Generators are useful appliances that supply electrical power during a power outage and prevent discontinuity of daily activities or disruption of business operations. Generators are available in different electrical and physical configurations for use in different applications. In the following sections, we will look at how a generator functions, the main components of a generator, and how a generator operates as a secondary source of electrical power in residential and industrial applications.

How does a generator work?

An electric generator is a device that converts mechanical energy obtained from an external source into electrical energy as the output.
It is important to understand that a generator does not actually ‘create’ electrical energy. Instead, it uses the mechanical energy supplied to it to force the movement of electric charges present in the wire of its windings through an external electric circuit. This flow of electric charges constitutes the output electric current supplied by the generator. This mechanism can be understood by considering the generator to be analogous to a water pump, which causes the flow of water but does not actually ‘create’ the water flowing through it.

The modern-day generator works on the principle of electromagnetic induction discovered by Michael Faraday in 1831-32. Faraday discovered that the above flow of electric charges could be induced by moving an electrical conductor, such as a wire that contains electric charges, in a magnetic field. This movement creates a voltage difference between the two ends of the wire or electrical conductor, which in turn causes the electric charges to flow, thus generating electric current.

History

Before the connection between magnetism and electricity was discovered, electrostatic generators were used. They operated on electrostatic principles. Such generators generated very high voltage and low current. They operated by using moving electrically charged belts, plates, and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms:

Electrostatic induction
The triboelectric effect, where the contact between two insulators leaves them charged.

Because of their inefficiency and the difficulty of insulating machines that produced very high voltages, electrostatic generators had low power ratings, and were never used for generation of commercially significant quantities of electric power. The Wimshurst machine and Van de Graaff generator are examples of these machines that have survived. In 1827, Hungarian Anyos Jedlik started experimenting with the electromagnetic rotating devices which he called electromagnetic self-rotors, now called the Jedlik's dynamo. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. He formulated the concept of the dynamo at least 6 years before Siemens and Wheatstone but didn't patent it as he thought he wasn't the first to realize this. In essence the concept is that instead of permanent magnets, two electromagnets opposite to each other induce the magnetic field around the rotor. It was also the discovery of the principle of self-excitation.


Faraday disk, the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this induced an electric current radially outward from the center toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the center of the disk through the axle.
In the years of 1831–1832, Michael Faraday discovered the operating principle of electromagnetic generators. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux. He also built the first electromagnetic generator, called the Faraday disk, a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage.
This design was inefficient, due to self-cancelling counterflows of current in regions that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.
Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher, more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.


Dynamos are no longer used for power generation due to the size and complexity of the commutator needed for high power applications. This large belt-driven high-current dynamo produced 310 amperes at 7 volts, or 2,170 watts, when spinning at 1400 RPM.
The dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic induction to convert mechanical rotation into direct current through the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.
A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.
Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC electric motor, the AC alternator, the AC synchronous motor, and the rotary converter.
Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current. The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii.
Faraday developed the "rotating rectangle", whose operation was heteropolar - each active conductor passed successively through regions where the magnetic field was in opposite directions. The first public demonstration of a more robust "alternator system" took place in 1886.Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 Hz. In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated around 15 kHz). After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.
Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution. Before the adoption of AC, very large direct-current dynamos were the only means of power generation and distribution. AC has come to dominate due to the ability of AC to be easily transformed to and from very high voltages to permit low losses over large distances.

Electromagnetic generators

Dynamo

"Dynamo Electric Machine" (end view, partly section, U.S. Patent 284,110)
A dynamo is an electrical generator that produces direct current with the use of a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter. Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and cost reasons. A dynamo has the disadvantages of a mechanical commutator. Also, converting alternating to direct current using power rectification devices (vacuum tube or more recently solid state) is effective and usually economic.

