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An audio amplifier is an electronic device that amplifies low-power audio signals whose frequencies are confined to the audible frequency range, 20Hz to 20,000Hz. The output signal can reach thousands times the input signal. 
Early Stages of Audio Amplifiers
Audio Amplifiers were invented in 1909 by Lee De Forest. They were consisted of triodes vacuum tubes and were used to make the first AM radio. These tubes were formed of a sealed glass or metal enclosure from which the air has been withdrawn. Such early types allowed the development of radio broadcasting, telephone service, television and the first electronic digital computers. 
Figure : Vacuum Tubes Audio Amplifier
Modern audio amplifiers became based on transistors (BJTs, FETs, MOSFETs…) mainly in the late 1960s due to their reduced cost and wide availability. They have replaced vacuum tubes in almost all applications except in some high-frequency transmitters on space satellites.
Need for Amplification
Amplification is defined as the process that increases the strength of a signal in terms of current, voltage or power without altering the original signal at the input.
A sound is set to be audible if it is not too high or too low to be heard by the human ear that is between 20 Hz and 20 kHz. The microphone is a transducer that converts acoustic signals to electrical ones. The output signal derived from the microphone is very weak (in the order of 0.01V or less) and must be amplified before being processed in any other circuit. Once the electrical signal reaches the speaker, it will be converted into an audible sound wave.
1.4 Characteristics of Audio Amplifiers
1.4.1 Linearity and Efficiency
Linearity and efficiency are of the main characteristics of audio amplifiers. An audio amplifier is said to be linear if it preserves the detail of the signal waveform, that is:
Where Vo(t) and Vi(t) are the output voltage and input voltage respectively and A is the constant gain.
Real audio amplifiers are linear with some practical limitations. When the input signal is increased, the amplifier will amplify the signal until a certain point is reached where the amplifier becomes saturated and no more amplification can be produced. This phenomenon is known as clipping and leads to distortion. Some of the audio amplifiers are designed in a way to handle this result in a controlled way that causes a reduction in gain instead of distortion. The reduction in gain results in a compression effect that will sound much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is reached at the point where the input signal is amplified by an amount of 1 dB less than the small signal gain. In others words, audio amplifiers can't be linear over all input values because the amplification process stops at a certain point. The existence of the 1 dB compression is due to the presence of the non linear active transistors. If we keep increasing the input power beyond a certain extreme limit, the components might be destroyed. The 1 dB compression is defined and represented as follows: P1dB output = P1dB input + (Gain - 1) dBm
Figure 2: P1 dB Compression Curve
The amplifier's efficiency is the ability of the amplifier to convert the dc supply power into the output signal power delivered to the load. Efficiency therefore can be represented by the following equation:
The power delivered to the load in an ideal amplifier is equal to the power supplied from the source. Thus, the efficiency of the ideal amplifier is equal to one. This case is hard to achieve in reality. Losses occur in the amplification process especially in high frequency systems.
Note that there is a trade-off between these two characteristics. Improving the amplifier's linearity will degrade its efficiency and vice versa.
The audio amplifier gain 'G' is defined as the ratio of its output power, output voltage or output current to its input power, voltage or current respectively.
It is measured in decibels dB and represented as follows: 
1.4.3 Bandwidth and Cut-off Frequencies
The bandwidth 'BW' of an audio amplifier is the interval of frequencies where the amplifier functions with high performance. It is evaluated down from the peak value by 3 dB.
Professional amplifiers are equipped with an input and/or output filtering in order to limit frequency response beyond 20 Hz-20 KHz, otherwise much of the output power would be wasted on infrasonic and ultrasonic frequencies and the risk of AM radio interference might increase.
Note that the low frequency gain is the gain of the amplifier at the low cutoff frequency whereas the high frequency gain is at the high cutoff frequency.
Figure 3: Bandwidth and Cut-off Frequencies
1.4.4 Total Harmonic Distortion
A perfectly linear amplifier will amplify the input signal without any signal distortion at the output. However, there are always spurious signal components that are added to a signal in the form of harmonics or intermodulation distortion. Total Harmonic Distortion compares the output signal of the amplifier with its input signal and measures the harmonic frequency between the two. This difference is the total harmonic distortion (THD) which is measured as percentage where lower percentages are better. In other words, THD is the value of harmonic components excluding the fundamental one expressed as the percentage of the rms value of the fundamental.
