The
microphone is a device which is used to capture sound signals and forms
an essential part of most of the electronic gadgets. The microphone
converts the sound signals in the environment to their corresponding
electrical signals. These electrical signals are actually very small in
amplitude and they need to be amplified several hundred times before it
could be reproduced through a loudspeaker. Hence in between a normal
microphone and a normal loudspeaker there comes a multi-staged amplifier
section which amplifies both the voltage and power of the weak audio
signals generated at a microphone.
This article discusses about a simple circuit that can reproduce the sound signals captured through a microphone on a loudspeaker. This circuit is made with the help of two stage transistor amplifier and a op-amp
based loudspeaker driver amplifier. To demonstrate its working, music
is played in a mobile phone which is kept near the microphone and the
same music with more loudness is generated at the loudspeaker connected
to the circuit.
The
microphone is connected to a circuit which helps to couple out the
audio signals generated at the microphone so that it can be used for
amplification. The microphone coupling circuit is followed by two stages
of transistor based amplifier and in between the amplifier there is a
volume controller included. The amplified signals may not have enough
power to operate the loudspeaker and hence the voltage amplified signals
need to be current amplified also. It is done with the help of an
op-amp based amplifier circuit with only increases the current sourcing
capability of the audio signals.
The
microphone coupler is a circuit which helps to couple out the weak
audio signals generated at the microphone. There are different kinds of
microphones which have different working principle, but all of them have
a diaphragm which vibrates according to the sound signals. As the
diaphragm vibrates the current flowing through the microphone varies
according to the sound signals amplitude which made the diaphragm to
vibrate. Here in this circuit a condenser microphone is used which and
the varying current is made to flow through a resistor across which the
equivalent voltage get generated due to the current flow. This voltage
across the resistor will be having a DC voltage on to which the varying
voltage gets added up. This varying voltage is separated out from the DC
voltage with the help of a coupling capacitor and fed to the following
amplifier circuits.
With a condenser microphone a 10K resistor and a 0.1uF coupling capacitor is used in most of the circuits.
Here
a single transistor based amplifier circuit is used as the first stage
amplifier for the audio signals coupled out from the microphone. This
circuit is designed to have extremely high gain so that the audio
signals are get amplified enough. The transistor is connected in a
common emitter configuration and fixed bias technique is used for
biasing the transistor.
As
the value of the Rc increases the gain of the circuit increases and it
should be taken care of that when there is no input signals present the
amplifier must be in its quiescent state, means in case of a transistor
based circuit the output voltage without any input signal should be
exactly the half of the total supply voltage.
Here
a 2.2K ohm resistor is selected, which will allow to flow more than one
mille ampere current through the transistor and the resistor itself in
series with it, creating around 2.8 volts across Vce.
Vce = 5 – (2200 * 1mA) = 2.8 V; (almost quiescent voltage)
Since
the expected output current Ic is fixed at 1mA, the input current at
quiescent state that will produce that output current can be calculated
with the help of the relation of the hfe of a transistor with the input
and the output currents. The hfe is generally called the current gain
and is given by the equation
hfe = Ic / Ib; where Ic is the output collector current and the Ib is the input base current
The
hfe of the transistor BC548 has a maximum value of 300, and applying
the values of Ic and hfe on to the above equation the Ib can be
calculated around 4uA.
The
voltage Vb across base resistor Rb will be the supply voltage minus 0.7
volts for a silicon transistor at quiescent state. Here since the
supply voltage is 5V, the Vb can be calculated as 4.3 V. Now since the
voltage Vb across the resistor and the current Ib flowing through the
resistor is known, the required value of the resistor can be calculated
using the ohms law;
Rb = 4.3 V / 4.3 uA = 1M
A 0.1uF capacitor is commonly used to couple the audio signals in between the amplifier stages.
The
volume controller used here in between the transistor stages is a
simple potentiometer which will attenuates the output of the first stage
amplifier before it is fed to the second stage amplifier. This volume
controller circuit helps to keep the amplitude of the signal within the
input range of the second stage amplifier circuit.
A
1K variable resistor is used as the volume controller in this circuit
in which the output from the first stage amplifier is connected to one
of the terminals and the input to the second stage is coupled out from
the variable pin through a capacitor and the third pin is grounded.
The
second stage amplifier is exactly similar in design with the first
stage amplifier. This amplifier simply amplifies the signal more and at
the output of this stage one can obtain a good enough voltage amplified
signal which is ready to be current amplified by the following current
amplifier circuit.
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