how to make Class B audio amplifier

PARTS AND MATERIALS

• Four 6 volt batteries
• Dual operational amplifier, model TL082 recommended (Radio Shack catalog # 276-1715)
• One NPN power transistor in a TO-220 package -- (Radio Shack catalog # 276-2020 or equivalent)
• One PNP power transistor in a TO-220 package -- (Radio Shack catalog # 276-2027 or equivalent)
• One 1N914 switching diode (Radio Shack catalog # 276-1620)
• One capacitor, 47 µF electrolytic, 35 WVDC (Radio Shack catalog # 272-1015 or equivalent)
• Two capacitors, 0.22 µF, non-polarized (Radio Shack catalog # 272-1070)
• One 10 kΩ potentiometer, linear taper (Radio Shack catalog # 271-1715)

Be sure to use an op-amp that has a high slew rate. Avoid the LM741 or LM1458 for this reason.

The closer matched the two transistors are, the better. If possible, try to obtain TIP41 and TIP42 transistors, which are closely matched NPN and PNP power transistors with dissipation ratings of 65 watts each. If you cannot get a TIP41 NPN transistor, the TIP3055 (available from Radio Shack) is a good substitute. Do not use very large (i.e. TO-3 case) power transistors, as the op-amp may have trouble driving enough current to their bases for good operation.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 4: "Bipolar Junction Transistors"

Lessons In Electric Circuits, Volume 3, chapter 8: "Operational Amplifiers"

LEARNING OBJECTIVES

• How to build a "push-pull" class B amplifier using complementary bipolar transistors
• The effects of "crossover distortion" in a push-pull amplifier circuit
• Using negative feedback via an op-amp to correct circuit nonlinearities

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

This project is an audio amplifier suitable for amplifying the output signal from a small radio, tape player, CD player, or any other source of audio signals. For stereo operation, two identical amplifiers must be built, one for the left channel and other for the right channel. To obtain an input signal for this amplifier to amplify, just connect it to the output of a radio or other audio device like this:

This amplifier circuit also works well in amplifying "line-level" audio signals from high-quality, modular stereo components. It provides a surprising amount of sound power when played through a large speaker, and may be run without heat sinks on the transistors (though you should experiment with it a bit before deciding to forego heat sinks, as the power dissipation varies according to the type of speaker used).

The goal of any amplifier circuit is to reproduce the input waveshape as accurately as possible. Perfect reproduction is impossible, of course, and any differences between the output and input waveshapes is known as distortion. In an audio amplifier, distortion may cause unpleasant tones to be superimposed on the true sound. There are many different configurations of audio amplifier circuitry, each with its own advantages and disadvantages. This particular circuit is called a "class B," push-pull circuit.

Most audio "power" amplifiers use a class B configuration, where one transistor provides power to the load during one-half of the waveform cycle (it pushes) and a second transistor provides power to the load for the other half of the cycle (it pulls). In this scheme, neither transistor remains "on" for the entire cycle, giving each one a time to "rest" and cool during the waveform cycle. This makes for a power-efficient amplifier circuit, but leads to a distinct type of nonlinearity known as "crossover distortion."

Shown here is a sine-wave shape, equivalent to a constant audio tone of constant volume:

In a push-pull amplifier circuit, the two transistors take turns amplifying the alternate half-cycles of the waveform like this:

If the "hand-off" between the two transistors is not precisely synchronized, though, the amplifier's output waveform may look something like this instead of a pure sine wave:

Here, distortion results from the fact that there is a delay between the time one transistor turns off and the other transistor turns on. This type of distortion, where the waveform "flattens" at the crossover point between positive and negative half-cycles, is called crossover distortion. One common method of mitigating crossover distortion is to bias the transistors so that their turn-on/turn-off points actually overlap, so thatboth transistors are in a state of conduction for a brief moment during the crossover period:

This form of amplification is technically known as class AB rather than class B, because each transistor is "on" for more than 50% of the time during a complete waveform cycle. The disadvantage to doing this, though, is increased power consumption of the amplifier circuit, because during the moments of time where both transistors are conducting, there is current conducted through the transistors that is not going through the load, but is merely being "shorted" from one power supply rail to the other (from -V to +V). Not only is this a waste of energy, but it dissipates more heat energy in the transistors. When transistors increase in temperature, their characteristics change (V_be forward voltage drop, β, junction resistances, etc.), making proper biasing difficult.