Why Evaluating The THD?
In reality, total harmonic distortion is hardly perceptible to the human ear. Every component adds some level of distortion, but most distortion is insignificant and small differences in specifications between components mean nothing. Some components have distortion so low it cannot be accurately measured. Listening to a component and evaluating its sound characteristics is the most important way to judge a product. Other considerations such as selecting the right speakers are more important than the percentage of total harmonic distortion. 
1.5 Power Amplifier Classification
Power amplifiers are the output stages in the transmission chain. They are classified into classes according to their circuit configurations, methods of operation and efficiency considerations.
For analog design, the classes are A, B, AB and C. These classes range from entirely linear with low efficiency to entirely non-linear with high efficiency. Classes D and E represent switching designs based upon the conduction angle (or angle of flow), Θ, of the input signal cycle through the output amplifying device. In other words, Θ corresponds to the interval of the input signal cycle where the amplifier is in conduction mode.
1.5.1 Class-A Audio Amplifier
This class of amplifiers has the highest linearity over the other classes. It functions in the linear portion of its characteristic and is equivalent to a current source. To achieve high linearity and gain, the amplifier's base and drain DC voltage should be chosen properly in order to maintain the amplifier's operation in the linear region.
The conduction angle of this class is Θ = 360°, therefore the amplifier remains conducting all the time. This type of amplifier is less complex than other types, typically more linear but less efficient. Theoretical maximum efficiency obtained with inductive output reached 50% but with capacitive loads, 25% was the highest efficiency obtained. Even if there is no input at all, power will keep drawing from the supply. In addition to that, the amplifier is always conducting and operated over the most linear part of its transfer characteristic making the device less efficient and producing heat.
C:\Documents and Settings\Norma Said\Desktop\Electronic amplifier - Wikipedia, the free encyclopedia_files\180px-Electronic_Amplifier_Class_A.png
Figure 4: Class-A Audio Amplifier
1.5.2 Class-B Audio Amplifier
Class-B audio amplifiers operate ideally at zero quiescent current so that the DC power is small resulting in higher efficiency than Class-A amplifiers. Operation occurs in one half of the input wave cycle, thus Θ=180° which results in a large amount of distortion. The amplifier is switched off half of the time and doesn't dissipate power so that improves the efficiency of this class compared to Class-A.
Electronic Amplifier Class B fixed.png
Figure 5: Class-B Audio Amplifier
A practical circuit of Class-B amplifiers is the complementary pair also known as 'Push-Pull' arrangement. In this configuration, complementary devices are used to amplify the opposite halves of the input signal then to recombine them at the output. One of the transistors 'pushes' current into the load while the other transistor 'pulls' current from it. The current level is therefore larger than the one at the input.
Figure 6: Class-B Push- Pull Audio Amplifier
Although this arrangement provides good efficiency, it can face a mismatch problem when the two halves join. This phenomenon is called crossover distortion. To avoid this problem we need to bias the devices in a way not to allow complete turning off when out of use. This is the Class-AB operation.
1.5.3 Class-AB Audio Amplifier
The operation of this Class corresponds to the operation of the Class-B amplifier over half of the waveform. In addition to that, the amplifier conducts a small amount on the other half. The transistor will be on for more than half a cycle but less than a full cycle of the input signal. As a result, the zone where both devices simultaneously are nearly off is reduced. When the waveforms from the two devices are combined, the crossover is sharply minimized.
Class-AB is therefore a compromise between Class-A and Class-B in terms of efficiency and linearity. It is much more efficient than Class-A but less efficient than Class-B. Modern Class AB amps are commonly between 35-55% efficient with a theoretical maximum of 78.5% .
1.5.4 Other Classes
The three Classes previously cited are considered linear amplifiers where the output signal's amplitude and phase are linearly related to the input signal's amplitude and phase. In application where efficiency is critical, Classes-C, D, E or F are used.
