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Running head: AUDIO AMPLIFIER 1

Audio amplifier

Student’s Name

Institution’s Name

AUDIO AMPLIFIER 2

Audio Amplifier

Objective

The aim of this lab was to measure the highest amplification of an audio amplifier and

find the frequency response of the amplifier.

Introduction

Usually, an audio amplifier Integrated Circuit (IC) is located at the center of a circuit.

Ordinarily, an IC contains many transistors connected to amplify the small input signal to a

stronger output signal capable of driving a speaker. This ability of an amplifier to increase the

amplitude of an input signal and obtain an output is known as a gain. Specifically, the increase in

the input to output is a result of energy addition to an input signal. Accordingly, gain is defined

as the mean ratio of a signal output to a signal input, and it is expressed in logarithmic decibel

(dB) units.

Voltage gain (dB) = 20log (









) Equation 1

It is crucial for an output to be an exact copy of the input but have a larger magnitude.

For this to be accomplished, feedback is used to keep the gain constant. In this experiment, the

gain was made useful by the use of a potential divider on each of the two channels fed into the IC

(R2+R5 and R2+R6) as in figure 1. The dividers work by reducing the input signal to a portion

of the initial signal. Since the potentiometer (R2) is variable, the part of the input signal obtained

can be varied, which, in turn, changes the output volume. Capacitor C3 is connected to the

supply to ensure stability, while other capacitors play a role in filtering to cut out high-frequency

noise.

AUDIO AMPLIFIER 3

Figure 1: Circuit diagram of the audio amplifier

Additionally, an amplifier has a bandwidth within the range of frequencies that it is

meant to amplify. Otherwise, a narrow bandwidth results in the loss of some signal frequencies.

Conversely, a wide bandwidth introduces unwanted signals. For an audio amplifier, this would

lead to a humming or mechanical noise at low frequencies and an audible hiss at high

frequencies. In this manner, an amplifier acts as a filter with the bandwidth being given by a -3

dB line as in figure 2 below.

AUDIO AMPLIFIER 4

Figure 2: A graph showing the magnitude of a bandpass filter’s gain using the −3 dB bandwidth

concept.

If the chart in figure 2 is plotted with voltage gain, this corresponds to a reduction of

(1/2) or half the power in voltage. Ultimately, these concepts are utilized in the measurement

of amplification. In the same manner, by plotting a graph of voltage gain as a function of

frequency input, the frequency response of an audio amplifier is determined.

Equipment

The equipment required to perform this experiment included a built stereo audio

amplifier, one signal generator, one oscilloscope, one 3.5mm stereo metal coupler, one 3.5 mm

extension cable, and crocodile clips as well as the necessary cables.

AUDIO AMPLIFIER 5

Procedures

Firstly, the stereo coupler and the extension cable were connected to the audio amplifier

input. Then, the signal generator was connected to the black and red connector on the jack

extension cable. After that, the oscilloscope was connected to the signal generator and the

speakers denoted by channel one and channel two respectively. The experimental setup was as in

figure 3 below.

Figure 3: Experimental setup

Maximum Amplification

The volume of the speaker was set to maximum by making adjustments on the

potentiometer. After that, the input signal was set to a sine wave of about 50 mV in peak to peak

amplitude (V

) and 17 kHz in frequency. Once this was done, observations of the input and

output signal were made from the oscilloscope. The amplitude of the input was then increased up

to the point where distortion (saturation point) was observed in the output signal. The amplitude

AUDIO AMPLIFIER 6

was then set at a value just below the saturation point. Lastly, the amplitude of the output signal

was measured, and the maximum voltage gain of the amplifier was calculated.

Frequency Response and Bandwidth

The amplifier volume was adjusted to half using the potentiometer. Subsequently, the input

signal was set to a sine wave of approximately 100 mV (V

). After that, the output amplitude

was recorded while changing the frequency from 20 Hz to 500 kHz. From the results obtained, a

graph of output voltage gain as a function of input frequency was plotted on a semi-log graph

paper. Furthermore, the bandwidth of the audio amplifier was calculated.

Signal to Noise Ratio (SNR)

The volume level was set to about half the maximum volume and the noise level measured

without connecting the input. Subsequently, the input was set to a sine wave of 100 mV (V

)

amplitude and the output signal was measured. Afterward, the SNR was calculated in dB. The

procedure was then repeated with a lower volume that was just audible, and the two resultant

SNR values were compared.

Limits of Circuit Design

The set up was changed to a square wave of amplitude 100 mV (V

) and 150 Hz. Next, the input

and output waveforms were recorded. Finally, the output waveforms were noted down as

frequency was increased from to 1500 Hz, 6 kHz, 12 kHz, 40 kHz, and 120 kHz.