Alternator

Without a commutator, a dynamo becomes an alternator, which is a synchronous singly fed generator. Alternators produce alternating current with a frequency that is based on the rotational speed of the rotor and the number of magnetic poles.
Automotive alternators produce a varying frequency that changes with engine speed, which is then converted by a rectifier to DC. By comparison, alternators used to feed an electric power grid are generally operated at a speed very close to a specific frequency, for the benefit of AC devices that regulate their speed and performance based on grid frequency. Some devices such as incandescent lamps and ballast-operated fluorescent lamps do not require a constant frequency, but synchronous motors such as in electric wall clocks do require a constant grid frequency.

When attached to a larger electric grid with other alternators, an alternator will dynamically interact with the frequency already present on the grid, and operate at a speed that matches the grid frequency. If no driving power is applied, the alternator will continue to spin at a constant speed anyway, driven as a synchronous motor by the grid frequency. It is usually necessary for an alternator to be accelerated up to the correct speed and phase alignment before connecting to the grid, as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.
Typical alternators use a rotating field winding excited with direct current, and a stationary (stator) winding that produces alternating current. Since the rotor field only requires a tiny fraction of the power generated by the machine, the brushes for the field contact can be relatively small. In the case of a brushless exciter, no brushes are used at all and the rotor shaft carries rectifiers to excite the main field winding.

Induction generator

An induction generator or asynchronous generator is a type of AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as minihydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.
To operate an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self excited by using phase correcting capacitors.

MHD generator


A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25 MW demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980s, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time.MHD generators operated as a topping cycle are currently (2007) less efficient than combined cycle gas turbines.

Other rotating electromagnetic generators
Other types of generators, such as the asynchronous or induction singly fed generator, the doubly fed generator, or the brushless wound-rotor doubly fed generator, do not incorporate permanent magnets or field windings that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies.
The full output performance of any generator can be optimized with electronic control but only the doubly fed generators or the brushless wound-rotor doubly fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, a feature which, by itself, offers cost, reliability and efficiency benefits.

Homopolar generator


Faraday disk, the first homopolar generator
A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation and the orientation of the field. It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage. They are unusual in that they can source tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.
Excitation


A small early 1900s 75 kVA direct-driven power station AC alternator, with a separate belt-driven exciter generator.
Main article: Excitation (magnetic)
An electric generator or electric motor that uses field coils rather than permanent magnets requires a current to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all.
Smaller generators are sometimes self-excited, which means the field coils are powered by the current produced by the generator itself. The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This "bootstrap" process continues until the magnetic field in the core levels off due to saturation and the generator reaches a steady state power output.
Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.

Electrostatic generator

A Van de Graaff generator, for class room demonstrations


Suppose that the conditions are as in the figure, with the segment A1 positive and the segment B1 negative. Now, as A1 moves to the left and B1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A1 and neighboring sectors comes opposite the segment B2 of the B plate, which is now in contact with the brush Y, they will cause a displacement of electricity along the conductor between Y and Y1 bringing a negative charge, larger than the positive charge in A1 alone, on Y and sending a positive charge to the segment touching Y1. As A1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X, has a new charge, this time negative, driven into it by induction from B2 and neighboring sectors. As the machine turns, the process causes exponential increases in the voltages on all positions, until sparking occurs limiting the increase.
An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. By the end of the 17th Century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity. Electrostatic generators operate by using manual (or other) power to transform mechanical work into electric energy. Electrostatic generators develop electrostatic charges of opposite signs rendered to two conductors, using only electric forces, and work by using moving plates, drums, or belts to carry electric charge to a high potential electrode. The charge is generated by one of two methods: either the triboelectric effect (friction) or electrostatic induction.

Wimshurst machine

The Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst (1832–1903). It has a distinctive appearance with two large contra-rotating discs mounted in a vertical plane, two crossed bars with metallic brushes, and a spark gap formed by two metal spheres.

Van de Graaff generator


A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved in modern Van de Graaff generators can reach 5 megavolts. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance, so it can produce a visible electrical discharge to a nearby grounding surface which can potentially cause a "spark" depending on the voltage.

The basic purposes of an amplifier is to do just that, to amplify (a signal). However, the way different types of amplifiers do this can vary greatly, and an amplifier can do other tasks besides amplifying. Generally when we speak of amplifiers we mean increase the power of a signal. We are starting to get into the building blocks that make up part of a radio system.

