This document reports on the lab exercise which was performed on an audio amplifier with the main aim of obtaining its frequency response characteristics and maximum gain. The frequency response characteristics of the amplifier include its bandwidth, signal to noise ratio, voltage gain and power gain. Amplifiers are employed in acquiring electric devices which are capable of accessing signals power based on either voltage or current characteristics. Amplification enables either the amplitude or any other parameter of the input signal to be increased by adding energy that is converted from some power supply in what is referred to as gain. Besides, despite the fact that amplifier configurations seem to have a complicated circuitry, their basic principle of operation is a simple process. This involves the concept of voltage and current gains. With a power amplifier, the input signal is generated from a source for example mobile phones audio amplification system. The amplifier output is in the form of audio speaker that is received as the audible sound.
Background Theory
Audio amplifier is defined as an electronic device that increases the gain of input audio signals to stronger levels that can drive devices such as loudspeakers (Self, 2009). These kinds of amplifiers are usually fabricated in an integrated circuit. Transistors form the major component of these ICs. It is the connection of these transistor components that permits amplification of the low level amplitude signals at the input to high level amplitude output signals capable of operating a loudspeaker (Marston, 2011). Importantly, audio amplifiers have been proven both on digital technology as well as analog devices. For instance, analog devices provide numerous audio amplifiers such as line drivers, microphone drivers, receivers and microphone preamps. Also, other products of ADI are namely Class-B, Class-A as well as Class-D offerings.
A good audio amplifier is one that is characterized by a flat frequency specification to within 1 dB within a frequency range of 20Hz to 20 kHz at a specified power. In these conditions, human hear will barely detect 1dB change in the sound level (Self, 2009). A good audio amplifier should not introduce any form of distortion in its own sound system. 1dB change in the in the amplifier output voltage however corresponds to about 12% change in the voltage that is measured by an oscilloscope (Bhargava, Gupta, " Kulshreshtha, 1984).
The measure of the ability of these amplifiers to increase the amplitude levels of a signal is referred to as gain. The gain refers to ratio of signal output to signal input of a similar system. In most cases, it is expressed in logarithmic decibels (dB) units. Being a ratio of two parameters, it is a dimensionless quantity. When this ratio is for output voltage to input voltage, it is referred to as voltage gain. In a similar fashion, when the ratio is for output current to input current, it becomes current gain
and the ratio of output power to input power is power gain (Cripps, 2002).
Mathematically, these gains can be expressed as:
In decibels,
Voltage gain in dB = 20 log ………………….2
In decibels,
Power gain in dB = 10 log
Fig 1: Circuit Diagram of Audio Amplifier
In order to ensure that the amount of gain is kept constant in an amplifier, negative feedback is employed. This ensures that the output is an exact replica of the input signal with the only variation being in the output being the magnitude.
For most amplifier circuits, each channel of the IC has a potential divider circuits for example in R2+R$ and R2+R6 shown in the diagram above. These dividers minimize the input to signal to a percentage of the original signal. Audio amplifiers also have variable potentiometer which can be used in varying the obtained percentage which helps in varying the output volume. A capacitor C3 is connected across the supply to ensure that the system remained stable. Other capacitor performs filtering roles to minimize the frequency noise.
Amplifiers are characterized by a bandwidth that is relevant to a range of frequencies it is supposed to amplify. Narrow bandwidth is characterized by frequency loss while extremely wide bandwidth result into introduction of unwanted signals. For audio amplifier, these include mechanical noise frequency hum and audible hiss.
Audio amplifiers may be classified depending on various factors like nominal gain or output power or type of packaging etc. (Self, 2012). These amplifiers find application in sound systems such as in home audio systems, PC sound cards, guitars etc.
Objectives
i) To obtain and verify the Frequency Response Characteristics curve of an audio amplifier.
ii) To measure the maximum gain of the audio amplifier under investigation.
Equipment required
Table 1
S/N
Name
Quantity
1
Signal generator
1
2
3.5 mm Stereo metal coupler
1
3
Oscilloscope
1
4
Built stereo audio amplifier
1
5
Extension cable
1
6
Crocodile Clips
-
7
Connecting Wires
-
Procedure
Connections were made as per the figure below and series of measurements made and recorded as in results.