In this experiment, the transistors operate in pure class B mode. That is, they are never conducting at the same time. This saves energy and decreases heat dissipation, but lends itself to crossover distortion. The solution taken in this circuit is to use an op-amp with negative feedback to quickly drive the transistors through the "dead" zone producing crossover distortion and reduce the amount of "flattening" of the waveform during crossover.

The first (leftmost) op-amp shown in the schematic diagram is nothing more than a buffer. A buffer helps to reduce the loading of the input capacitor/resistor network, which has been placed in the circuit to filter out any DC bias voltage out of the input signal, preventing any DC voltage from becoming amplified by the circuit and sent to the speaker where it might cause damage. Without the buffer op-amp, the capacitor/resistor filtering circuit reduces the low-frequency ("bass") response of the amplifier, and accentuates the high-frequency ("treble").

The second op-amp functions as an inverting amplifier whose gain is controlled by the 10 kΩ potentiometer. This does nothing more than provide a volume control for the amplifier. Usually, inverting op-amp circuits have their feedback resistor(s) connected directly from the op-amp output terminal to the inverting input terminal like this:

If we were to use the resulting output signal to drive the base terminals of the push-pull transistor pair, though, we would experience significant crossover distortion, because there would be a "dead" zone in the transistors' operation as the base voltage went from + 0.7 volts to - 0.7 volts:

If you have already constructed the amplifier circuit in its final form, you may simplify it to this form and listen to the difference in sound quality. If you have not yet begun construction of the circuit, the schematic diagram shown above would be a good starting point. It will amplify an audio signal, but it will sound horrible!

The reason for the crossover distortion is that when the op-amp output signal is between + 0.7 volts and - 0.7 volts, neither transistor will be conducting, and the output voltage to the speaker will be 0 volts for the entire 1.4 volts span of base voltage swing. Thus, there is a "zone" in the input signal range where no change in speaker output voltage will occur. Here is where intricate biasing techniques are usually introduced to the circuit to reduce this 1.4 volt "gap" in transistor input signal response. Usually, something like this is done:

The two series-connected diodes will drop approximately 1.4 volts, equivalent to the combined V_be forward voltage drops of the two transistors, resulting in a scenario where each transistor is just on the verge of turning on when the input signal is zero volts, eliminating the 1.4 volt "dead" signal zone that existed before.

Unfortunately, though, this solution is not perfect: as the transistors heat up from conducting power to the load, their V_be forward voltage drops will decrease from 0.7 volts to something less, such as 0.6 volts or 0.5 volts. The diodes, which are not subject to the same heating effect because they do not conduct any substantial current, will not experience the same change in forward voltage drop. Thus, the diodes will continue to provide the same 1.4 volt bias voltage even though the transistors require less bias voltage due to heating. The result will be that the circuit drifts into class AB operation, where both transistors will be in a state of conduction part of the time. This, of course, will result in more heat dissipation through the transistors, exacerbating the problem of forward voltage drop change.

A common solution to this problem is the insertion of temperature-compensation "feedback" resistors in the emitter legs of the push-pull transistor circuit:

This solution doesn't prevent simultaneous turn-on of the two transistors, but merely reduces the severity of the problem and prevents thermal runaway. It also has the unfortunate effect of inserting resistance in the load current path, limiting the output current of the amplifier. The solution I opted for in this experiment is one that capitalizes on the principle of op-amp negative feedback to overcome the inherent limitations of the push-pull transistor output circuit. I use one diode to provide a 0.7 volt bias voltage for the push-pull pair. This is not enough to eliminate the "dead" signal zone, but it reduces it by at least 50%:

