model circuit and whare we can use large signal model circuit for analysing the analog circuit ?.

#### sunking difference between small signal vs large signal

small signal: ac
large signal: dc

and you can check the analog circuit design books.

#### nagag

##### Newbie level 4 when to use large signal circuit

Normally any active device which we use for linear applications(ex:amplifier) will have non linear transfer function(input vs output).In order to use those devices for linear applications we have to limit our operation to a small segment where you can consider the transfer curve as linear.Large signal model can be applicabale any where in the input range and it will consider all non linearities.Small signal model will be applicable at that pirticaular operating point only for small signals.
"Micro electronics" by Sedra and smith will give you a very good idea.

#### ashaheer

##### Junior Member level 2 #### gigle

##### Junior Member level 1 large signal equation

small signal model : model for small signal
large signal model : model for large signal
I am not kidding. Because of the nonlinear behavior of transistors, we need to linearlize the model for small range input. Actually you can get the small signal equation by differentiating large signal equation.

Once you fix the operating point, small signal parameters are fixed from large signal equation. From the point you can easily estimate the small signal performance. For the low freq. small signal model you can ignore some parasitic capacitance while you can not do that for high freq. small signal model.

For example, in op-amp when you solve slew rate you will use large signal model. When you wnat to know the DC gain (low freq gain) or BW you will use small signal model.

#### pixel Small signal analysis is valid for small enough signals that produce linear response at the output.
This is not a case if a signal was large enough to change component paremeters significally(i.e. gain, impedance...), or in the worst case region of operation.

#### suria3

##### Full Member level 5 What is smalsignal model ?

Small signal model is for ac analysis where this model is to see all the output parameters like gain and BW when it produces linear responses as in saturation.

What is large signal model ?

Large signal model is more to dc analysis when you circuit is not working in linear response like in triode. Example when you want to find out slew rate of your circuit, you will apply large step signal input with correspond to large dc current.

what is actual difference between the smal signal model and large signal model ?and what is the necessity of that models ?

The necessity of these models is, they will help you to find out your circuit parameters before you start design. Let say you wanna design circuit which will need a gain of certain dB, then by using the models, you can know where to tune the circuit in order to achieve your target.

Whare we can use smal signal model circuit and whare we can use large signal model circuit for analysing the analog circuit ?.

Both models you have to use in the early stage of desigining. Thanks

##### Member level 5 These are the teminologies used for active circuites (generally the ones invloving semiconductors).

Small Signal Model deals with the situation where DC Voltage sources are shorted and current sources opened and only the effect of AC is observed on the circuit. This analysis involves capacitance, inductance etc. at various points of the active circuit. This is mainly meant to study the actual operation of the circuit to the input.

Large signal model is the DC model where ac sources are neglected depending upon their nature and only the effect of DC is studies on the circuit. This is mainly meant for the stability and distortion analysis of the circuit when large inputs are applied.

#### pixel Large signal model is the DC model where ac sources are neglected depending upon their nature and only the effect of DC is studies on the circuit.
Large signal model is used to find DC operational point, but it is not same as DC model. You can also have signal in large signal model.

#### electronics_kumar small signal :signal of value less than 1V and for linear applications
Large signal: signal of value greater than 1V and meant for non linear applications

#### pixel electronics_kumar said:
small signal :signal of value less than 1V and for linear applications
Large signal: signal of value greater than 1V and meant for non linear applications

:!: 1V have nothing with this...
Linear range depends on used components, and configuration.

#### ferrite

##### Member level 1 small signal means that you wanna investigate the response of a circuit when it is already biased at certain operationg point. on this point, the system is approximated as a linear system, just as you approach a complex function by its Taylor's expansion.

#### electronics_kumar pixel said:
electronics_kumar said:
small signal :signal of value less than 1V and for linear applications
Large signal: signal of value greater than 1V and meant for non linear applications

:!: 1V have nothing with this...
Linear range depends on used components, and configuration.
i couldn't understand your wordings.... whether you r objecting my say or else....

#### pixel Are you sure that for input signal range 1V you will have linear response, and for greater signals than 1V non-linear?
There are much systems that can become non-linear for much smaller voltages than 1V, and I pointed that this part of your statement was not correct.
(put 1V as a input of high gain differential amplifie i.e gain=1000, and see will you have linear output response - 1000V). Also you can have systems with linear response for input voltages greater than 1V.

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