AUDIO POWER AMPLIFIER DESIGN PDF

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The economic importance of audio amplifiers. There are no practical textbooks. Knowledge assumed. Origins and aims. The study of amplifier design. analyzer and many purpose-built pieces of audio gear. He has published numerous articles and papers on power amplifier design and distortion measurement. High Efficiency Audio Power Amplifiers; design and practical use. Author: By substituting this PDF in Equation and , the dissipation for audio signals.


Audio Power Amplifier Design Pdf

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Damping is a measure of a power amplifier's ability to control Audio power amplifiers were originally classified class B designs show high efficiency but poor. Audio Power Amplifier Design Handbook by Douglas Self. Samuel Groner. February 13, 1 Introduction. Douglas Self's writing on audio power amplifiers. PDF | This paper reports the design and implementation of a Watt audio amplifier. The system features practical audio power amplifier. Figure 2 shows a.

The triode vacuum amplifier was used to make the first AM radio. Audio power amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late s.

Transistor-based amplifiers are lighter in weight, more reliable and require less maintenance than tube amplifiers. In the s, there are still audio enthusiasts, musicians particularly electric guitarists , electric bassists , Hammond organ players and Fender Rhodes electric piano players, among others , audio engineers and music producers who prefer tube-based amplifiers, and what is perceived as a "warmer" tube sound.

Design parameters[ edit ] Three rack-mounted audio power amplifiers used in a sound reinforcement system. Key design parameters for audio power amplifiers are frequency response , gain , noise , and distortion. These are interdependent; increasing gain often leads to undesirable increases in noise and distortion.

Audio power amplifier

While negative feedback actually reduces the gain, it also reduces distortion. Most audio amplifiers are linear amplifiers operating in class AB. Until the s, most amplifiers were tube amplifiers which used vacuum tubes. During the s, tube amps were increasingly replaced with transistor -based amplifiers, which were lighter in weight, more reliable, and lower maintenance. Nevertheless, there are still niche markets of consumers who continue to use tube amplifiers and tube preamplifiers in the s, such as with home hi-fi enthusiasts, audio engineers and music producers who use tube preamplifiers in studio recordings to "warm up" microphone signals and electric guitarists, electric bassists and Hammond organ players, of whom a minority continue to use tube preamps, tube power amps and tube effects units.

While hi-fi enthusiasts and audio engineers doing live sound or monitoring tracks in the studio typically seek out amplifiers with the lowest distortion, electric instrument players in genres such as blues , rock music and heavy metal music , among others, use tube amplifiers because they like the natural overdrive that tube amps produce when pushed hard.

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Simple, the base-emitter junction of a transistor is a diode, and even when conducting it will retain non-linearities. These are often sufficient to enable the input stage to act as a crude AM detector, which will be quite effective with high-level TV or CB radio signals.

Adding the external resistance again swamps the internal non-linearities, reducing the diode effect to negligible levels. This is not to say that it will entirely eliminate the problem where strong RF fields are present, but will at least reduce it to 'nuisance' rather than 'intolerable' levels. UPDATE: I have been advised by a reader who works in a transmitting station that connecting the capacitor directly between base and emitter in conjunction with the stopper resistor is very effective.

He too found that the traditional method was useless, but that when high strength fields are encountered, the simple stopper is not enough. In all both cases it is essential to keep all leads and PCB tracks as short as possible, so they cannot act as an antenna for the RF. Needless to say, a shielded and grounded equipment case is mandatory in such conditions.

There are a number of traps here, not the least of which is that it is commonly assumed that the load from the output stage is infinite. Assuming an output transistor combination with a current gain of 50 for the driver, and 20 for the power transistor , with an 8 Ohm load, the impedance presented to the Class-A stage will be about 2k Ohms, which is a little shy of infinity.

Added to this is the fact that the impedance reflected back is non-linear, since the driver and output transistors change their gain with current - as do all real-life semiconductors. There are some devices available today which are far better than the average, but they are still not perfect in this respect. The voltage gain is typically about 0. It must be noted that this figure will only be true for mid-range currents, and will be reduced at lower and higher values.

Figure 5 shows the basic stage type - the same basic amplifier we used before, with the addition of a current source as the collector load. Also common is the bootstrapped circuit not shown here, but evident on many ESP designs. There is not a lot of difference between current source and bootstrap circuits, but the current source has slightly higher gain. With either type, there are some fairly simple additions which will improve linearity quite dramatically.

Figure 5 shows the typical arrangement, including the pF dominant pole stabilisation capacitor connected between the Class-A transistor's collector and base. Figure 5 - Typical Class-A Driver Configuration It is important to try to make the Class-A stage capable of high gain, even when loaded by the output stage.