Since the voltage drop of a single diode will always be less than the combined voltage drops of the two transistors' base-emitter junctions, the transistors can never turn on simultaneously, thereby preventing class AB operation. Next, to help get rid of the remaining crossover distortion, the feedback signal of the op-amp is taken from the output terminal of the amplifier (the transistors' emitter terminals) like this:

The op-amp's function is to output whatever voltage signal it has to in order to keep its two input terminals at the same voltage (0 volts differential). By connecting the feedback wire to the emitter terminals of the push-pull transistors, the op-amp has the ability to sense any "dead" zone where neither transistor is conducting, and output an appropriate voltage signal to the bases of the transistors to quickly drive them into conduction again to "keep up" with the input signal waveform. This requires an op-amp with a high slew rate (the ability to produce a fast-rising or fast-falling output voltage), which is why the TL082 op-amp was specified for this circuit. Slower op-amps such as the LM741 or LM1458 may not be able to keep up with the high dv/dt (voltage rate-of-change over time, also known as de/dt) necessary for low-distortion operation.

Only a couple of capacitors are added to this circuit to bring it into its final form: a 47 µF capacitor connected in parallel with the diode helps to keep the 0.7 volt bias voltage constant despite large voltage swings in the op-amp's output, while a 0.22 µF capacitor connected between the base and emitter of the NPN transistor helps reduce crossover distortion at low volume settings:

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how to make 4W Audio Power Amplifier with TDA1013B

4W Audio Power Amplifier with TDA1013B

Audio power amplifiers with TDA1013B need few external components and can have a wide supply voltage range. The IC has a high signal-to-noise ratio and is equipped with thermal overload protection.

Tda1013B Datasheet (PDF)

Fig 1: 4W audio power amplifier schematic diagram with DC volume control

VP = 18 V
RL = 8Ω
Vctr = 2V to 6.5 V.

how to make 20 Watt Audio Power Amplifier

TDA2005 - 20 Watt Audio Power Amplifier

The TDA2005 is a class B dual audio power amplifier specifically designed for car radio applications. Its high current capability (up to 3.5 A) and the ability to drive very low impedance loads (down to 1.6 ohms) makes this circuit a good choice for cheap power booster amplifiers. The IC is sold in two variants: TDA2005M for bridge and TDA2005S for stereo applications.

ABSOLUTE MAXIMUM RATINGS
Symbol	Parameter	Value
Vs	Operating Supply Voltage	18 V
Vs	DC Supply Voltage	28 V
Vs	Peak Supply Voltage (for 50 ms)	40 V
Io	Output Peak Current (non repetitive t=0.1ms)	4.5 A
Io	Output Peak Current (repetitive f>10 Hz)	3.5 A
Ptot	Power Dissipation at Tcase = 60 °C	30 W
Tstg, Tj	Storage and Junction Temperature	-40 to 150°C

The Tda2005 IC has a high output power: PO=10W+10W @ RL=2ohm, d=10% (stereo) and PO=20W @ RL=4ohm, d=1% (bridge) for Vs=14.4V. The high reliability of the IC comes from the various protections against: output DC and AC short circuit to ground, overrating chip temperature, load dump voltage surge, fortuitous open ground and very inductive loads.

The IC can be used used in bridge (TDA2005M) or stereo (TDA2005S) amplifiers - with or without boostrap - with easily programmable gain and bandwidth. The Tda2005 needs a very low number of external components, and has a very simple mounting system with no electrical isolation between the package and heatsink.

Schematics with TDA2005

2 x 10W stereo Amplifier Schematic

Fig 1: 2 x 10W Stereo Amplifier Schematic circuit with TDA2005S

20W Bridge Amplifier Circuit (G_V=34dB)

Fig 2: 20W Bridge Amplifier Circuit with TDA2005M (Voltage Gain = 34dB)

Transformer

Most of the electronic circuits used in Circuitstoday.com have different applications of the transformer. Therefore, it is important to know the working principle, construction and types of transformers used in different analog circuits.

Transformer – Working Principle

A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings.

The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance. The working principle of the transformer can be understood from the figure below.

