Introduction

Since the invention of the vacuum tube, the design of electronic circuits has indissolubly been connected with the development of electronic devices. In fact, the design of electronic circuits cannot be performed without knowledge of the physical operation of the devices from which they will be constructed. However, the approach to the design of application-specific amplifiers, as presented in this book, does not depend on the technology in which the circuits finally will be realized. The design approach, the concepts and the techniques presented can be used for designing vacuum tube amplifiers, amplifiers realized with discrete components or amplifiers realized in modern CMOS IC processes. Different technologies, simply introduce different design constraints and offer different possibilities for the implementation of the concepts. The designer’s task is to deal with the limitations and maximally exploit the specific implementation possibilities of the selected technology. For this reason, knowledge of the operation and modeling of electronic devices is regarded indispensable for designers of electronic circuits. However, physics and modeling of electronic devices has become a field of knowledge on its own. A full treatment would require a bookshelf full of material and is considered outside the scope of this work. The reader is assumed to have basic knowledge of the operation and modeling of discrete semiconductor devices, of passive devices and of CMOS and BiCMOS IC technology.

During circuit design, the designer must know in which way the performance aspects of the circuit are related to those of the electronic devices and how they can be affected by design. In general, the performance aspects of electronic devices depend on the device type and on its operating conditions. In IC technology, device scaling may also be used for performance optimization. Hence, our aim is to briefly summarize topics related to device selection, device scaling and selection of an operating point. This will be done for so-called active semiconductor devices only, hence for semiconductor devices that, when applied in conjunction with power sources, exhibit amplifying capabilities. We will confine ourselves to Bipolar Junction Transistors (BJTs), Junction Field Effect Transistors (JFETs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). We will describe their basic operation and device models.

In this book, we will advocate the use of different device models at different stages of the design process. At an early stage, we usually want to use relatively simple models that describe performance aspects of interest accurate enough to motivate early stage design decisions. Based on the analysis of relevant performance aspects, we will be able to select the device and its operating region. Parameters for these simplified models can be found from simulation with more complete models, or by estimating them from the parameters of those models or graphs. At a later stage, numeric simulation with more elaborate models is required to give an accurate prediction of the circuit operation. Nowadays, automated circuit optimization is used to obtain the best possible performance with the lowest cost factors and sensitivity for production process variations. Automated circuit optimization, however, is outside the scope of this book.

Design equations and symbolic circuit analysis

Setting up design equations for specific performance aspects, such as, noise behavior, small-signal dynamic behavior and temperature stability requires symbolic circuit analysis. SLICAP (Symbolic Linear Circuit Analysis Program) is an open-source symbolic simulator that can be used for this purpose.

Based on Python and Maxima Computer Algebra Systems.

It uses linearized (small-signal) device models of which the parameters can be related to the technology, the geometry and the operating point of a device. SLICAP will be used throughout this text book.

Numeric circuit analysis

The development of device models and the development of circuit simulators is an ongoing process that helps to increase the predictability of the behavior of electronic circuits. The first version of Berkeley SPICE (Simulation Program with Integrated Circuit Emphasis) was released in 1973. The program was written in Fortran by Nagel (see [20] and [21]). Within a few years, SPICE2 was released and widely accepted by circuit manufacturers. SPICE3 was written in C by Quarles (see [22]), it was released in 1989. Nowadays many SPICE-like simulators have been integrated in CAD software packages.

Most SPICE-like simulators can perform DC, AC + Noise and Transient analysis. The DC analysis uses nonlinear instantaneous device descriptions. During DC analysis all dynamic effects are discarded. The AC analysis uses linearized dynamic device descriptions. The operating point for the linearization is found from a DC analysis that always precedes the AC analysis. The AC analysis is in fact a small-signal frequency-domain analysis.

The influence of stationary noise sources can be evaluated with a small-signal noise analysis.

The transient analysis performs a time-domain analysis thereby using nonlinear dynamic device descriptions.

Some SPICE-like simulators are also capable of performing time domain noise analysis.

In this chapter, we will briefly describe basic models for SPICE3X versions. For a more detailed study of device modeling for SPICE, the reader is referred to literature: [23]. From these SPICE models, we will derive simplified descriptions that can be used for hand calculations for estimating dominant behavioral properties of the device. Some SPICE versions as well as other simulators support more elaborate models.

Most simulators also provide operating point information that can be used for the analysis of linearized circuits. If not, design engineers have to use IC design manuals or data sheets and extract these parameters from measured device characteristics. Alternatively, they can be determined from simulations with more complex models.

Simulation accuracy

The validity of the simulation results is limited by the level of detailing in the device models, the accuracy of the model parameters and the numerical routines that have been implemented in the simulator. Both the numerical routines and the device models are continuously improved. The validity of the model parameters, however, is not always obvious; particularly when using discrete components. Many CAD packages include device libraries with parameter values that are represented by more than three digits. However, the suggested accuracy may be far beyond reality. On\ the one hand, physical reproducibility does not justify the suggested accuracy, while on the other hand the characteristics obtained from simulation with typical parameter values may not correspond to those obtained from typical measurement data. Parameter extraction is a critical process and the designer should be aware of the validity of the models and the parameters. In case of any doubts the designer should check whether results obtained with measurements given in data sheets, can be reproduced with simulations.

This chapter

Basic models for the BJT, the JFET and the MOSFET will be discussed in sections Bipolar transistors, Junction Field Effect Transistors and MOS transistors, respectively. For all these devices will start with a brief description of the available device types, their symbols and their simplified physical structure. We will then focus on the device models that are implemented in SPICE and derive simplified models for hand calculations.

Section SLiCAP device models is devoted to the implementation of these device models in SLICAP.