Introduction to amplifier design#
Amplification is the most important basic electronic information processing function. Electronic amplifiers are the physical objects that embody this function.
Application areas for electronic amplifiers are numerous. Amplifiers are applied in radio receivers to raise the level of an antenna signal to such an extent that digitization and/or demodulation can be carried out. In all kinds of electronic equipment, amplifiers are applied to adapt the level of sensor signals to the input range of analog to digital converters, or to adapt the output level of a digital to analog converter to the driving level of actuators. Amplification is also implemented in other basic functions, such as voltage and current references, oscillators, active filters, comparators, limiters and memory circuits. As a matter of fact, the principle of amplification is applied in basic digital cells, such as inverters, gates and digital memory circuits. Amplification of electrical signals is thus indissolubly connected with electronic information processing, and amplifiers can be found in many manifestations.
Functionality#
Intuitively, one might say that an amplifier provides its load with an enlarged copy of the source signal. According to this assumption, amplifiers would need one input port and one output port to which the signal source and load are connected, respectively. This, however, is not true. The signal power that an amplifier can deliver exceeds the available power of the signal source. Amplifiers therefore, have an additional port to which a power source needs to be connected. Due to the power from this source and the amplification mechanism embodied in the amplifying devices, the amplifier’s so-called available power gain can exceed unity.
This property makes amplifiers fundamentally different from transformers and lossless electrical networks, such as, matching networks. Transformers and matching networks can have either a voltage or a current gain that exceeds unity, but their available power gain is limited to unity.
The amplification mechanism itself is embodied in the biased amplifying devices from which the amplifier is constructed. Biased amplifying devices are combinations of power sources and devices such as MOS transistors, bipolar transistors and/or vacuum tubes. The principle of amplification will be elucidated in Chapter Amplification Mechanism.
Fig. 11 shows a model of an amplifier with its three ports: the input port, the output port and the power port. It gives a simple representation of the amplification mechanism: the source signal modulates the power transfer from the amplifier’s power source to the load.
Ideally, the information at the input port will be copied to the output port and the input and the output signals will not be affected by variations of the power supply. For this reason, amplifiers are functionally modeled as two-ports. In a functional description, the power port is simply omitted.
In most cases, we do not want any reverse signal transfer from the output port to the input port. Amplifiers with this property are called unilateral.
Definition#
According to the previous description, we can define an amplifier as follows:
Definition
An amplifier is a physical object having at least three electrical ports:
An input port to which the source signal will be connected (referred to as the input)
An output port to which the load will be connected (referred to as the output)
An input port to which the power supply will be connected (referred to as the supply)
An amplifier provides the load with an accurate copy of the source signal, and its available power gain exceeds unity.
This definition is a good starting point for a discussion of the quality of information transfer by the amplifier.
Information-processing quality#
The quality of information processing tells us something about the amount of correspondence between the load signal and the source signal. By their physical nature, electronic amplifiers generate noise and are sensitive to electromagnetic fields (EMI
). These and other effects will adversely affect the correspondence between the source signal and the load signal. Fortunately, a unique correspondence between signal values at the load and signal values at the source at any time instant is not required. The correspondence between the load and the source signal has to be such that an observer can retrieve the relevant information from the load signal. Errors due to the fundamental physical limitation of speed, power and noise and those due to technological limitations are thus allowed, as long as the information can be retrieved. The manifestation of these information processing errors also depends on the physical construction of the amplifier. This physical construction, in its turn, shows a strong interaction with the cost price of the amplifier. Hence, the amplifier’s information processing characteristics, its physical construction and the economical constraints
show a strong interaction.
Physical appearance#
Audio amplifiers, antenna amplifiers and operational amplifiers are names for different objects that have at least one thing in common: they are all amplifiers. The meaning of the word amplifier\ thus depends on one’s perspective. A designer of audio power integrated amplifier circuits might use the term audio amplifier for a single IC, while the user of an audio amplifier refers to a complete product with housing and its user interface.
Generally, electronic amplifiers can have the following physical appearance:
A complete product, including housing, power supply and user interface
A hard wired electromechanical assembly
A printed circuit board assembly (PCA)
A thick or thin film assembly
An integrated circuit (IC)
A part of an integrated circuit.