Class-C audio amplifiers are biased in a way the output current is zero for more than one half of the input sinusoidal signal cycle. 90% efficiency can be achieved accompanied with high output distortion. On the other hand, Class-D amplifiers allow the conversion of the input signal into a sequence of output pulses. The frequency of these pulses is ten or more times the highest input frequency, they contain inaccurate spectral components, known as harmonics, which must be removed using low-pass passive filter.
Finally, Classes-E and F are high efficient switching power amplifiers.
1.6 Examples of Audio Amplifiers
1.6.1 Transistor Audio Amplifiers
Transistor circuits are frequently used as amplifiers. Some of them deal with current amplification with a small load resistance; others provide voltage amplification with high load resistance while the rest of them amplify power. For example, Voltage Amplification occurs by producing a large change in the collector current for a small change in the base current. The load resistor placed in series with the collector responds to the large changes in the collector current and yield in large variations in the output voltage.
The amount of magnification is the forward gain that we determine by the external circuit design of the amplifier.
Transistor amplifiers can consist of bipolar junction transistors (BJTs) or metal oxide semiconductor field effect transistors (MOSFETs). Such types of amplifiers are realized using numerous configurations. With BJTs, common base, common collector or common emitter can be realized whereas MOSFETs configurations are common gate, common source or common drain amplifiers. Each configuration type has its own advantages and disadvantages as well as particular gain, impedances…Biasing transistors can be accomplished in three different ways. The three types of BIAS used to properly bias a transistor are base-current bias known as fixed bias, self-bias and combination bias. Combination bias is the one most widely used because it improves circuit stability and at the same time overcomes some of the disadvantages of base-current bias and self-bias.
1.6.2 Operational Audio Amplifiers
Operational audio amplifiers, called op-amps, are high gain electronic voltage amplifiers with differential inputs and a single output. The large gain is controlled with the negative feedback of the amplifiers. Op-amps are widely used in numerous applications such as scientific, industrial…
They come in the form of integrated circuits IC and might have very special performances at low costs. As an example, the LM386 audio amplifier designed for use in low voltage consumer applications. Its gain is internally set by varying the capacitor between pins 1 and 8. The gain may therefore increase from 20 to 200 Hz. This type of audio amplifiers is widely used in TV sound systems and AM-FM radio amplifiers.
Figure 7: LM386 Audio Amplifier
Four Transistors Audio Amplifier
2.1 General Description of The Circuit Design
Four transistors audio amplifier is one of the simplest types of audio amplifiers using transistors. In addition to the four transistors, the circuit consists of several resistors, capacitors and two diodes.
The configuration is shown below:
Figure 8: Four Transistors Audio Amplifier
A DC source of 9V is chosen to drive the 8â„¦ speaker. The choice of the voltage is made in a way to supply the transistor with the minimum voltage to turn on. However, it was
noticed that the distortion is high on the value 9V. On the other hand, variations in the components parameters showed that few limited deviation of all the indicated values will not affect the performance of the circuit nor degrade its efficiency.
Typical sounds produced by human speech have frequencies on the order of 300 to 3000 Hz. The frequency chosen for the input AC voltage source was 1000Hz, preventing the interference with the electrical frequency 50 or 60Hz.
The Tables below show how to identify the sound's intensity according to its nature and how to identify its corresponding frequency respectively: 
Sound Intensity (dB)
Threshold of hearing
Whispering at 5 feet
Coffee maker, library, quiet office
Dishwasher, electric toothbrush, large office
Normal Conversation, Television
Shouted Conversation, Electric Drill, Truck
Band Concert, Heavy Machinery
Threshold of Pain, Jet airplane taking off
Table 1: Sound Intensity in dB
Sound Frequency (Hz)
Voice frequency for understanding speech
Maximum sensitivity of the human hearing
Lower limit of human hearing
Table 2: Sound Frequency in Hz
Lowering the 33Kâ„¦ resistor to about 10Kâ„¦ will move the voltage on the output electrolytic capacitor to about half the supply voltage which will result in more signal swing but this change is not necessary if the volume is adequate. Both 4.7â„¦ resistors may be replaced with a single 10â„¦ resistor in series with either emitter.