Signals that we actually amplify in radio and communications are very often not sine waves, though many times they are. For our discussion we will consider that we are amplifying a sine wave(s) of alternating voltage or current. 

When discussing alternating signals it is usual to speak of a full cycle as being equivalent to 360 degrees.  Fractions of a full cycle can then be more easily described.  As an illustration, if a full 360 degrees sine wave of alternating current is sent to a rectifier, only 180 degrees of current will appear at the output.  A half wave rectifier or diode could be said to have an operating angle of 180 degrees.  Similarly, different types of amplifier will provide at their output only a portion of the amplified input signal.  An amplifier with an operating angle of 180 degrees will behave similar to a half wave rectifier except that the output is amplified.

This is usually the hardest concept about amplifiers to grasp. It is bad to make the assumption: "what goes into an amplifier is the same as what comes out, only bigger”. True in many cases, audio amplifiers for example, we want exactly what goes in to come out. We just want it to be louder or amplified.  However, in radio circuits there are all sorts of other considerations. One of the main ones is efficiency of the amplifier, and a number of techniques are used to improve the efficiency. Typically, audio (sound) amplifiers have a very low efficiency, probably at best 30%.

We have covered the actual principle of amplification in the last two readings. A smaller voltage or current is used to control a much larger voltage or current. Most of the time this is done using one of the active devices , BJT, JFET, Triode etc. Now we cannot get power for nothing. The power supplied to the amplifier comes from the power supply, so if an amplifier has an efficiency of only 25%, that means for 25 watts of useable signal output we have to provide it with 100 Watts of power from the power supply. So efficiency, particularly at radio frequencies where the power tends to be high (perhaps 100,000 watts for a TV station), becomes important. Efficiency is not just important in terms of say running down batteries in portable equipment, but also heat. If a power supply is only 25% efficient, then 75% of the power is being converted into heat and this heat brings with it all sorts of other problems, particularly with semiconductors, and keeping them cool.

A little more on efficiency. An active device (in practice several active devices in a multistage amplifier) say a BJT, is forward biased which is normal. Even with no input signal there is still em itter-collector current. S o even though we are not doing any amplifying, the active device is consuming power from the power supply. 

So accept just for now, that not all amplifiers actually amplify the entire input signal fed to them. Like the example given above, if an amplifier only amplified half of one cycle it would have an operating angle of 360/2 = 180 degrees. It may seem that if an amplifier does not have an operating angle of 360 degrees then what we put in (the input signal) will not be what we get out, and this is true. An amplifier that does not have an operating angle of 360 degrees distorts, or is non-linear . Sometimes this does not matter, other times it does.

Amplifiers fall into four basic categories called Classes, and you need to remember all (though I can never remember a question on Class AB). It is important you remember the classes and their efficiency, and the basic principles, and in a couple of cases you will need to identify a circuit. This does not mean you need to know the complete circuit operation.

CLASSES OF AMPLIFIER AND THEIR OPERATING ANGLES

Class A       360 degrees

Class B       180 degrees

Class AB    270 degrees

Class C        90-120 degrees

At the outset you should see that a Class A amplifier with an operating angles of 360 degrees amplifies all of the input signal - this is the basic definition of a linear amplifier, though more formally "without distortion" should be added. However, Class A operation is the least efficient. We will discuss efficiency later. Class A is used for small signal applications because of its poor efficiency, but nevertheless it is a quality amplifier. However we can do tricks with the others as well, to make linear amplification possible. In some cases we don’t care if we have linear amplification.

A LOOK AT HOW THE CLASSES DIFFER

 Take an active device such as an electron tube. If the control grid is made negative enough the anode current will fall to zero .  This point is called cutoff .  Decreasing the negative bias applied to the control grid increases the anode current linearly, until at zero bias the anode current is almost at maximum.  Making the control grid positive will increase anode current slightly, but the electron tube quickly saturates. 

(Saturation is when an active device is conducting its maximum current). 

