Analog Signal and Audio Amplifier: A Practical Guide with Exercises, Exercises of Electronics

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EIA Stream, Industrial Centre. The Hong Kong Polytechnic University

PCB Assembly

Analog Circuit Testing

Rev 2.

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Product Development Flow

Analog Circuit

Design

Learning Outcomes: 1.

Understand what analog signal is and its characteristics.

Learn about the parameters of an audio amplifier.

Applying the learnt soldering technique to assemble an audioamplifier with digital volume control.

Learn how to measure the analog amplifier parameters.

1. What is an Analog Signal: •^

Unlike digital signal which is discrete with values = either 1 or 0.

-^

An analog signal has continuous voltage values which vary withtime:

A digital signal example^ An analog signal exampleTime

V

1.1 Characteristics of an Analog Signal: •^

Any continuous analog signals can be actually constructed by asummation of series of sine waves with different frequencies &amplitudes. They are called the

Frequency Components

of this

signal

=

• +^ …….

A^1

sin (

πf^1

t)

A^2

sin (

πf^2

t)

An^

sin (

πfn

t)

•^

The spread of these

frequencies (f

n^ – f

1 )^

is called the

Bandwidth

Signal (t) = A^ of the signal

1 sin (

πf^1

t) + A

2 sin (

πf^2

t) + ……… + A

n^ sin (

πfn

t) , where f

1 < f

2 < …. < f

n

1.2 Audio Signals: •^

Audio signal is an analog signal with frequency componentsranging from 20Hz to 20 000Hz, which is hearable by humanbeings.

-^

Therefore, the bandwidth of an audio signal is around 20kHz.

-^

Below is an example of a girl’s voice and frequency spectrum.

2. Audio Amplifier:2.1 What is an amplifier? •^

An amplifier will deliver a

gain

from input signal to output

signal.

Input voltage

Vin

Output voltage

Vout

Power SupplyAmplifier

•^

Normally,

gain

of an amplifier refers to the

Voltage Gain

Av

Av

=

 

•^

Important notes:^ •^

The amplifier must get external power supply to give energy to increasethe voltage at the output.

-^

The maximum output will be limited by the power supply voltage.

2.2 What is an audio amplifier? An audio amplifier is a kind of amplifier which: •^

Amplifies the AC signals with frequency range from 20Hz to20kHz.

-^

Does not amplify any DC voltages.

Input Signal

Vin

Output Signal

Vout

Power Supply Audio Amplifierfor frequencies20Hz-20kHz

DC blockingcapacitor

AC signal

2.3 Basic Parameters of an Audio Amplifier: •^

Frequency Response & Bandwidth: Amplifiers do not have the same gain at all frequencies. When we plot the gainagainst the signal frequencies, we will get the following chart – “

Frequency

Response”

:

We define another parameter

Bandwidth

which describes the frequency band

that the amplifier can give a “useful” gain (the gain is not less than 3dBcompared to the maximum gain).In the above frequency response, the bandwidth is from 10Hz to 20kHz.

F^1

F^2

2.3 Basic Parameters of an Audio Amplifier: •^

Some notes on “

3dB Point

-^

In terms of

voltage gain

,^ -3dB =

= 0.707 of the maximum gain

.

-^

When we calculate the power output:^ 



= (



(^2) ) /R^

=^

( (^)
)= ^

^ Max Power Output

-^

Hence this point is also known as

Half-power point.

3dB point, orHalf-power Point

2.3 Basic Parameters of an Audio Amplifier: Summary: •^

Gain Av

=

 

-^

Decibel

dB

is a better unit to describe gain of an audio amplifier due to the

actual human perception of sound intensity.

-^

Frequency Response & Bandwidth

describes the behaviour of the gain with

respect to the signal frequencies.

-^

Maximum Output Swing without clipping

indicates the maximum output

signal level with good quality.

3. Audio Amplifier Example:

-^

You will now build an audio amplifier circuit with

digital volume control

through a DIP switch

.

-^

The circuit consists of two major blocks:^ •^

Attenuator with a

3-bit digital

control

to provide 5-step attenuation

-^

Audio amplifier which provides a high gain and drives a 8-Ohm speaker

LM386Audio AMP

VolumeControlAnalogMultiplexer74LV

+5V^

+5V

8 OhmLoudspeaker

DIPSwitch

Signal Input

DigitalControl

3.2 Stage 2 - Audio Amplifier Chip:

-^

The 2

nd^ stage will amplify the signal from the attenuation block and drive the

loudspeaker.

Input fromst 1 stage output

3.3 Measurement of Amplifier Parameters:

Following the steps in the worksheet, measure the

parameters

of

the audio amplifier with digital volume control: •^

Output levels and gain

at different volume steps

•^

Frequency response plotting

at maximum volume

•^

Bandwidth

-^

Maximum output swing

without clipping at maximum volume