Simple and easy to understand.

1. Public Address System (Amplifier):
Material Required:
1. Transistors2N3904 , 2N3906 & BD135
2. Mike or Cell Phone
3. Capacitors10 & 100 micro Farad
4. Diodes1N4001
5. Two Batteries
6. Resistors 1k, 6.8k, 22k, 33k, 47k, 130, 220& 150 ohm
7. Variable Resistor 5k
8. Vero Board & Jumper wires
9. Speaker 8 ohm & 5 Watt
10.Ox Cable
Theory:
A single stage of Amplification is often not enough for a
particular application. The overallgain can be increased by using more than
one stage, so when two amplifiersare connected in such a way that the
outputof first drives inputbase signal of the second. The Amplifiersare said
to be cascaded.
The most common cascaded arrangementis the ground-emitter
to ground-emitter configuration. They are used in sound-producing systems
as Audio Amplifiers in T.V receivers as video (picture) amplifiersand in many
other applications.

2. Interesting Application:
BJT Two Stage Pre-Amplifier Circuit:
The Two stage amplifier circuitcontains a voltage Divider circuit
wit BJT. An inputsignal is applied by using mike or through CellPhone.
By using formula, we can calculate required values

3. VCE = VCC – ICRC
VB = R2/R1 + R2 * VCC
VE = VB - VE
Voltage gain = Vout / Vin
DarlingtonPair:
One way to Boost up input resistance is to use a
Darlington Pair. The Collectorsof two transistorsare connected and the emitter
of first drives the base of second.
An Emitter Follower using a Darlington pair can be used to interfere the
amplifier and the circuit.
This Darlington pair is very helpful for obtaining maximum gain
output. Beta is a major factor for determining the inputresistance.
This configuration achievesbeta multiplication.

4. Transistors:
The 2N3904 is a bipolar junction used for generalpurpose low power
amplifying or switching applications. It is used in the variety of analog and
switching applications. It has a minimum beta and currentgain of 100 ata
collector currentof 10 milli ampere.
Class AB Power Amplifier Circuit:
The primary advantage of ClassAB power
amplifier is the maximum outputpower for the given amountof input power.
A particular Diode arrangementisused to get the outputwithoutdistortion.
The signal voltage at the base is out of phase with the signal of feedback voltage
and has the effectof stabilizing the gain.

5. Complete Audio Amplifier:
At the end, we combine the pre-amplifier circuitwith
the push-pullpower amplifier. The circuitis a Class AB amplifier with Darlington
configurationsand a diode currentmirror.
Both a TraditionalDarlington pair and a
Complementary Darlington pair are used to provide sufficient currentto an 8 ohm
speaker load.
The signalfor pre-amp is capacitive coupled to the
drive stage which is used to preventexcessive loading on the pre-amp and provide
additionalgain.

6. Simulation Diagram:
Conclusion:
In the end, we conclude thata public addresssystem can be
used for any sound generating system. Itis widely used in many applicationssuch
as T.V receiversand video amplifiers.