A fixed DC voltage applied to the control grid (bias) determines the class of operation. That’s so important I will say it again - bias determines the class of operation.  For Class A operation, the electron tube (or any active device) is biased at the centre of the linear portion (straight-line section) of the operating curve . Class B is biased at, or near, cutoff. In Class C operation the bias is about twice cutoff, though it can be higher.  For Class AB, bias is about halfway between cutoff and the centre of the linear portion of the operating curve.

Radio hobbyists and professionals frequently use the term "linear amplifier" as a synonym for "power amplifier".  Such usage is incorrect as power amplifiers can be linear or non- linear, depending on their application.  For audio, Class A operation is the only single ended amplifier that provides linear amplification.  (Single ended means only one active device employed). Linear amplifiers do not distort the amplitude of the signal being amplified (within specified practical limits at least) .  It is possible to use Class AB and B as linear radio frequency (RF) amplifiers for amplitude modulated signals, since the output power is proportional to the input power.  The missing cycle can be restored in the tank circuit (flywheel effect). Amplitude modulated signals are those which contain intelligence (or information) in their amplitude. 

Since a Class B amplifier has an operating angle of 180 degrees, it is possible by using a pair of active devices, and to arrange for each device to amplify half of the sine wave, to combine the two amplified halves in the output to achieve linear amplification.  Such an arrangement is called a class B push-pull and is most useful for audio amplifiers and RF amplifiers.

Class B is non-linear and has an operating angle 180 degrees. However, we can arrange two such amplifiers to look after each half of the input, and combine them after amplification in the output, to obtain the full signal with linear amplification Why bother? Well we discussed that Class B was biased at or near cutoff, so with no input signal (the signal to be amplified), little or no current flows in the anode, collector or drain circuit - no power taken from the supply when no amplification is taking place means higher efficiency

AMPLIFIER EFFICIENCY

CLASS A          25-30%

Use:

Audio - low distortion

RF:    Very inefficient

CLASS B          30-35%

Use:

Audio Power (push-pull)

RF power - all modes (modes are to be discussed)

CLASS AB       50-60%

Use:

RF Power Amplifiers all AM modes

CLASS C          60-80%

Use:

RF power - FM

We have not covered transmission modes. Most will be familiar with AM and FM radio stations. AM is amplitude modulation, the information, music, intelligence, is in the amplitude of the signal. FM is frequency modulation, the information, music; intelligence is in the frequency of the wave.  Remember, a sine wave has two characteristics - think back to your basic sine wave. These characteristics are

amplitude and frequency . Either can be modified to carry information (modulated), the amplitude or the frequency, and we can disregard the other without losing any information.

INPUT-OUTPUT CURVES

CLASS A – OPERATING CURVE

The characteristic curve diagram of figure 1 is that of an electron tube Class A amplifier. It could just as easily be for a FET or BJT.  In an electron tube a negative bias is applied to the grid. In the diagram I have labelled the grid bias (above zero) as shown. The actual value in volts of the grid bias is of no significance, as it will vary from device to device. The graph is a plot of grid bias versus plate (anode) current. It is important to understand this graph. Forget the waveforms for a moment and just concentrate on the line on the graph, which is a plot of grid bias verses plate current . The plate (anode) current is on the vertical axis while the negative grid volts are on the horizontal axis. You can see on the graph that there is a point where the grid bias is so negative that no plate current will flow, and this point is labelled cutoff . Further along the curve is a spot marked operating point. This operating point is the amount of bias required for Class A operation. Note that the characteristic curve is really a curve, for if I had extended the curve a bit further upward it would have quickly flattened out, this is called saturation . Importantly, notice that a portion of the characteristic curve is a straight line. If the active device (here an electron tube) isoperated on the straight line portionof its characteristic curve, then variations to the bias by the input signal superimposed on it will cause

proportional variations in the larger anode current. Operating an amplifier on the linear portion (straight-line portion) means the amplifier will be linear and faithfully reproduce the output signal the same as the input signal. If I seem to be highlighting a lot it is because this must really be understood.

Figure 1 – Operating Curve for Class A Amplifier.