Transformer Working

 As shown above the transformer has primary and secondary windings. The core laminations are joined in the form of strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core. These staggered joints are said to be ‘imbricated’. Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force. The so produced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic Induction as

e=M*dI/dt

If the second coil circuit is closed, a current flows in it and thus electrical energy is transferred magnetically from the first to the second coil.

The alternating current supply is given to the first coil and hence it can be called as the primary winding. The energy is drawn out from the second coil and thus can be called as the secondary winding.

In short, a transformer carries the operations shown below:

1. Transfer of electric power from one circuit to another.
2.  Transfer of electric power without any change in frequency.
3. Transfer with the principle of electromagnetic induction.
4. The two electrical circuits are linked by mutual induction.

 Transformer Construction

For the simple construction of a transformer, you must need two coils having mutual inductance and a laminated steel core. The two coils are insulated from each other and from the steel core. The device will also need some suitable container for the assembled core and windings, a medium with which the core and its windings from its container can be insulated.

In order to insulate and to bring out the terminals of the winding from the tank, apt bushings that are made from either porcelain or capacitor type must be used.

In all transformers that are used commercially, the core is made out of transformer sheet steel laminations assembled to provide a continuous magnetic path with minimum of air-gap included. The steel should have high permeability and low hysteresis loss. For this to happen, the steel should be made of high silicon content and must also be heat treated. By effectively laminating the core, the eddy-current losses can be reduced. The lamination can be done with the help of a light coat of core plate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, the thickness of the lamination varies from 0.35mm to 0.5mm for a frequency of 25 Hertz.

Types of Transformers

The types of transformers differ in the manner in which the primary and secondary coils are provided around the laminated steel core. According to the design, transformers can be classified into two:

1.      Core- Type Transformer

In core-type transformer, the windings are given to a considerable part of the core. The coils used for this transformer are form-wound and are of cylindrical type. Such a type of transformer can be applicable for small sized and large sized transformers. In the small sized type, the core will be rectangular in shape and the coils used are cylindrical. The figure below shows the large sized type. You can see that the round or cylindrical coils are wound in such a way as to fit over a cruciform core section. In the case of circular cylindrical coils, they have a fair advantage of having good mechanical strength. The cylindrical coils will have different layers and each layer will be insulated from the other with the help of materials like paper, cloth, micarta board and so on. The general arrangement of the core-type transformer with respect to the core is shown below. Both low-voltage (LV) and high voltage (HV) windings are shown.

Core Type Transformer Cruciform Section

Core Type Transformers

The low voltage windings are placed nearer to the core as it is the easiest to insulate. The effective core area of the transformer can be reduced with the use of laminations and insulation.

2.      Shell-Type Transformer

In shell-type transformers the core surrounds a considerable portion of the windings. The comparison is shown in the figure below.

Core Type and Shell Type Transformer Winding

The coils are form-wound but are multi layer disc type usually wound in the form of pancakes. Paper is used to insulate the different layers of the multi-layer discs. The whole winding consists of discs stacked with insulation spaces between the coils. These insulation spaces form the horizontal cooling and insulating ducts. Such a transformer may have the shape of a simple rectangle or may also have a distributed form. Both designs are shown in the figure below:

Shell Type Transformers Rectangular Form

Shell Type Transformers Distributed Form

 A strong rigid mechanical bracing must be given to the cores and coils of the transformers. This will help in minimizing the movement of the device and also prevents the device from getting any insulation damage. A transformer with good bracing will not produce any humming noise during its working and will also reduce vibration.

A special housing platform must be provided for transformers. Usually, the device is placed in tightly-fitted sheet-metal tanks filled with special insulating oil. This oil is needed to circulate through the device and cool the coils. It is also responsible for providing the additional insulation for the device when it is left in the air.

