These parts of the Scots Guide is intended to cover a range of electronic techniques which are likely to be of some use or interest to physical scientists and engineers. The topics covered in these notes focus on the ‘Analog’ side of things. In general, electronic systems tend to be made up by combining a range of simpler elements, circuits, and components of various kinds. We will examine a number of these in detail in later sections, but for now we’ll start with a simple over-view of amplifiers and their uses.
The uses of amplifiers.
At the most basic level, a signal amplifier does exactly what you expect – it makes a signal bigger! However the way in which this is done does vary with the design of the actual amplifier, the type of signal, and the reason why we’re wanting to enlarge the signal. We can illustrate this by considering the common example of a ‘Hi-Fi’ audio system.
In a typical modern hi-fi system, the signals will come from a unit like a CD Player, FM Tuner, or a Tape/MiniDisc unit. The signals these produce have typical levels of the order of 100mV or so when the music is moderately loud. This is a reasonably large voltage, easy to detect with something like an oscilloscope or a voltmeter. However the actual power levels of these signals is quite modest. Typically, these sources can only provide currents of a few milliamps, which by means powers of just a few milliwatts. A typical loudspeaker will require between a few Watts and perhaps over 100 Watts to produce loud sounds. Hence we will require some form of Power Amplifier to ‘boost’ the signal power level from the source and make it big enough to play the music.
We usually want to be able to control the actual volume – e.g. turn it up to annoy neighbours, or down to chat to someone. This means we have to have some way of adjusting the overall Gain of the system. We also want other functions – the most obvious being to select which signal source we wish to listen to. Sometimes all these functions are built into the same piece of equipment, however is often better to use a separate box which acts as a Pre-Amp to select inputs, adjust the volume, etc. In fact, even when built into the same box, most amplifier systems use a series of ‘stages’ which share the task of boosting and controlling the signals.
The system shown in figure 1.2 is similar to the previous arrangement, but in this case is used for taking signals from microphones and ‘mixing’ them together to produce a combined output for power amplification. This system also include an ‘EQ’ unit (Equalising) which is used to adjust and control the frequency response.
Microphones tend to produce very small signal levels. typically of the order of 1 mVrms or less, with currents of a few tens of microamps or less – i.e. signal powers of microwatts to nanowatts. This is similar to many other forms of sensor employed by scientists and engineers to measure various quantities. It is usual for sensors to produce low signal power levels. So low, in fact, that detection and measurement can be difficult due to the presence of Noise which arises in all electronic system. For that reason it is a good idea to amplify these weak, sensor created, signals, as soon as possible to overcome noise problems and give the signal enough energy to be able to travel along the cables from the source to its destination. So, as shown in figure 1.2, in most studios, the microphone sensor will actually include (or be followed immediately) by a small Low-Noise Amplifier (LNA).
Due to the range of tasks, amplifiers tend to be divided into various types and classes. Frequently a system will contain a combination of these to achieve the overall result. Most practical systems make use of combinations of standard ‘building block’ elements which we can think of as being the ‘words’ of the language of electronics. Here we can summarise the main types and their use. Most types come in a variety of forms so here we’ll just look at a few simple examples.
The Voltage Amplifier
Figure 1·3 shows four examples of simple voltage amplifier stages using various types of device. In each case the a.c. voltage gain will usually be approximated by
provided that the actual device has an inherent gain large enough to be controlled by the resistor values chosen. Note the negative sign in expression 1.1 which indicates that the examples all invert the signal pattern when amplifying. In practice, gains of the order of up to hundred are possible from simple circuits like this, although for reasons we will discuss in later sections (on feedback and distortion) it is usually a good idea to keep the voltage gain below this. Note also that vacuum state devices tend to be called “valves” in the UK and “tubes” in the USA.
Many practical amplifiers chain together a series of voltage amplifier stages to obtain a high overall voltage gain. For example a PA system might start with voltages of the order on 0·1mV from microphones, and boost this to perhaps 10 to 100 V to drive loudspeakers. This requires an overall voltage gain of × 109, so a number of voltage gain stages will be required.
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