REPORT

AUDIO AMPLIFIER

Project Report

Fri Jan 16, 2015

Mudasser Raheem 2013-EE-36 SECTION “A”

Electrical Engineering Department

University of Engineering & Technology

Lahore-Pakistan

Description

SPECIFICATIONS:

Voltage gain / mid band gain (76db)

Input from USB player

Input impedance greater than 10k ohm

Lower cut off (2.572Hz)

Upper cut off(852.56kHz)

Volume control

Nice loud output from speaker

Circuit excited using our own dual-power supply

An audio power amplifier is an electronic amplifier that amplifies low-power audio signals(signals

composed primarily of frequencies between 20 - 20 000 Hz, the human range of hearing) to a level

suitable for driving loudspeakers. Our audio amplifier is capable of taking input from USB

player as well as from aux wire this increases its importance from other amplifiers. But

amplifier needs DC +15V and -15 V to power up circuit. The sole amplifying device used in

this circuit is BJT. The circuit comprises of three gain stages made using BJT’s 2N3904 ,

(they are designed such that gives maximum output swing), and one power stage

Complementary-symmetry push–pull circuit using Darlington transistors (class B), whose cross

over distortion is removed by technique of changing resistors presents above and below 4

diodes connected in series. Diodes helps in maintaining the Vbe voltage of Darlington pair

in the amplifier. Power transistors TIP122 (NPN) and TIP127 (PNP) were used in power

stage. This part of circuit provides maximum power to drive the speaker. For the protection

of these transistors, good quality heat sinks were used to avoid excessive heating. 1ohm 5

watt resistor was connected between power transistors to limit the current flowing through

it. The output volume (gain control of an amplifier) is varied by means of a variable

resistor. The PCB circuit is based on following layouts. It holds the main circuit and

incorporating the circuit for constant +5 Volts needed to excite USB player.

PROBLEMS:

A few problems while designing were faced. It was difficult to achieve 15V between

collector and emitter of transistors, which was necessary for max swing. To avoid crossover

distortion was one of the major problem solved using variable resistors and after achieving

no distortion variable resistors were replaced by fixed value resistors. Otherwise everything

was all right.

CIRCUIT DIAGRAM

PCB-LAYOUT

PCB LAYOUT was designed on PROTEUS software. Tracks thickness was set to 70TH

to get good bold impression on PCB.

BOTTOM COPPER + TOP SILK SCHEMATIC OF AUDIO AMPLIFIER PCB:

SIMULATION results

To achieve good and satisfactory results we simulated our circuit on PROTEUS as

well as on MULTISIM.

DC_Q-POINTS:

DC_Q-points (Vce) were set in such manners that they give maximum output swing.

Vce was probably set very close to 15V. As our simulation results satisfied the desired

voltage. Output DC_voltage is also less than 50mV with respect to ground as shown

in the figure below.

GAIN:

Three gain stages along with power amplifier stage was designed as per required

condition that is more than 1000. Simulated results also satisfied our gain as shown

in the figure below.

When input signal with amplitude 1mV was set it gives gain more than 1000 most

probably near (6500).

FREQUENCY RESPONSE:

Multisim was used to draw BODE_PLOT which shows midband gain in db. Which is

76db.

Lower cut-off frequency at -3db also satisfied the results i.e. Lower cutoff frequency ≤ 10 Hz with RL = 100 ohms.

Upper cut-off frequency at -3db also satisfied the results i.e. upper cutoff frequency ≥ 50 KHz with RL = 100 ohms.

INPUT RESISTANCE:

But in simulations beta value calculated was very close to 190.

Which also satisfies resistance greater than 10k ohm.