There may be cases when the smooth tank surface will not be able to provide the needed cooling area. In such cases, the sides of the tank are corrugated or assembled with radiators on the sides of the device. The oil used for cooling purpose must be absolutely free from alkalis, sulphur and most importantly moisture. Even a small amount of moistures in the oil will cause a significant change in the insulating property of the device, as it lessens the dielectric strength of the oil to a great extent. Mathematically speaking,  the presence of about 8 parts of water in 1 million reduces the insulating quality of the oil to a value that is not considered standard for use. Thus, the tanks are protected by sealing them air-tight in smaller units. When large transformers are used, the air tight method is practically difficult to implement. In such cases, chambers are provided for the oil to expand and contract as its temperature increases and decreases. These breathers form a barrier and resists the atmospheric moisture from contact with oil. Special care must also be taken to avoid sledging. Sledging occurs when oil decomposes due to over exposure to oxygen during heating. It results in the formation of large deposits of dark and heavy matter that clogs the cooling ducts in the transformer.

The quality, durability and handling of these insulating materials decide the life of the transformer. All the transformer leads are brought out of their cases through suitable bushings. There are many designs of these, their size and construction depending on the voltage of the leads. Porcelain bushings may be used to insulate the leads, for transformers that are used in moderate voltages. Oil-filled or capacitive-type bushings are used for high voltage transformers.

The selection between the core and shell type is made by comparing the cost because similar characteristics can be obtained from both types. Most manufacturers prefer to use shell-type transformers for high-voltage applications or for multi-winding design. When compared to a core type, the shell type has a longer mean length of coil turn. Other parameters that are compared for the selection of transformer type are voltage rating, kilo-volt ampere rating, weight, insulation stress, heat distribution and so on.

Transformers can also be classified according to the type of cooling employed. The different types according to these classifications are:

1.      Oil Filled Self-Cooled Type

Oil filled self cooled type uses small and medium-sized distribution transformers. The assembled windings and core of such transformers are mounted in a welded, oil-tight steel tanks provided with a steel cover. The tank is filled with purified, high quality insulating oil as soon as the core is put back at its proper place. The oil helps in transferring the heat from the core and the windings to the case from where it is radiated out to the surroundings. For smaller sized transformers the tanks are usually smooth surfaced, but for large size transformers a greater heat radiation area is needed, and that too without disturbing the cubical capacity of the tank. This is achieved by frequently corrugating the cases. Still larger sizes are provided with radiation or pipes.

2.      Oil Filled Water Cooled Type

This type is used for much more economic construction of large transformers, as the above told self cooled method is very expensive. The same method is used here as well- the windings and the core are immersed in the oil. The only difference is that a cooling coil is mounted near the surface of the oil, through which cold water keeps circulating. This water carries the heat from the device. This design is usually implemented on transformers that are used in high voltage transmission lines. The biggest advantage of such a design is that such transformers do not require housing other than their own. This reduces the costs by a huge amount. Another advantage is that the maintenance and inspection of this type is only needed once or twice in a year.

3.      Air Blast Type

This type is used for transformers that use voltages below 25,000 volts. The transformer is housed in a thin sheet metal box open at both ends through which air is blown from the bottom to the top.

E.M.F Equation of a Transformer

Transformer EMF Equation

Let,

 N_A =  Number of turns in primary

 N_B = Number of turns in secondary

Ø_max  = Maximum flux in the core in webers = B_max X A

f   = Frequency of alternating current input in hertz (H_Z)

As shown in figure above, the core flux increases from its zero value to maximum value Ø_max  in one quarter of the cycle , that is in ¼ frequency second.

Therefore, average rate of change of flux = Ø_max/ ¼ f = 4f Ø_maxWb/s

Now, rate of change of flux per turn means induced electro motive force in volts.

Therefore, average electro-motive force induced/turn = 4f Ø_maxvolt

If flux Ø varies sinusoidally, then r.m.s value of induced e.m.f is obtained by multiplying the average value with form factor.

