The Design Of Class A Amplifier Computer Science Essay

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Communication has been the important part in human's daily life and the most common manner of communication is through conversation. In real life, there is a physical limitation for a person to communicate with a big crowd of people. The voice of the person who speaks is not loud enough till all the audience can hear clearly. Hence, such physical limitation leads to a development of various devices which help human extend their capabilities of talking and listening.

In the early days and even nowadays, some people use the simple microphone and loud speaker in order to make the speech audible to a big crowd of audience. Those devices enable human to communicate in a more efficient way. During the early 19th century, with the advent of vacuum tubes, the primitive form of transistor, the engineers immediately searched for the usage of vacuum tubes in amplification. With the invention of the silicon semiconductor, the engineers searched for more applications of transistor.

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Consequently, the engineers managed to use the transistor to amplify sound and they even could manipulate the sound in many ways in order to produce high fidelity sound. There was a tremendous development in the field of audio, where high precision and quality of audio equipment was required to amplify and manipulate the music.

Nowadays, the audio mixer is the most important audio equipment available in the market. It enables the disk jockeys to combine the multiple audio sources into a harmonious output. Furthermore, it also enables the musicians to control the frequency bands, loudness and other individual music instruments' parameters to generate the desired music.

In this project, a basic audio mixer is built based on the fundamental knowledge about the operation of operational amplifier and its applications in cross-fading, amplifying and filtering. In addition, the application of bipolar junction transistor in power amplifier is also applied to the audio mixer.

1. INTRODUCTION

The operational amplifier, µA741 is the main electronics component mostly utilized in the project of audio mixer. A basic audio mixer involves various applications of operational amplifier such as cross fader, active filters, inverting summer and voltage follower or buffer. Other than that, the operational amplifier is also applied in the circuit construct of voltage level indicator.

In this project, other model or series of operational amplifier is disallowed to be used in the circuit, and this situation has become a great challenge as µA741 operational amplifier itself has high or more internal noise compared to other model of operational amplifier. Nevertheless, this challenge is good for the students to learn on how to reduce the noise instead of just using good operational amplifier which has very low internal noise.

Generally, the basic audio mixer is totally not good enough as its output contains a lot of noise. In order to reduce the noise, the wire used to connect the electronics components has to be as short as possible since the wire itself has internal resistance which will lead the noise to occur. However, sometimes the wire used to connect is too short and it causes loose connection occur. Therefore, every connection has to be connected properly, if not the circuitry might have been short-circuited.

Somehow, the failure of the circuit is not due to the connection. The most probably reason is caused by the components that are not reliable such as the potentiometer and IC (Integrated Circuit) of operational amplifier. Hence, the troubleshooting should be made on the components as well if the circuit does not work. The voltage of each component connected is measured to determine which component is not reliable.

As mentioned before, the fundamental design of audio mixer is not good enough even though all the possible error of the circuit is minimized as small as possible. Consequently, the enhancement is required for the better performance of audio mixer. The enhancement includes adding two more equalizer bands and class AB amplifier, and replacing the initial speaker with a higher power speaker.

After enhancing, the noise has been reduced a lot and the sound quality significantly becomes much clear and louder than the output of the previous design. However, there is still a limitation of the new design that the circuit can only handle a certain range of current. The output will be noisy once the current flowed exceeds the maximum current that the circuit allows.

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Overall, the noise is managed to be reduced a lot and the sound quality has been improved to be much better. The enhancement and the other attempts to enhance will be discussed in more detail for the following content.

2. ENHANCEMENT

2.1 5-BAND EQUALIZER

Two additional equalizer bands are added in order to cover human's audible frequency range from 20 Hz to 20 kHz. Originally, the basic audio mixer contains a 3-band graphic equalizer and each band controls the bass, midrange and treble respectively. 5-band graphic equalizer is designed to cover some ranges of frequency that have not been covered by the 3-band graphic equalizer. After enhancing, the equalizer now has 2 bands controlling the bass, 1 band controlling the midrange, and 2 bands controlling the treble.

The equalizer bands are categorized according to their frequency response. The bass handles low frequency response less than 500 Hz, the midrange handles frequencies from 500 Hz till around 3 kHz, and the treble handles high frequency response from 3 kHz to 20 kHz. The type of equalizer used in the design is graphic equalizer and each equalizer band is actually a cascaded and active first-order RC band pass filter.

For each equalizer band, the range of frequency is designed by using the formula of computing the cutoff frequency. The formula used to obtain lower and upper cutoff frequency is same, which is as follow:

where fc is cutoff frequency, R is resistance and C is capacitance.

The formula of center frequency, fo, of a band pass filter is given by:

where:

fL is the lower cutoff frequency.

fH is the upper cutoff frequency.

∆f is the equal distance from fc to fL or fH, therefore

The formula of quality factor, Q, is given by:

Q = fc / BW = fc / ( fH - fL ), where BW is the bandwith.