There have been many different methods used to achieve this, but none is completely successful. The output stage is not a simple impedance, and it varies as the load impedance changes. Bipolar transistors reflect the load impedance back to the base, adjusted according to the device's gain. A potential problem is that some designers seem completely oblivious to this problem area, or create such amazingly complex 'solutions' as to make stabilisation against oscillation very difficult.

The gate capacitance is not affected by the load impedance, and nothing is reflected back to the Class-A driver. This will typically allow it to have higher gain - especially when low load impedances are involved. The Class-A driver needs only to be able to charge and discharge the gate capacitance of the MOSFETs, and this is not influenced by the output current or load.

This straightforward addition of an emitter follower to the Class-A driver with the 1k 'bootstrap' resistor has increased the combined LTP and Class-A driver gain to 1,, yes, 1. Open loop output impedance is about 10k, again without the cap. Once the latter is in circuit, gain is reduced to a slightly more sensible 37, at 1kHz with the pF value shown. Output impedance at 1kHz is now comparatively very low, at about Ohms. Note that in the above, I have used a 5k resistor instead of the more usual current source to bias the long-tailed pair.

This is for clarity of the drawing, and not a suggestion that the current source should be forsaken in this position. I have often seen amplifier designs where the circuit is of such complexity that one must wonder how they ever managed to stop them from becoming high power radio frequency oscillators. The maze of low value capacitors sometimes used - some with series resistance - some without, truly makes one wonder what the open loop frequency and phase response must look like.

Couple this with the fact that many of these amps do not have wonderful specifications anyway, and one is forced to ponder what the designer was actually trying to accomplish being 'different' is not a valid reason to publish or promote a circuit in my view, unless it offers some benefit otherwise unattainable. Having carried out quite a few experiments, I am not convinced that vast amounts of gain from the input stage and Class-A amplifier stage are necessary or desirable.

As long as the circuit is linear i. I have seen many circuits with far more open loop gain than my reference amp Project 3A , that in theory should be vastly superior - yet they apparently are not.

There are essentially two ways to create a constant current feed to the Class-A driver stage. The active current source is one method, and this is very common. It does introduce additional active devices, but it is possible to make a current source that has an impedance so close to infinity that it will be almost impossible to measure it without affecting the result just by attaching measurement equipment. For more detailed information on current sources, see the article Current Sources Sinks and Mirrors.

Figure 6A shows an active current source for reference. A simpler way is to use the bootstrap circuit, where a capacitor is used from the output to maintain a relatively constant voltage across a resistor.

If the voltage across a resistor is constant, then it follows that the current flowing through it must also be constant. Figure 6a shows the circuit of a bootstrap constant current source. Unlike a true current source, the current through the bootstrap circuit will change with the supply voltage.

This is a gradual change, and is outside the audio spectrum - or at least it should be if the circuit is designed correctly. Under quiescent conditions, the output is at zero volts, and the positive supply is divided by Rb1 and Rb2.

As the output swings positive or negative, the voltage swing is coupled via Cb, so the voltage across Rb2 remains constant. The current through Rb2 is therefore constant, since it maintains an essentially constant voltage across it. Note that this applies only for AC voltages, as the capacitor cannot retain an indefinite charge if there is a DC variation. The overall difference is not great in a complete design.

Although the current source is theoretically better, a bootstrap circuit is simpler and cheaper, and introduces no additional active devices.

كتاب Audio Power Amplifier Design Handbook

The capacitor needs to be large enough to ensure that the AC across it remains small less than a few hundred millivolts at the lowest frequency of interest. Both configurations in basic form, since there are many variations are shown in Figure 7. There are two main areas where the Darlington configuration is inferior, and we shall look at each. In the following, bias networks and Class-A driver s are not included, only the output and driver transistors As can be seen, the component count is the same for those shown, but instead of using two same polarity both PNP or both NPN , the compound pair also called a Sziklai pair uses one device of each polarity.

The final compound device assumes the characteristics of the driver in terms of polarity, and the Emitter, Base and Collector connections for each are shown. The ohm resistor or other value determined by the design is added to prevent output transistor collector to base leakage current from allowing the device to turn itself on, and also speeds up the turn-off time. Omission of this resistor is not a common mistake to make, but it has been done. In some cases, you'll see a comparatively high value used.

Audio Power Amplifier Design, 6th Edition

The results are degraded distortion figures, especially at high frequency, and poor thermal stability. The value must be selected with reasonable care, if it is too low, the output transistor will not turn on under quiescent no signal conditions, the driver transistor s will be subject to excessive dissipation, and crossover distortion will result. If too high, turn-off performance of output devices will be impaired and thermal stability will not be as good.

