Small Signal Model for transistors

In summary, the small-signal model tells you the AC characteristics around a biased point. It is used when you need the AC characteristics (gain, impedance) of the transistor so that you can calculate the AC gain or impedance of the circuit that incorporates that transistor. The large-signal model tells you the AC characteristics around a non-biased point. It is used when you must calculate the AC characteristics of the circuit or device.
  • #1
nebbione
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Hi everyone, at my university (Computer engineering) we are studying the small signal model, but i didn't understand the practical application, i mean, why and when should it be used ?

For example at home i usually make electronics circuits, so i wanted to know how can i use the small signal model in my experiments ?

And when to use the large signal model ?

Thank you,
 
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  • #2
The small-signal model tells you the AC characteristics around a biased point. It is used when you need the AC characteristics (gain, impedance) of the transistor so that you can calculate the AC gain or impedance of the circuit that incorporates that transistor.
 
  • #3
And what's the difference between large signal model?
 
  • #4
nebbione said:
And what's the difference between large signal model?

... a small signal model, which lends itself well to small signal design and analysis. ... charge control model, which is particularly well suited to analyze the large-signal transient behavior ...


http://ecee.colorado.edu/~bart/book/book/chapter5/ch5_6.htm
 
  • #5
nebbione said:
And what's the difference between large signal model?

The issue is that it is VERY difficult to calculate things like gain, bandwidth, and distortion directly from first principles. You end up with Volterra series which are notoriously difficult to deal with.

By making some assumptions (the main one being the signal is small enough not to change the device's operating point) you can make the calculations MUCH easier, and also more insightful.

In practice you use the small-signal model whenever you can, and the large-signal model when you must.
 
  • #6
nebbione said:
And what's the difference between large signal model?
The characteristic curves of a transistor are very nonlinear. Its β varies with IC, its VBE varies exponentially with IB, etc. The small signal model is a linear approximation that works well for small excursions around a fixedpoint, and allows much easier analysis & design. See below.

The best idea is to think of the diode's exponential curve. For small excursions around a point on that curve, we can represent the exponential's behaviour as a DC voltage source in series with a fixed resistor, we have linearised the characteristic. The resistor value needed? It's determined as being equal to the slope of the exponential at that operating point.

Suppose you see that your transistor's collector current is 100mA when its base current is 1mA. You'd probably say its β = 100. But then you notice that for a collector current of 115mA the base current needed is 1.3mA. So it seems β here is only about 90? But when designing for small changes, we say IC changed by 15mA when IB changed by 0.3mA, giving a small signal β = ΔIC / ΔIB = 50
 

1. What is a small signal model for transistors?

A small signal model for transistors is a simplified representation of a transistor's behavior, focusing on the small changes in voltage and current around a bias point. It allows for easier analysis and understanding of a transistor's characteristics.

2. How is a small signal model different from a large signal model?

In a small signal model, the transistor is considered to be operating around a bias point with small changes in voltage and current, while a large signal model takes into account the full range of voltage and current values that the transistor can handle. The small signal model is useful for analyzing the transistor's linear behavior, while the large signal model is more appropriate for non-linear analysis.

3. What are the main components of a small signal model for transistors?

The main components of a small signal model for transistors are the input resistance, output resistance, and voltage gain. The input resistance represents the resistance seen by the input signal, the output resistance represents the resistance seen by the output signal, and the voltage gain represents the amplification of the input signal.

4. How is a small signal model used in circuit design?

A small signal model is used in circuit design to predict the behavior of a transistor in a specific circuit. By analyzing the input resistance, output resistance, and voltage gain, engineers can determine the best operating conditions for the transistor and ensure that the circuit will function as desired.

5. What are the limitations of a small signal model for transistors?

A small signal model is only accurate for small changes in voltage and current, and does not take into account non-linear effects that may occur at higher voltages or currents. It also assumes that the transistor is operating in a linear region, which may not always be the case in practical circuits. Additionally, the model may not account for parasitic components and other real-world factors that can affect transistor behavior.

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