These physical realizations of amplifiers can be nested in a hierarchical way. A complete audio amplifier, for example, may consist of several printed circuit board assemblies and other parts that are mechanically and electrically interconnected. One or more of these printed circuit board assemblies can be an electronic amplifier itself. Such an amplifier, in its turn, may be built with discrete components, as well as\ with integrated circuit operational amplifiers. These integrated circuits also comprise interconnected electronic devices. The ability to amplify signals that finds its origin in the biased amplifying devices, is used in these different forms of amplifiers.
Cost factors and environmental conditions#
The processing of information by amplifiers is not free of cost. It requires physical resources such as power, matter and space. The amplifier also generates heat and electromagnetic interference signals. Among others, these are called costs factors.
The quality of information processing should not be affected by normal changes in the operating conditions. The environmental operating conditions, or shortly environmental conditions, such as, interference signals, temperature range, humidity, shock and vibration should be specified.
The aim of a good design is to provide the required information processing capability for an acceptable level of cost factors under specific environmental conditions.
Figure of merit#
In order to compare the performance-to-cost ratio of various amplifiers, it would be nice to have some figure of merit (FOM). Moreover, such a figure of merit, would help the designer in selecting solutions for specific design problems. There are various kinds of figure of merit. They usually relate the most important performance aspects to the most important cost factors, the degrees of importance usually being defined by the user. Some examples are listed below.
Output power versus dimensions or weight
Output power versus consumed power
Output power versus product costs
Noise figure versus power consumption
Dynamic range versus power consumption
Bandwidth versus power consumption
etc.
This chapter#
In section Amplifier port requirements, we will start with the description of the functional behavior of an amplifier. At the start of the design, questions that need to be answered follow from the information that needs to be processed, the character of the source and the character of the load of the amplifier:
Which electrical quantity (current or voltage) should be selected as the electrical input quantity of the amplifier?
Which electrical quantity should be delivered to the load?
Is the signal source electrically connected to the power supply or is it floating with respect to the power supply?
Is the load electrically connected to the power supply or is it floating with respect to the power supply?
Based on this knowledge, we will make an inventory of amplifier types (or concepts) and model their (ideal) behavior with the aid of a two-port. This will be done in section Modeling of the ideal behavior.
Due to fundamental physical and technological limitations, the behavior of practical amplifiers will deviate from that of their idealized function concept and information processing errors are inevitable. However, as long as the load signal shows a sufficiently large correspondence with the source signal, the observer will be able to retrieve the relevant information. Predictable errors do not necessarily degrade the information handling capability of the amplifier. Known errors can be compensated for in pre- or post-processing functions, thereby restoring the correspondence between the source signal and the load signal. Reproducible errors due to nonlinearity, for example, can be compensated for by adding opposite pre- or post-distortion to input or output signal, respectively. This also holds for reproducible errors that result from bandwidth limitation. However, we will consider the\emph{ conceptual amplifier} to behave instantaneously, linearly and time-invariantly, and model it accordingly.
In section Modeling of the non-ideal behavior, we will introduce performance parameters that describe the non-ideal behavior of amplifiers. We will discuss:
Errors arising from imperfect isolation between the input port and the output port, and between the power supply port and the two signal ports
Errors due to a nonzero transfer from the power port to the input port\ or vice versa, and from the power port to the output port or vice versa
Signal processing errors due to fundamental physical and technological limitations. We will study different kinds of errors resulting from:
The addition of noise and interference
Limitation of the current and voltage handling capability due to the physical limitation of power and due to the nonlinear signal transfer of the characteristics of biased amplifying devices
Small-signal bandwidth limitation and slew rate limitations due to the physical limitation of the rate of change of electrical currents and voltages
Ageing due to changes in the properties of electronic devices over time
Temperature changes
In section Cascaded Amplifiers, we will describe the performance aspects of cascaded amplifiers and learn about error propagation in cascaded amplifiers. This helps us to relate properties of individual amplifiers to those of a chain of amplifiers.