The final value depends to some extent on the current in the Class-A driver stage and the gain of the driver transistor, but the final arbiter of quiescent is the Vbe multiplier stage. These comments apply equally to the Darlington and compound pairs. Values of between Ohms up to a maximum of perhaps 1k should be fine for most amplifiers, with lower values used as power increases.

High power creates higher currents throughout the output stage and makes the transistors harder to turn off again, especially at high frequencies. This can lead to a phenomenon called 'cross-conduction', which occurs because the transistors cannot switch off quickly enough, so there is a period where both power transistors are conducting simultaneously.

It won't happen at normal audio frequencies, although you may get slightly higher than normal current drawn from the power supply even at 20kHz. If an amp is driven to any reasonable power at higher frequencies, it can spontaneously self-destruct if there is sufficient cross conduction happening. The easiest way to reduce it is to use smaller resistors between base and emitter of the power transistors, but be aware that this will increase the demands on the drivers.

For example, with ohm resistors as shown above, the resistors will only pass around mA, but if they are reduced to say 47 ohms, that increases to perhaps 16mA or more. A heatsink for the drivers becomes a necessity. If you want to get full power at kHz or more why?

You will also need to increase the power rating for the Zobel network resistor, or it will overheat at high frequencies.

Since each has its own thermal characteristic a fall of about 2mV per degree C , the combination can be difficult to make thermally stable. In addition, the gain of transistors often increases as they get hotter, thus compounding the problem.

The bias 'servo', typically a transistor Vbe multiplier, must be mounted on the heatsink to ensure good thermal equilibrium with the output devices, and in some cases can still barely manage to maintain thermal stability. If stability is not maintained, the amplifier may be subject to thermal runaway, where after a certain output device temperature is reached, the continued fall of Vbe causes even more quiescent current to flow, causing the temperature to rise further, and so on.

Since the single Vbe is that of the driver which should not be mounted on the main heatsink, and in some will have no heatsink at all , the requirements for the Vbe multiplier are less stringent, mounting is far simpler and thermal stability is generally very good to excellent. I have used the compound pair since the early s, and when I saw it for the first time, it made too much sense in all respects to ignore. However, there were a couple of other tricks used at the time to guarantee stable operation.

It is a very basic Vbe multiplier circuit, and seemingly, nothing can go wrong. This is almost true, except for the following points. Figure 9 - The Basic Bias Servo Circuit The design of many amps especially those using a Darlington output stage requires that the bias servo be made adjustable, to account for the differing characteristics of the transistors.

An 'inverting' amplifier produces an output degrees out of phase with the input signal that is, a polarity inversion or mirror image of the input as seen on an oscilloscope. A 'non-inverting' amplifier maintains the phase of the input signal waveforms.

An emitter follower is a type of non-inverting amplifier, indicating that the signal at the emitter of a transistor is following that is, matching with unity gain but perhaps an offset the input signal. Voltage follower is also non inverting type of amplifier having unity gain.

Other amplifiers may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub-systems and rarely to individual stages. Amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages. Different types of these include:. Depending on the frequency range and other properties amplifiers are designed according to different principles.

Frequency ranges down to DC are only used when this property is needed. Amplifiers for direct current signals are vulnerable to minor variations in the properties of components with time.

Special methods, such as chopper stabilized amplifiers are used to prevent objectionable drift in the amplifier's properties for DC. Depending on the frequency range specified different design principles must be used. Up to the MHz range only "discrete" properties need be considered; e.

For example, a specified length and width of a PCB trace can be used as a selective or impedance-matching entity. Above a few hundred MHz, it gets difficult to use discrete elements, especially inductors. In most cases, PCB traces of very closely defined shapes are used instead stripline techniques.

The power amplifier classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. The angle of flow is closely related to the amplifier power efficiency.

The practical amplifier circuit to the right could be the basis for a moderate-power audio amplifier. It features a typical though substantially simplified design as found in modern amplifiers, with a class-AB push—pull output stage, and uses some overall negative feedback.

Bipolar transistors are shown, but this design would also be realizable with FETs or valves. The input signal is coupled through capacitor C1 to the base of transistor Q1. The capacitor allows the AC signal to pass, but blocks the DC bias voltage established by resistors R1 and R2 so that any preceding circuit is not affected by it.

Q1 and Q2 form a differential amplifier an amplifier that multiplies the difference between two inputs by some constant , in an arrangement known as a long-tailed pair. This arrangement is used to conveniently allow the use of negative feedback, which is fed from the output to Q2 via R7 and R8.

