[Moved]: Small Signal Analysis

Can anyone give me detailed explanation for
What is Small signal and how its differ from large signal... Why small signal gain is necessary.. why we are not considering v_out(pp)/v_in(pp) as like measuring large signal gain.

Re: Small Signal Analysis

Analog_Domain said:

What is Small signal and how its differ from large signal...

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The small signal analysis is in general performed at the most inner circuits, where operations, let's say, algebraic with one or more signals ( multiplication, addition, gain, etc. ...) are made in the analog domain. The term "small signal" leads to the concept that we can simplify some mathematical models of the functions that describe such components, once in these cases we can, for example consider as practically linear the excursion of a signal around a specific value when its variation is much smaller than its absolute value ( e.g: V±Δv, where V>>Δv ), therefore for this analysis we should first determine the polarization values ( e.g In_(V), Vn_(V) ) so that the exact gain/factor can be calculated for this exact setpoint. In short words, the small signal analysis allows you to determine the expected behaviour of a circuit yielding a notable accuracy, without the need of performing a complete computational nodal analysis, such as is made by well-known simulators.

Re: Small Signal Analysis

Well, Lets say you are trying to analyze a first few stages of multi-stage amplifier system. For first stage, signal source could be a transducer producing few micro volts of signal voltage. It is very important for you to preserve the wave shape of the input signal. So you need, out of your first amplifier stage, an input output relation of the form:- output= Gain x Input. i.e equal increments in output corresponding to equal increments in input. But this is possible only if you bias your transistor in linear region. In BJT terms Q point to be set into linear region. Now, Q point is (VCEQ, ICQ). Here, if your input signal doesn't disturb your operating point Q, i.e. input (micro-volts) & supply voltage (A few volts), then the signal could be called as small signal.
As to why is it necessary, the reason is mathematical simplicity in analyzing as pointed out by "andre_teprom" above.

Re: Small Signal Analysis

Analog_Domain said:

Can anyone give me detailed explanation for
What is Small signal and how its differ from large signal... Why small signal gain is necessary.. why we are not considering v_out(pp)/v_in(pp) as like measuring large signal gain.

Click to expand...

All amplifier circuits are non-linear - and you cannot define the ratio V-out/V_in because both wave forms are not identical (large signal distortions).
That is the reason we define "small-signals" (and assume no waveform distortions) which apply to a limited amplitude region only.
Mathematically, this means that we approach the non-linearity by the corresponding tangent to the transfer curve at the selected operating point.
How large this region is depends on the amplifier type: For a single transistor amplifier it is rather small and for an opamp with feedback is rather large.

Re: Small Signal Analysis

So it Means, Small signal is the signal given to the gate of the amplifier r any circuit such that the mosfet tends to operate only in linear region for those signal. Is this correct?

Re: Small Signal Analysis

Analog_Domain said:

So it Means, Small signal is the signal given to the gate of the amplifier r any circuit such that the mosfet tends to operate only in linear region for those signal. Is this correct?

Click to expand...

Not only do you operate in the linear region but you mathematically assume the device is perfectly linear in that region.
Thus, for example, you can't do a small-signal distortion analysis because the small-signal analysis assumes everything is linear and thus there can't be any distortion.