The power amplifier circuit included the 2N4401 and 2N4403 transistors in order to know the amount of power that is delivered to the load or supplied to the circuit and are specified to be general purpose switching and small signal amplifier transistors. They have a low base spreading resistance which makes them suitable for low noise amplifiers, so that when using these transistors, the signal to noise ratio is improved. The 1N914 diodes used are fast switching diodes; they have a 100V reversed voltage and accept a current of about 500mA. Their use was limited to the protection of the power amplifier that is the last amplifier in the transmission chain (output stage) that requires the most attention to power efficiency.
The circuit is suitable to be used in a wide variety of applications including receivers, intercoms, microphones and telephone pick-up coils.
2.3 Simulation and Measurements
2.3.1 Gain and Cut-off Frequencies
Once the circuit is designed in Multisim, AC analysis is used to calculate the frequency response of the audio amplifier. Below is the simulation output result:
Figure 9: Amplifier's Gain
In order to evaluate the gain of the audio amplifier, we evaluate the value of the maximum gain obtained. Then by subtracting 3dB from 'Gmax' we obtain the low frequency gain and high frequency gain. The gain of the amplifier 'G' is the constant value 'Mid Band Gain' between both low and high frequency gains. It is found to be: G= 39.493 dB
The cut off frequencies, as previously defined, represent the frequencies over which the gain is decreased by 3dB of its maximum value. The low cut off frequency was found to be: fc,Low = 5.65Hz while the high cut off frequency was fc,High = 100KHz
2.3.2 Total Harmonic Distortion
A perfect linear amplifier can be described using the following formula: Y = AX, where Y is the output signal, X is the input signal and A is the amplifier gain.
The general expression including higher order terms is given by the following:
Y = AX + BX2 + CX3 + DX4 + …, where B and C, etc. are the constant coefficients for the higher order terms.
The second term in the above equation is known as the second-order component, the third term is the third-order component and so on. Harmonic distortion can be analyzed by applying a spectrally pure signal source to a circuit design. By analyzing the output signal and its harmonics the distortion can be determined. Multisim will calculate the node voltages and branch currents at the harmonic frequencies 2f and 3f and display the results against the input frequency f as it is swept across the user defined frequency range.
Total Harmonic Distortion has been evaluated over the load voltage and found to be:
THD (Load Voltage) = 40.6015%
2.4 Improving The Four Transistors Audio Amplifier
Improving the gain of the voltage amplifier is quite a good feature that was obtained in two different ways: Using compound devices such as the Darlington Pair Configuration or lowering all the resistor values of the circuit.
18.104.22.168 Darlington Configuration
In order to increase the current gain 'β' of the output-stage transistors, and thus reduce the required base current drive, the Darlington configuration is frequently used to replace the NPN transistor and the PNP transistor of the Class-AB. The Darlington configuration is in fact equivalent to one transistor having β ≈ β1 - β2 and twice the value of VBE. 
The Darlington Pair Configuration is shown below with the simulation output result of the gain:
Figure 10: Darlington Transistor Pair
Figure 11: Improved Gain using Darlington Transistor Pair
The amplifier's gain has increased to G = 42.1725 dB
22.214.171.124 Varying The Resistors Values
In an attempt to increase the current delivered to the load the resistors values were decreased. Below is illustrated the improved gain using one third of the initial resistor values:
Figure 12: Improved Gain by Decreasing Resistor Values
In this way the audio amplifier's gain has been increased to G = 43.256 dB.
This method is to be adopted with several limitations. Increasing the voltage gain should take into account the excessive high current produced that may go beyond the rating values of the components used and therefore damage them as well as the circuit. See Appendices B, C and D for rating values of the transistors and diodes used.
In addition, increasing the efficiency will decrease the amplifier's linearity as previously shown. In order not to drop the functioning of the audio amplifier in the non-linear zone, one must avoid extremely increasing the efficiency and maintain a certain compromise among linearity and efficiency.
2.4.2 Cut-off Frequencies
In applications that recommend varying the Cut-off frequencies, the capacitors need to be changed. To affect the high Cut-off frequency, internal transistors capacitance need to be varied but when dealing with the low Cut-off frequency, modifying the value of the coupling capacitors in the first stage of the amplifier will change the low Cut-off frequency.