The negative feedback into the difference amplifier allows the amplifier to compare the input to the actual output. The amplified signal from Q1 is directly fed to the second stage, Q3, which is a common emitter stage that provides further amplification of the signal and the DC bias for the output stages, Q4 and Q5.

R6 provides the load for Q3 a better design would probably use some form of active load here, such as a constant-current sink.

So far, all of the amplifier is operating in class A. The output pair are arranged in class-AB push—pull, also called a complementary pair.

They provide the majority of the current amplification while consuming low quiescent current and directly drive the load, connected via DC-blocking capacitor C2. The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimized. That is, the diodes push the output stage firmly into class-AB mode assuming that the base-emitter drop of the output transistors is reduced by heat dissipation.

This design is simple, but a good basis for a practical design because it automatically stabilises its operating point, since feedback internally operates from DC up through the audio range and beyond.

Further circuit elements would probably be found in a real design that would roll-off the frequency response above the needed range to prevent the possibility of unwanted oscillation.

A common solution to help stabilise the output devices is to include some emitter resistors, typically one ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amp. Any real amplifier is an imperfect realization of an ideal amplifier. An important limitation of a real amplifier is that the output it generates is ultimately limited by the power available from the power supply.

An amplifier saturates and clips the output if the input signal becomes too large for the amplifier to reproduce or exceeds operational limits for the device. The power supply may influence the output, so must be considered in the design. The power output from an amplifier cannot exceed its input power. The amplifier circuit has an "open loop" performance. This is described by various parameters gain, slew rate , output impedance , distortion , bandwidth , signal-to-noise ratio , etc.

Many modern amplifiers use negative feedback techniques to hold the gain at the desired value and reduce distortion. Negative loop feedback has the intended effect of lowering the output impedance and thereby increasing electrical damping of loudspeaker motion at and near the resonance frequency of the speaker. When assessing rated amplifier power output, it is useful to consider the applied load, the signal type e.

In high-powered audio applications that require long cables to the load e. This avoids long runs of heavy speaker cables. To prevent instability or overheating requires care to ensure solid state amplifiers are adequately loaded. Most have a rated minimum load impedance. All amplifiers generate heat through electrical losses. The amplifier must dissipate this heat via convection or forced air cooling. Heat can damage or reduce electronic component service life.

Designers and installers must also consider heating effects on adjacent equipment. Different power supply types result in many different methods of bias. Bias is a technique by which active devices are set to operate in a particular region, or by which the DC component of the output signal is set to the midpoint between the maximum voltages available from the power supply.

Most amplifiers use several devices at each stage; they are typically matched in specifications except for polarity. Matched inverted polarity devices are called complementary pairs. Class-A amplifiers generally use only one device, unless the power supply is set to provide both positive and negative voltages, in which case a dual device symmetrical design may be used.

Class-C amplifiers, by definition, use a single polarity supply. Amplifiers often have multiple stages in cascade to increase gain.

Each stage of these designs may be a different type of amp to suit the needs of that stage. For instance, the first stage might be a class-A stage, feeding a class-AB push—pull second stage, which then drives a class-G final output stage, taking advantage of the strengths of each type, while minimizing their weaknesses.

From Wikipedia, the free encyclopedia. This article is about electronic amplifiers. For other uses, see Amplifier disambiguation.

Main article: Amplifier figures of merit. Main articles: Operational amplifier and Instrumentation amplifier. Distributed amplifier. See also: Power amplifier classes. Electronics portal.

Electronics, 2nd Ed. CRC Press. Foundations of Analog and Digital Electronic Circuits. Morgan Kaufmann. Introduction to Circuit Analysis and Design.

Springer Science and Business Media. In Glen Ballou. Handbook for Sound Engineers: The New Audio Cyclopedia. Howard W. From Marconi's Black-Box to the Audion. MIT Press. Retrieved January 7, Eliminating the Bridging Inverter.

This arrangement is also used as the input presents a high impedance and does not load the signal source, though the voltage amplification is less than one. Stability is very important to me, and I tend towards an amp which absolutely does not oscillate, even at the expense of a little more distortion.

In the days of the integrated circuit a few more transistors on a chip are much cheaper and smaller than a capacitor. There are usually easily measured differences between NPN and PNP devices from the same family, and datasheets will quickly disabuse you of the notion that they are the same. In the light of this aim, specific background of audio amplifier types and definitions of necessary concepts are explained in introduction part of the report.

Needless to say, a shielded and grounded equipment case is mandatory in such conditions. Telegraph and Telephone Age. This is not always the case, and some layouts may include more than enough track length to not only act as an inductor, but as an antenna as well.