If the input signal drives the bias point into cutoff or saturation then the amplifier will produce distortion, and the output anode current will not be an exact representation of the input signal.

As an example, audio amplifiers (low power ones) often operate in Class A. Think of your

TV or stereo system. If you turn the volume control up too high, the music will be louder

but it will begin to sound distorted.

The volume control adjusts the level of input signal into the audio amplifier. When you turn it up too high you overdrive the amplifier causing it to go into the cutoff and saturation regions of the curve, and you hear it as distortion.

So figure 1 above demonstrates Class A operation. The active devices are operated on the linear portion of the operating curve. Since all of the input signal is amplified in a Class A amplifier, it is said to have an operating angle of 360 degrees.

CLASS B - OPERATING CURVE

Have a look at the operating curve of a Class B amplifier in figure 2. Notice the operating point is at, or near, cutoff.

Cutoff is when the grid voltage is so negative that it prevents any electrons from flowing from cathode to anode. If the input signal is negative going, it takes the electron tube beyond cutoff and no anode (plate) current flows. However, if the input signal goes positive it will take the electron tube out of cutoff (to the right on the horizontal axis) and current will flow in the output and be amplified. As you can see by the diagram, only one half cycle of the input signal is amplified. A Class B amplifier is therefore not a linear amplifier. However, unlike a Class A, with no input signal there is no anode current in a Class B, so it is a more efficient amplifier. A  Class B amplifier has an operating angle of 180 degrees.  A combination of two Class B amplifiers configured correctly, one to amplify one half cycle and the other the other half cycle, can be used to produce linear amplification.

CLASS C - OPERATING CURVE

Have a look at the operating curve of a Class C amplifier in figure 3. Notice the operating

point is beyond cutoff, at or near cutoff.

Figure 3 – Operating Curve for Class C Amplifier.

Not even one half cycle of input signal is amplified (180 deg). About 90 to 120 degrees of the input signal is amplified. A Class C amplifier causes massive distortion to the amplitude of the input signal. Don't be confused by the size of the input signal and the output signal (plate current) shown on any of these curves, their physical size on the curves does not represent their power levels. On all the curves, plate current is much larger and at a much high voltage than the input signal.

If there is information in the amplitude of the input signal it will be severely distorted. However,

if there is information in the frequency of the input signal, there is no distortion, as the frequency of the signal that goes in, is the same as that which comes out. So if we have coded some information into the frequency (frequency modulation), the Class C amplifier is the only way to go, as it is the most efficient amplifier of all. A Class C amplifier can only be used at radio frequencies (RF).

We can restore the full cycle in the output of a Class C amplifier by using a tuned circuit in the plate of the electron tube. Such a circuit is called a tank circuit, and it will restore the other half cycle by utilising the flywheel effect.

CLASS AB

I will just mention that there is another Class of amplifier called Class AB - it is not in the syllabus. It is an improvement of the Class B. For linear amplification, two active devices like a Class B amplifier, are required. One problem with the Class B is that the operating curve is not quite straight for a little bit near cutoff. Even if you use two Class B amplifiers, one for each half cycle, you get some distortion when one amplifier turns off (after one half cycle) and the other Class B amplifier turns on. This is due to the non-linear part of the operating curve near cutoff. The distortion that results is called

crossover distortion . The Class AB is a minor modification of the Class B to overcome this problem.

ACTIVE DEVICES

I have used graphs of an electron tube to illustrate the operation of Class A, B and C, as I think it is easier to visualise. Any active device can be used for all of the classes of amplifier . For a FET we would have to speak of gate bias instead of grid bias, and drain current instead of plate current. The polarity of the bias voltage will be different depending on the type of active device used. Polarities are not important for this reading, I just used them because I was explaining the operation of the curves using an electron tube.

You may be presented with a circuit diagram of different amplifiers in the exam and basically have to identify them. The exam is multiple-choice, so you are presented with a circuit (schematic) diagram and asked to pick one of four choices of what it is. I have seen dozens of actual exam papers so I will show you a couple of schematics of amplifiers and give you the clues to identification. I may cover some of the operation only for the purpose of reinforcing some earlier material - a type of revision.