Figure 3 illustrate the experimental setup diagram, as it is shown that the stereo metal coupler plus the extension cable was connected to the input of the audio amplifier. Next, was to take the red connecter along with the black connector of the other side of the extension cable and banded together with the signal generator. Then, attach the oscilloscope to the signal generator (channel 1) and to one of the speakers (channel 2).
Fig 3: Experimental Circuit Diagram
Results and Discussion
PART A: Maximum Amplification
Results
Input amplitude = 50 mV
Output amplitude = 800 mV
Analysis and Discussion of the Results
Maximum voltage gain = 20 log
Comment: Beyond this peak value, the output of the audio amplifier saturates.
PART B: Frequency Response and Bandwidth
Results
Table 2
Frequency (Hz)
Output Voltage
Voltage-Gain (dB)
20
168.5 mV
3.7
50
170.5 mV
3.8
80
174.0 mV
4.0
100
187.0 mV
4.6
200
261.0 mV
7.5
800
348.0mV
10
1 k
404.0m V
11.3
2 k
1.96 V
25
4 k
2.04V
26.5
6 k
1.96 V
25
8 k
694m V
16
10 k
464mV
12.5
12 k
404mV
11.3
14 k
356mV
10.2
18 k
270mV
7.8
20 k
261mV
7.5
50 k
227mV
6.3
100 k
187 mV
4.6
200 k
170.5 mV
3.8
500 k
166.5 mV
3.6
Analysis and Discussion of the Results
I. Frequency Response Curve
From the response curve shown below, it can be seen that upper cut-off frequency is 7500Hz while the lower cut-off frequency is 1800Hz. The maximum gain of 26.5 dB is realised when the frequency of the audio amplifier is about 5000Hz.
II. Computation of bandwidth from the curve
Bandwidth = Upper cut-off frequency – Lower cut-off frequency
=7500Hz – 1800 Hz (as graphically illustrated)
= 5.7 kHz
III. Demonstration of how -3 dB corresponds to a voltage reduction of
Assuming an input voltage of V, the output voltage becomes
Thus, the voltage gain =
Similar case applies to the Power gain
PART C: Signal to Noise Ratio
Results
Measured Noise voltage, Vnoise = 70mV
Output voltage when the volume level is half = 1.40 V
Output Voltage when the volume level is less than half = 420 mV
Analysis and Discussion of Results
SNR when:
I. Output voltage when the volume level is half
SNR(signal to noise ratio) =
II. Output Voltage when the volume level is less than half
SNR =
Comment: The signal to noise ratio is directly proportional to the volume level.
PART D: Limits of Circuit Design
.
Results
Fig 4.1: 150 Hz input/output waveform with 10 (µS)
Fig 4.2: 1500 Hz input/output waveform with 100 (µS)
Fig 4.3: 6 kHz input/output waveform with 50 (µS)
Fig 4.4: 12 kHz input/output waveform with 20 (µS)
Fig 4.5: 40 kHz input/output waveform with 10 (µS)
Figure 4.6: 120 kHz input/output waveform with 2 (µS)
Comment: As therein the results, an increase in frequency of the signal yields a more distorted output signal. This is as a result of high frequency effects on the transistors leading to increased junction capacitances (Miller effect)
Conclusion
In a nutshell, the circuit configuration was set up, energized and a series of measurements made as in results. The maximum amplification and the frequency response of the audio amplifier were computed and a frequency response curve drawn on a semi- log paper. The yielded results conformed to the expected theoretical profile. In this respect, the experimental objectives were achieved.
References
Bhargava, N. N., Gupta, S. C., " Kulshreshtha. (1984). Basic electronics and linear circuits. New Delhi: Tata McGraw-Hill.
Cripps, S. C. (2002). Advanced techniques in RF power amplifier design. Boston,
Retrieved from http://public.eblib.com/choice/publicfullrecord.aspx?p=227601
Marston, R. (2011). Audio pre-amplifier circuits. Audio IC Circuits Manual, 46-64.
Self, D. (2009). The Voltage-Amplifier Stage. Audio Power Amplifier Design Handbook, 117-137.
Self, D. (2012). Audio Power Amplifier Design Handbook. Retrieved from https://books.google.co.ke/books?id=TLt0DO6LAokC"printsec=frontcover"dq=amplifier"hl=en"sa=X"ved=0ahUKEwizpPqz2KvfAhUSgHMKHbFiBCIQ6AEIKDAA#v=onepage"q=amplifier"f=false
