# Difference between large signal and small signal models

1. Mar 12, 2014

### 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]

2. Mar 12, 2014

### 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.

3. Mar 13, 2014

### milesyoung

What is small signal AC analysis?

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

4. Mar 13, 2014

### Staff: Mentor

In the small-signal model, the base-to-emitter path is represented by a resistance (of value 0.026β/IE 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.