All the computed values of the design of 5-band equalizer are shown in Table 1 below.

Equalizer Band

fL (Hz)

fH (Hz)

fc (Hz)

BW (Hz)

Q

1stBand

159.1549

530.5165

344.8357

371.3571

0.93

2ndBand

530.5165

1940.9139

1235.7152

1410.3974

0.94

3rdBand

1940.9139

6639.7557

4290.3348

4698.8418

0.91

4thBand

6639.7557

20095.3211

13367.5384

13455.5654

0.99

5thBand

11287.5846

31206.8516

21247.2181

19919.2670

1.07

Table1: Computed values of 5-band equalizer design

The circuit design was simulated by using a software called PSpice Student Version. This software is not good enough due to its limitation of nodes, which can be only up to 64 nodes. In order to get simulated, the circuit design was further simplified. The simulation result would be definitely different with the computed result of the actual design. All the simulation results obtained were just an approximation. The circuit designs were totally same for every equalizer band except for the resistance and capacitance.

First of all, let's analyze the previous 3-band equalizer and make the comparison between it and 5-band equalizer. All the theoretical values of the design of 3-band equalizer are shown in Table1.1.

Equalizer Band

fL (Hz)

fH (Hz)

fc (Hz)

BW (Hz)

Q

1stBand

159.1549

530.5165

344.8357

371.3571

0.93

2ndBand

1989.4368

5894.6275

3942.0322

3905.1907

1.00

3rdBand

4232.8442

12541.7607

8387.3025

8308.9165

1.00

Table1.1: The theoretical values of 3-band equalizer design

Figure 1 shows the example of one band equalizer circuit while Figure 1.2 and Figure 1.3 shows the summing circuit of all individual bands for 3-band equalizer and 5-band equalizer respectively.

Figure 1: One band equalizer circuit

Figure1.2: 3-band equalizer's summing circuit

Figure 1.3: 5-band equalizer's summing circuit

Simulation result of 3-band equalizer:

Figure 1.4: 1st band's frequency response (3-band equalizer)

Figure 1.5: 2nd band's frequency response (3-band equalizer)

Figure1.6: 3rd band's frequency response (3-band equalizer)

Figure 1.7: 3 individual bands' frequency response (3-band equalizer)

Figure 1.8: Summing frequency response of 3-band equalizer

Equalizer Band

Theoretical

Simulated

Percentage of difference (%)

Center Frequency (Hz)

Center Frequency (Hz)

1st

344.8357

291.9760

15.33

2nd

3942.0322

3347.8000

15.07

3rd

8387.3025

7103.7000

15.30

Table 1.2: Comparison between theoretical values and simulated values (3-band equalizer)

Simulation result of 5-band equalizer:

Figure 1.9: 1st band's frequency response (5-band equalizer)

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Figure 1.10: 2nd band's frequency response (5-band equalizer)

Figure 1.11: 3rd band's frequency response (5-band equalizer)

Figure 1.12: 4th band's frequency response (5-band equalizer)

Figure 1.13: 5th band's frequency response (5-band equalizer)

Figure 1.14: 5 individual bands' frequency response (5-band equalizer)

Figure 1.15: Summing frequency response of 5-band equalizer

Equalizer Band

Theoretical

Simulated

Percentage of difference (%)

Center Frequency (Hz)

Center Frequency (Hz)

1st

344.8357

291.9760

15.33

2nd

1235.7152

1023.1000

17.20

3rd

4290.3348

3584.7000

16.45

4th

13367.5384

11466.0000

14.23

5th

21247.2181

18507.0000

12.90

Table 1.3: Comparison between theoretical values and simulated values (5-band equalizer)

In comparing the summing frequency response between 3-band equalizer and 5-band equalizer, 5-band equalizer has wider range of bandwidth and higher center frequency and voltage level. The difference between the theoretical result and simulated result is quite high, which is more than 10 %. Hence, the design is required to be constructed and tested in order to get the actual result.

2.2 POWER AMPLIFIER

There are 3 types of power amplifiers considered to be used in the design of audio mixer, which are class A, class B, and class AB. The design of audio mixer is considered as medium power application. Thus, class A was chosen since its output had low distortion.

Class A amplifier contains a transistor which conducts during the entire cycle of the AC input signal. Its output signal varies for a full 360 degree (2π) of the cycle. Its disadvantage is that it produces the output with low efficiency, which can only be up to 25 %. It was generally used as low power amplifier.

Class B amplifier contains two transistors and each of them conducts for 180 degree of the AC input cycle. In the AC input, one transistor is conducting during the positive alternation while the other one is conducting during the negative alternation. The two half-cycles are combined as a full 360 degree output of the operation. It has an output that contains cross-over distortion and its theoretical maximum efficiency is 78.5 %. Generally, it is used as high power amplifier.

Class AB amplifier also consists of two transistors and it is actually a advanced version of basic class B amplifier. The transistor in this amplifier conducts for more than 180 degree but less than 360 degree of the input cycle. Its theoretical efficiency is within a range from 25 % till 78.5 %. It is preferred in the audio system compared with class A and B amplifier.