In Figures 13-a) and 13-b) the low Cut-off frequency has been modified by decreasing and increasing the capacitance values respectively:
Figure 13-a: Increasing The Low Cut-off Frequency
Figure 13-b: Decreasing The Low Cut-off Frequency
Initially, the low Cut-off frequency was: fc,Low =5.65Hz. Dividing the capacitance values by three increased this frequency value to fc,Low = 16.702Hz while multiplying the initial capacitances values by three has decreased the low Cut-off frequency to fc,Low = 2.29Hz
2.5 Hardware Implementation
The audio amplifier circuit was implemented and the gain was measured using an oscilloscope. Below is the figure of the design and the oscilloscope showing both input and output voltages:
Figure 14: Hardware Implementation
Figure 15: Input and Output Voltages on The Oscilloscope
The output voltage was obtained 72.8mV where the input was 20mV. The measured gain therefore was G = 36.88 dB.
Comparing the simulated value of the gain to the measured one G = 39.493 dB, we notice they are slightly different. In the implementation of the circuit, some losses occur due to the components heating and wiring losses.
2.6 Advantages and Disadvantages
The Four Transistors Audio Amplifier is immune to noise and operates as a Class-AB audio amplifier having a good efficiency and maintaining a linear operation mode.
The presence of the power isolation circuit at the output stage reduces the oscillations resulting in less distortion and unlike other Classes this audio amplifier doesn't conduct at all times which will reduce the power dissipation in the circuit and the excessive heating that affects the amplifier's performance and damages the electronic components. The use of diodes in the last stage has protected the transistors from handling extra heating and thus favored the efficient operation of the amplifier.
Finally, one of the Four Transistors Audio Amplifier's advantages resides in the simple modification upon the component values resulting in higher gain or matching with other application requirements.
On the other hand, the main disadvantage of this audio amplifier type is that its usage is restricted to 9V. This voltage causes stability problems and more distortion. However, if this DC supply is decreased, the amplification process will no more occur as the transistor won't turn on.
2.6.1 Thermal Dissipation and Distortion
When the junction temperature of a transistor increases, the collector current increases due to the increase in the saturation currents and DC conductances. Thermal instability occurs when junction temperature and collector current increase in regenerative and uncontrollable fashion. The limit depends on factors both within and external to the transistor. The internal factors are the thermal resistance, the current amplification factor, and the base lead resistance. The external factors are ambient temperature, collector voltage, circuit resistances, and the thermal coupling between the transistor and temperature compensation elements, if any.
Thermal Distortion is a major problem in audio amplifiers that appears as second or third harmonic distortion at low frequencies. Fortunately, this undesired phenomenon has its largest effects in Class-B audio amplifiers where dissipation varies greatly over one cycle.
In order to minimize this effect, one should provide more power dissipation in the circuit to reduce the extra heating generated. In the Four Transistors Audio Amplifier, the use of the two diodes in the output stage has handled a part of the power, thus reducing the negative effects generated from thermal distortion.
2.6.2 Physical Limitations
The description of amplifiers in the previous sections dealt with ideal devices. In reality, transistor amplifiers suffer from a number of limitations that influence amplifier operation and ultimately reduce their efficiency and output power. Increasing the amplifier's efficiency will expose it to larger current flow that will push the device toward non-linear mode. Here is a compromise that will be always done in order to maintain highest efficiency possible with corresponding linear operation of the audio amplifier. Furthermore, each component of the power amplifier stage, such as diodes and transistors, has specific voltage, current, temperature…to handle. Therefore neglecting these parameters will increase the possibility of decreasing the amplifier's performance or damaging it. 
One type of audio amplifiers, The Four Transistors Audio Amplifiers, was investigated in terms of its characteristics and methods of improvements. Hardware implementation has allowed proving that the simulated results are slightly different than those measured using the laboratory equipments.
Certain physical limitations decreased the performance efficiency of the amplifier therefore some compromises had to be taken aiming to achieve high amplification within the linear operation mode.
Wide uses of audio amplifiers require certain modification through components values such as varying the frequency range in order to avoid electrical interference with the input signal.