Designing an Audio Amplifier: Steps and Analysis

DESIGNING AN AUDIO AMPLIFIER.
By Name
Course
Instructor
Institution
Location
Date

INTRODUCTION.
This is brief report discussing the steps involved in designing an audio amplifier employing a
bipolar transistor long tail pair input stage and a 1W BJT class –AB complementary output stage
to drive an 8-ohm speaker using discrete transistors. The circuit was designed to step by step in
stages on the following specified requirements.
 A minimum voltage gain of 1500.
 Ensure a maximum output compliance using +16V supply rail.
 The audio should employ a Long Tail pair bipolar transistor at the input stage.
 The audio amplifier should also employ a class AB complementary feedback-pair output
transistors.
With the above-given specifications on the design of the audio amplifier, a perfect audio
amplifier can be designed and simulated. The designed amplifier can hence be employed in
several sectors like in home theater system, public address, and mobile phones.The design
was made theoretically then tested using Multisim. Some circuit diagrams were done by hand
then snipped. The report also contains datasheets of the selected transistors, measurements
and design schematics. An audio power amplifier design has three major stage and these
stages include the following;
i. Input stage
ii. Voltage amplification stage
iii. Output Stage

For several voltage amplification purposes, a single transistor doesn’t give enough voltage gain
for a perfect functioning of the audio amplifier. Hence several circuits ( stages of the
amplification) are required. If there are several stages in amplifier the total voltage gain is the
product of individual stage gain as shown below;
Overall voltage gain AV = AV1×Av2×Av3. Basically, this amplifier is employed to raise the
strength (amplitude) of the audio signal which passes through the simplifier. In many cases, the
input signal of this audio amplifier may only measure a few hundred microwatts. These
amplifiers are usually used in the home audio system, theatrical sound reinforcement, and
concert and also in the public address system. Basically, they are used where the input signal is
relatively low and a higher voltage signal is expected at the output, this is what is referred to as
amplification. There are three major classes of amplifiers, namely class A, class B, and class
AB, but there are others like class D, class G, class DG, in this design we employed the use of
class AB amplifier.
DEVICE DATASHEET
BC817
Symbols Parameter values unit
VcB0 Collector- Base voltage 50 V
VcE0 Collector- emitter -voltage 45 V
VEB0 Emitter- Base voltage 5 V
Ic Collector Current 800 mA
BC807
Symbols Parameter values unit

VcB0 Collector- Base voltage -50 V
VcE0 Collector- emitter -voltage -45 V
VEB0 Emitter- Base voltage -5 V
Ic Collector Current -800 mA
Reasons for class AB complementary transistors.
 They are easier to stabilize because they have only one controlling Vbe.
 They have a relatively high efficiency of 78%.
 They are not affected by crossover distortion.
 They are suitable for audio frequency driver and low power output stages.
 They are complementary to each other. For example BC817 and BC807.
BC548C (NPN)
Symbols Parameter values unit
VcB0 Collector- Base voltage 30 V
VcE0 Collector- emitter -voltage 30 V
VEB0 Emitter- Base voltage 5 V
Ic Collector Current 100 mA
Wtotal Total power dissipation 0.5 W
Cc Collector capacitance 4.5 pF
Ft Transmission Frequency 200 MHz