Difference between large signal and small signal models

[shawrix]

Please can someone explain the clear concept between large signal model and small signal models in BJT.
Please explain what to assume in large signal and small signals problem solving wise and also the concept.

Amongst vBE, VBE, vbe which is large signal?, small signal? and what we assume in the two models of BJT.

My go:

Is vBE = VBE in large signal model, or is vBE=vbe in small signal model? why do we neglect VBE?

Whats the purpose of two models? Also is BJT at DC equivalent to large signal model?[i know its dumb but you see i need all things cleared]

 

[Simon Bridge]

As you'd expect - the difference is in the signal size.
Consider the case: $v(t)=V_1+V_2\sin\omega t$

The signal is the $V_2\sin\omega t$ part and it's "size" is $V_2$.
If $V_1>>V_2$ then you can use the small signal model.

This situation, the voltage $v(t)\simeq V_1$ is approximately DC.

It is useful because it makes the math simple.

 

[milesyoung]

I did a small post about this not so long ago:
What is small signal AC analysis?

Usually you go through some material on linearization of nonlinear systems before discussing small-signal analysis.

 

[NascentOxygen]

Please can someone explain the clear concept between large signal model and small signal models in BJT.
Please explain what to assume in large signal and small signals problem solving wise and also the concept.

Amongst vBE, VBE, vbe which is large signal?, small signal? and what we assume in the two models of BJT.

My go:

Is vBE = VBE in large signal model, or is vBE=vbe in small signal model? why do we neglect VBE?

Whats the purpose of two models? Also is BJT at DC equivalent to large signal model?[i know its dumb but you see i need all things cleared]

In the small-signal model, the base-to-emitter path is represented by a resistance (of value 0.026β/I_E or similar). In the large-signal or DC model, the base-to-emitter path is represented as a diode junction, and this often simply approximated as a fixed voltage drop of about 0.6v. It's the large-signal model that is used to calculate biasing. The small-signal model (available in various complexities) is then used for calculations of gain at frequencies of interest.

 

[alphy]

The difference between large signal and small signal models in BJT (Bipolar Junction Transistor) is primarily related to the input signal amplitude and the resulting output response.

In a large signal model, the input signal amplitude is relatively high, causing significant changes in the BJT's operating point and resulting in nonlinear behavior. This means that the output response is not a linear amplification of the input signal, and the transistor may enter saturation or cutoff regions. In this model, we consider the transistor's DC characteristics and use the transistor's full operating range.

In contrast, a small signal model assumes a low input signal amplitude, resulting in minimal changes in the BJT's operating point. This allows us to approximate the BJT's behavior as linear, and the output response is a linear amplification of the input signal. Here, we only consider the transistor's AC characteristics and use only a small portion of the transistor's operating range.

In terms of notation, vBE represents the voltage between the base and the emitter, while VBE represents the DC voltage between the base and the emitter. In a large signal model, vBE is equal to VBE, as the input signal amplitude is high. In a small signal model, we use vbe to represent the small-signal component of the voltage between the base and the emitter, as it is a small change from the DC voltage VBE.

We neglect VBE in small signal models because it is a constant DC voltage and does not change with the input signal. In small signal models, we are only interested in the small-signal component of the input signal, so we neglect the DC component.

The purpose of having two models is to simplify the analysis of the BJT in different operating conditions. In some cases, the large signal model is more appropriate, while in others, the small signal model is more useful. For example, in designing amplifiers, we often use the small signal model to analyze the circuit's performance.

Lastly, it is important to note that the BJT's DC equivalent is equivalent to the large signal model, as we are considering the transistor's DC characteristics. However, the BJT's AC equivalent is equivalent to the small signal model, as we are only considering the transistor's AC characteristics.

I hope this helps clarify the difference between large signal and small signal models in BJT and the assumptions made in each model. It is important to understand both models and when to use