PUSH-PULL AMPLIFIERS

We spoke of a Class B amplifier as having an operating angle of 180 degrees. To obtain linear amplification we could use two Class B amplifiers, one to look after and amplify each half cycle. This type of amplifier is called a push-pull . Class A amplifiers can also be used

in push-pull. The diagrams below are labelled Class A push-pull but they are virtually the

same. If you do see a diagram like this it won't be ambiguous, the examiner will be testing

you to see if you know what a push-pull configuration looks like. The bias would be set to

about 0.66 of cutoff for Class A operation. For Class B operation (the most likely use) the

bias is about 0.95 of cutoff.

Following is a brief description of the operation that will give you a bit more of an idea of

what push-pull is, rather than just circuit identification.

Refer to the vacuum (electron) tube circuit of figure 4 and assuming class A operation

(established by the Cathode resistor). With no signal applied to the grids, equal anode

current will flow through V1 and V2 since the bias is the same to both tubes.

Both tubes are biased to the centre of the linear portion of their operating curves (Class A).

Assume that I1 and I2 each register an anode current of 50 mA,  then I3 will be I1 + I2 =

100 mA.  Now suppose an input signal is applied via T1, which makes the top of the

secondary 5 volts positive with respect to the centre tap and the bottom five volts negative

with respect to the centre tap.

 Figure 4.

In other words, the grid voltage on V1 will be 5V less negative while on V2 the grid voltage becomes 5V more negative.  Assume that the reduced negative bias on V1's grid causes V1's anode current to

increase by 10 mA, then the increased negative bias on V2's grid will cause the anode current to fall

by 10 mA. I1 will now be 60 mA and I2 will be 40 mA. Notice that as one tube draws more current the other tube draws less, and the current I3 will remain constant.

On the next half cycle V2's anode current will increase and V1's anode current will decrease by a corresponding amount. T2's secondary voltage will be the resultant of each of the top and bottom primary currents. T2 also serves to match the high output impedance of the vacuum tubes to the low impedance of speaker.

The operation of the transistor circuit in figure 5 is the same, except that voltage divider bias is shown.  With Class A operation the signal currents that flow through R are equal E and opposite, and as such, net degenerative signal voltage will appear across the resistor, making the usual emitter bypassing unnecessary.

An important advantage of push-pull operation over the single ended circuit is the tendency to cancel

even order harmonics that may develop in the stage.  Such even order (ie 2 , 4 , 6 ) harmonics are 180 degrees out of phase in the output and completely  cancel.

HARMONICS- Harmonics are distortion products (signals) produced on multiples of the operating frequency. So a radio frequency amplifier operating say on 10 MHz will produce some output on 20, 30, 40 etc. MHz. It is a fact that harmonics always exist, are caused by non-linearity in an amplifier, and that every amplifier has some non-linearity . The advantage of the push-pull circuit is that even order, 2 nd , 4 th, 6 th etc. harmonics are less of a problem.

COMPLEMENTARY SYMMETRY AMPLIFIER

For identification purposes only, a complementary symmetry amplifier can look at first glance like a push-pull amplifier.

This amplifier takes advantage of semiconductor devices that are fabricated as the complement of each other.  A NPN transistor is the complement of a PNP transistor. A push-pull amplifier normally has an input transformer to provide out-of-phase signals to the input elements of two active devices. With complementary symmetry, phase inversion is unnecessary.

In the figure 6 schematic notice that the base connections of the two transistors are connected directly together. If the input goes negative, the PNP transistor (Q) will be 1 biased harder and its collector current will increase. However, a negative-going input signal will reduce the bias current of the NPN transistor (Q) causing its collector current to 2 decrease by the same amount. When the input signal swings positive, Q 's base and collector current is decreased, while 1 Q 's base and collector currents undergo a corresponding increase. 2 The resistor networks on each transistor look complicated, but close inspection shows that R and R provide voltage divider bias for Q , while R

and R provide the same function 1 3 1 2 4 for Q2.