2.2.1 THE DESIGN OF CLASS A AMPLIFIER

DC analysis:

Vcc = 12V

VCEQ = 0.5 Vcc = 6V

Vcc = IcRc + VCEQ + IERE , Ic ≈ IE

Vcc = Ic (Rc + RE) + VCEQ

Vth = Vcc [R2 / (R1 + R2)]

Rth = R1 || R2

Vth = IBRth + VBE

Given R1 = 50kΩ, R2 = 10kΩ, Rc = 3.6kΩ, RE = 1kΩ, and RL = 8Ω.

2 = (25000 / 3)IB + 0.7

IB = 0.156 mA

Ic = βIB

Let β = 110, Ic = (110) (0.156m) = 17.6 mA

AC analysis:

Assume ro = ∞.

gm = Ic / VT = 17.6m / 26m = 0.6769

rπ = β / gm = 110 / 162.5Ω

Rin = (R1 || R2) || rπ = 159.3919Ω = RC1

RC2 = Rc + RL = 3.6k + 8 = 3.608kΩ

RCE = RE || [(rπ + R1 || R2) / (1+β)] = 71.1Ω

Let fL = 20 kHz,

fCE ≈ fL ≈ 20 kHz, fC1 = fC2 = 0.25fL = 5 kHz

C1= 1 / (2πRC1fC1) = 199.7nF

C2 = 1 / (2πRC2fC2) = 8.82nF

CE = 1 / (2πRCEfCE) = 111.92nF

Voltage gain = - gm / (RC || RL) = 0.085 ≈ 0.09

Figure 2: Class A power amplifier circuit

DC power = (12)(201.32u + 1.276m) = 17.72 mW

AC power = Vcc2 / 8RL = (122)(8 - 8) = 2.25W

Figure 2.1: The input and output waveforms of class A power amplifier

According to the waveforms shown, the voltage gain = 4.6149m / 50m = 0.092 ≈ 0.09. The gain is too small because the output is attenuated for 90.8%. The design circuit was constructed by using the closest practical values of resistance and capacitance in order to see if the result was similar to simulation result. Eventually, the design circuit failed during circuit testing because the output sound was very low just like the simulation result though the volume control was turned to the maximum level. Moreover, the output sound was noisy. As a result, the progress of designing class A power amplifier was terminated and the next target was changed to class AB power amplifier. Class B power amplifier had not been tried due to its output with cross-over distortion.

2.2.2 THE DESIGN OF CLASS AB POWER AMPLIFIER

Class AB power amplifier is much better than class A and class B as it combines the advantages of class A and class B amplifiers. It produces the output with low distortion and high efficiency. However, it is not easy to be designed. Consequently, a few circuit diagrams of class AB amplifier were found through the Internet. These few circuit diagrams were then simulated to see their gain and power of outputs.

Figure 2.2: Circuit diagram of class AB without diode

DC power = (12)(1.681m + 143.89m) = 1.75 W

Figure 2.3: The input and output waveforms of class AB without diode

Voltage gain = (216.274m - 167.131m) / 50m = 0.98286

Figure 2.4: Circuit diagram of class AB with diodes

DC power = (12)(1.672m + 213.74u) = 22.63 mW

Figure 2.5: The input and output waveforms of class AB with diodes

The voltage gain = 3.5908m / 50m = 0.072

Figure 2.6: Circuit diagram of Darlington class AB

Dc power = (12)(1.673m + 197.2u) = 22.44 mW

Figure 2.7: The input and output waveforms of Darlington class AB

Voltage gain = 1.9131m / 50m = 0.038

The AC power is same to all these three circuits, which is computed as follow:

AC power = Vcc2 / 8RL = (122) / (8 - 8) = 2.25 W

Among these three circuits, the class AB amplifier without diode is the best based on the simulation results because it has the highest voltage gain and DC power. In fact, none of them amplifies the input signal. Nevertheless, the circuits were still constructed and tested one by one in order to look for the best one.

Out of expectation, the class AB amplifier with diodes gave the best sound quality among the others even though the sound was not as loud as the outputs of the other two circuits. The sound produced by the class AB amplifier without diode was very loud but still a bit noisy. The Darlington class AB amplifier also produced very loud and a bit noisy sound. Besides that, its bipolar junction transistors (BJTs) consumed heat and became so hot within very short of time due the high current flowed through it. After the circuit testing, the class AB amplifier with diodes was chosen as the power amplifier used in the design.

Again, the simulation results are not accurate and not precise because the various models of BJTs used in the circuit diagrams are available in the simulation software. The simulations were done by placing the other models of BJTs available in the simulation software. Actually, most of the electronics components have their own different characteristics or specification in the datasheets. Therefore, circuit testing is a must to be done in order to get the real result.

2.3 HIGHER POWER SPEAKER