I have always been intrigued by stories of motivation by persons who have come up with ideas that have caused us to change the way we think or live. In many cases, there appears to be no obvious path that led to these revelations; instead, they seem to have become manifest of their own accord. At other times, rather ordinary occurrences can be the source of inspiration. For example, the observance of a falling apple motivating Sir Isaac Newton to seek out the answer to an object’s path of descent when falling, which ultimately led to our basic understanding of what is now called classical physics.
The study and explanation of the movement of planetary bodies, which classical physics does, requires the use of models and mathematics that allow for the simulation of physical behavior in lieu of or prior to experimental verification. This same methodology is utilized for virtually all research, design and development done today; including electrical circuitry and electronic components. Undoubtedly, the standard for modeling electronic components; such as transistors, is SPICE. This tool can not only be used to model the behavior of transistors but to easily and accurately determine transistor parameters, as well. Before, exploring how to accomplish this, let’s take a look at some common transistor model types.
Types of Transistor Models
I can vividly recall being told once that a good way to look at a transistor is as two diodes. Whereas a diode is composed of N-type, where the majority carriers are negatively charged electrons, and P-type, where the majority carriers are positively charged holes, materials or regions transistors may have three regions, NPN or PNP. Diodes, also typically have two states: ON and OFF, while transistors may have multiple states; such as saturation, cutoff, active and reverse [active]. And similar to the operating point of a diode, transistors have an operating or quiescent point, which is defined by its DC biasing.
Transistor operation is not overly complicated. Basically, as long as the operating point falls within a specific region the device will perform as intended for that specific operational state (e.g. active or saturation). However, if the operating point crosses into another region, the transistor’s operation will change. Transistor models are developed to define the ranges for those regions and to select the best or optimal operating point or quiescent (Q) point around which operation can be maintained.
Typically, there are two classes of models for transistors:
Large Signal Models for Transistors
These models are used to determine the DC biasing of the transistor based upon its configuration. For example, bipolar junction transistors (BJTs) have three common-mode configurations:
Common-emitter (CE) - where DC current flows from collector to emitter and base to emitter. AC signal input is applied to the base and output taken from the collector.
Common-base (CB) - where DC current flows from collector to emitter and collector to base. AC signal input is applied to the emitter and output taken from the collector.
Common-collector (CC) - where DC current flows from base to collector and collector to emitter. AC signal input is applied to the base and output taken from the emitter.
Small Signal Models for Transistors
Small signal models are used after the large signal model has been determined to provide more precision. When a small signal is applied, it moves the operating point away from the bias point along the I-V characteristic curve based upon the amplitude of the signal. For proper operation, this deviation from the DC operating point must not cause the device to change its mode; for example, go from the active region into cutoff. Small signal models are usually two-port and may be of one of the following common types:
Both large signal and small signal analysis of transistors necessitates that you select a model, specify the knowns or fixed values and mathematically solve equations for the unknown parameters. These equations can range from linear equations to boundary value problems and iterative solution methods. In either case, it is preferable to utilize a software tool that can easily and accurately provide solutions. And the best of these is the Simulation Program with Integrated Circuit Emphasis, better known as SPICE.
Easy SPICE Model Parameter Determination for Your Model
In many cases, transistors are used much as diodes are, that is to switch electrical or electronic circuits ON and OFF. And just as for diodes, this is done by driving the transistor from one state of operation to another. For this type of functionality, large signal model transistor parameter determination is usually sufficient; coupled with knowledge of the given transistor’s configuration. Again, using the BJT as an example, in the CE configuration, active region operation requires that IB ≠ 0A; otherwise, there is no collector current flowing through the device as it is equal to the base current times the gain (IC = βIB).
When using SPICE, you are freed from the need to perform any mathematical calculations. Instead, you can utilize SPICE Model libraries and data taken from the device data sheet or that is specified by you to define the transistor model. Then the DC bias curve, as shown below, is obtained by simply simulating a simple circuit that contains your device.
Example VBE- IC Curve for BJT
SPICE Model Parameters for Transistors Accuracy Optimization
Probably, the greatest use of transistors is as amplifiers and it is highly likely that any RF PCB you design will contain one or more transistors. Although, large signal modeling and determination of DC biasing is still required, you will also need to utilize small signal analysis. And with the right SPICE program, you can refine the accuracy of your transistor model by including internal capacitances, resistances, gain variations, the early effect and other parameters; as shown below.
Example SPICE Model Parameters for BJT
The best SPICE program available is PSpice; included in Cadence’s PCB Design and Analysis packages; such as OrCAD and Allegro. With PSpice, you can accurately and quickly perform large and small signal modeling for transistors and verify performance through Analog/Mixed-Signal simulation prior to sending your boards off to be manufactured.
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