Form Factor = r.m.s. value/average value = 1.11

Therefore, r.m.s value of e.m.f/turn = 1.11 X 4f Ø_max = 4.44f Ø_max

Now, r.m.s value of induced e.m.f in the whole of primary winding

= (induced e.m.f./turn) X Number of primary turns

Therefore,

 E­_A = 4.44f N_AØ_max = 4.44fN_AB_mA

Similarly, r.m.s value of induced e.m.f  in secondary is

E­_B = 4.44f N_B Ø_max = 4.44fN_BB_mA

In an ideal transformer on no load,

V_A = E_A  and V_B = E_B  , where V_B is the terminal voltage

Voltage Transformation Ratio (K)

From the above equations we get

E_B/ E_A = V_B/ V_A = N_B/N_A = K

This constant K is known as voltage transformation ratio.

(1)   If N_B>N_A , that is K>1 , then transformer is called step-up transformer.

(2)   If N_B<1, that is K<1 , then transformer is known as step-down transformer.

Again for an ideal transformer,

Input V_A = output V_A

V_AI_A = V_BI_B

Or, I_B/I_A = V_A/V_B = 1/K

Hence, currents are in the inverse ratio of the (voltage) transformation ratio.

PCB Design and Layout Software

Printed Circuit Board (PCB) layouts are designed with help of PCB drawing software’s, commonly known as pcb layout design software. Here we are listing you a collection of free pcb design softwares which are simple, easy to use and available for ready download. PCB design tools fall into many categories based on their development platform, pricing strategies etc. We aim to give you a mixed collection free download softwares, open source pcb design softwares, enterprise level softwares (which requires license). All the softwares listed here contains links to download pages of software websites and you are free to download these tools based on their terms and conditions. So let’s begin the list.

Open Source PCB Design Software

Free PCB- is an open source pcb design software released under GNU license, made specifically for Micrososft Windows.  Free pcb is a simple, easy to use design tool, whose features include 1 to 16 copper layers, board size upto 60 inches by 60 inches, Footprint libraries and wizard etc. Though Free pcb seems like an easy, hobbyist type tool, it can be used for your basic professional works too.

BSch3V – is a basic pcb schematic drawing software, which can carry out your basic needs only. The software is developed for Windows operating system. Though its so basic, we list this pcb software here because of its simple nature so that a lay person can use it very easily and efficiently.

ExpressPCB – is a pcb design tool for the professional which is easy to learn and simple to use. This free cad software comes with two divisions – ExpressSCH for drawing schematics and ExpressPCB for drawing circuit board layouts.  This software is compatible for Windows Xp, Vista and 7. Drawing circuit diagram in ExpressPCB is as easy as placing components and connecting them.

Kicad – is an open source schematic diagram drawing software and printed circuit board design tool. This software is released under GPL license and is a combination 4 applications and a project management tool. Eeschema – is used for schematic entry, Pcbnew – as board editor, Gerbview – as photo plotter, Cvpcb – as footprint selector for pcb, and Kicad – as project manager. This application works with both Linux and Windows (2000 and XP).

PCB 123 - is a free circuit drawing and pcb layout software from Sunstone. We at CircuitsTodayuse this software for the last 3 years to draw circuits. And that’s why you see unique style diagrams in all our electronic circuits. Though there are many other good software’s available, we still stick on to PCB123 to maintain this unique style. Well, this software is worth your time for a try.

PCB Layout Tool – is a free, simple printed circuit board layout tool. You can use this extremely simple software to make layouts of even complex circuits. It is capable of producing RS-274X and Excellon NC-drill format output.

Eagle (Light Edition) – Eagle is one of the most popular Circuit drawing and PCB layout software available in the market. Most professionals,students and hobbyists find Eagle as an ideal choice for their PCB development needs. Eagle in fact is an enterprise PCB designing software but they are offering a lighter version (with limitations compared to enterprise application) for free download. And to your good news, Eagle will run on Windows, Linux and Mac.

PCB Editor – This is an Editor software from Gpleda, useful for editing printed circuit boards. This application runs on unix, linux, windows and macintosh operating systems. They offer an API (Application Programming Interface) to add new functionality and features. It facilitates schematics import, design rule checking, photo realistic design review images.

This list is not complete. We shall update this list soon with new additions. In the mean while, if I missed any software, please reply here in comments section.