AUDIO AMPLIFIER 7

Results and Discussion

Maximum Amplification

5) For a gain of 20dB in the audio amplifier,

Then, as per equation 1, the voltage gain (dB) = 20= 20log (









)

1= log (









)

10 log (









) = 10

(









) = 10

Therefore, a gain of 20dB in the amplifier means that the ratio of V

out

to V

would be equal to

10.

Input (Ch. 1) = 500 mV/ division

1.5 V/max

2.5 V= V

Output (Ch. 2) =10V/ division

30 V/ Vmax

50.8 V= V

9) Using equation 1, voltage gain (dB) = 20log (









)

AUDIO AMPLIFIER 8

= 20log (





)

= 26.02 dB

Frequency Response and Bandwidth

11) Resulting output amplitude with changing frequency is as follows

= 10V= input

Table 1

Resulting output amplitude with changing input frequency

Input Frequency (Hz)

Output amplitude (V

pp)

Output voltage gain

1.72

-15. 29

2.60

-11.70

2.96

-10.58

100

3.00

-10.46

200

3.24

-9.79

800

3.40

-9.37

1000

3.40

-9.37

2000

3.40

-9.37

4000

3.60

-8.87

6000

3.64

-8.78

8000

3.72

-8.59

10000

3.84

-8.31

12000

3.84

-8.31

14000

3.84

-8.31

18000

3.80

-8.40

20000

3.80

-8.40

50000

3.50

-9.12

100000

2.80

-11.06

200000

1.72

-15.29

500000

0.76

-22.38

AUDIO AMPLIFIER 9

12) The voltage gain is calculated using Equation 1 to plot a graph of output voltage gain versus

input frequency.

For the 20 Hz frequency, voltage gain = 20log (





) = -15. 29

In the same manner, the other voltage gain values are obtained as in Table 1.

A graph of output voltage gain vs. input frequency was plotted as follows

Figure 4: Output voltage vs. input frequency

13)

As the input frequency increases from 20 Hz to 500 kHz, the output voltage gain also

increases from -15.29 up to -8.31 at the frequency of 14 kHz. From there, as the frequency keeps

-25

-20

-15

-10

-5

1 100 10000 1000000

output voltage gain

Input frequency

output voltage gain vs. input frequency

output voltage gain vs.

input frequency

AUDIO AMPLIFIER 10

increasing to 500 kHz, the voltage gain drops from -8.31 to -22.38 at 500 kHz. The behavior of

voltage gain is directly proportional to the corresponding output amplitude.

Bandwidth = 500000- 20= 499980 Hz

14)

-3dB = 20 log(1/2)

I= V/R=









=   



and I









=   



Power, P

= IV=   



 (  



) =   



And P

out

= I

V=   



 (  



) =   



Therefore power gain (dB) = 10log (









) = 10log (









)= 18 dB

Signal to Noise Ratio

15)

Noise level without connecting the input was measured as

= V

rms

2.19mV and V

= 10.8mV

16) At 100 mV input the output = V

480 mV

And V

rms

= 139 mV

As such SNR (dB) = 20log (139/2.19)= 36.05

17)

AUDIO AMPLIFIER 11

Noise level, V

rms

= 1.66 mV at V

= 7.6 mV

And V

rms

= 8.15 mV at V

= 23.4mV

Therefore, SNR (dB) = 20log (15/1.66)= 13.82

The SNR value is higher for the higher volume.

Limits of Circuit Design

18)

Input was set to 150Hz and 200 mV

And output Vmax= 56mV, V

min

= 66 mV, V

= 122 mV

19)

The output waveforms with increasing frequency are as follows

Table 2

The output waveforms for a square wave

Frequency (Hz)

max

(mV)

min

(mV)

1500

150

6000

148

12000

150

40000

146

120000

136

AUDIO AMPLIFIER 12

Conclusion

In this experiment, the maximum amplification of an audio amplifier was successfully

measured by the use of voltage gain. Maximum amplification was observed at an input

frequency of 10 kHz to 14 kHz where the voltage gain was highest at -8.31 dB. As evidenced by

the graph, the voltage gain remains constant at some points as the frequency increases.

Accordingly, the output at these points is exactly as the input, but the former is larger.

Additionally, the frequency response of the amplifier is determined by the behavior of voltage

gain with the increasing frequency. Overall, the voltage gain increases with increasing input

frequency up to some point when it drops. For a square wave, the resulting waveforms have

decreasing voltages as the frequency increases.

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