AN INTERMEDIATE FREQUENCY AMPLIFIER

When we come to cover receivers and transmitters, we will come across a stage called the intermediate frequency amplifier.  The intermediate frequency or IF amplifiers are a little different from other amplifiers, in that they are designed to operate on one frequency only. Typical intermediate frequency amplifiers operate on 10.7 MHz and 455 kHz.

Because this amplify operates only on one frequency, it can be designed with tuned circuits to only pass that frequency, actually a narrow band of frequencies.  An intermediate frequency amplifier is just like any other amplifier, however, you may need to identify the circuit in an exam.  The circuit that I've shown in figure 7 is one that was actually used in an exam, and the question was to identify what the circuit was.

The clue to the identification of the circuit is the use of the transformers (radio frequency transformers) T and T . They are ferrite slug tuned (indicating radio frequencies) and 1 2 shielded (the dashed box).  This is a circuit of a fixed tuned radio frequency amplifier, or an intermediate frequency amplifier. The tuned transformer coupling provides greater selectivity. The transformer primaries form part of a parallel tuned circuit and have all the properties that go with this.

FOR INTEREST

Notice how the collectors are connected to a tapping on the transformer primaries.  This is to prevent the low impedance of the transistors loading the tuned circuits and reducing  Q and hence the selectivity.  The turns ratio of T and T also serve to match the output and 1 2 input impedances of each stage. R and R provide voltage divider bias for Q . R is the emitter stabilisation resistor and C

is the emitter bypass capacitor.

This circuit really has much more than you need to know, and as long as you can identify it as an intermediate frequency amplifier by the clues I have given, you will do fine.  If the  circuit looks a little odd, it is because two different types of transistors have been used, NPN and PNP.

FREQUENCY MULTIPLIER

While frequency multipliers are not amplifiers, it seems like an appropriate place to discuss them.  The purpose of a frequency multiplier is to do just that, multiply frequency.  A 1 MHz signal can be converted to a 2 MHz signal using a frequency multiplier, or to any multiple of 1MHz.

(Frequency multipliers should not be confused with mixers, which is another method of converting a signal from one frequency to another). 

Frequency multipliers can only convert to multiples of harmonics of the input signal. If you remember (if not go back and have a look at the reading on tuned circuits) that a parallel tuned circuit has a property called the flywheel effect .  A parallel tuned circuit inthe output of an amplifier is commonly referred to as a tank circuit.  Frequency multipliers are designed with output tuned circuits (tanks) that are resonant on either the second, third, and less frequently the fourth, harmonic of the input frequency. 

The output waveform of a multiplier is a sign wave produced by the " flywheel effect" of the output tank.  Multipliers are also referred to as doublers, triplers, and quadruplers. Class C operation is essential, as current pulses into the output tank are required to keep the tank oscillating.  In a doubler the output tank is tuned to twice the input frequency, and will receive one input current pulse, and then go through two complete oscillations before the next current pulse arrives just in time to overcome the lost energy (remember damping). Output tanks in quadruplers require a much higher Q since they must sustain four full cycles of oscillation without appreciable attenuation or damping, before receiving a newcurrent pulse.

Multipliers can only be used to multiply signals that have no intelligence in their amplitude, as the stage is nonlinear.  I emphasise again that multipliers operate in Class C which is nonlinear.

BUFFER AMPLIFIER

An oscillator is a circuit (to be discussed) which provides weak low power signals on a certain frequency. The power output of an oscillator is very low. If we were to connect something like a BJT amplifier after an oscillator, the oscillator would probably cease to function, as a BJT requires current from the source to be amplified.

A buffer amplifier is any amplifier (typically after an oscillator) designed to amplify the input signal, without placing any great demands, power wise, on the stage supplying the signal. 

In other words, a buffer amplifier requires very little or no power to drive it, and it prevents overloading the previous stage. Some microphones have a small pre-amplifier actually built into them, as the microphone insert (crystal, dynamic, etc) cannot provide much power out. A pre-amplifier after a microphone is never called a buffer, but in effect, it is doing the same job as a buffer

amplifier.