本文讨论了简单的电路,能够重现的声音信号s captured through a microphone on a loudspeaker。该电路是在两个阶段晶体管放大器和A的帮助下制成的运算放大器基于扬声器驱动器放大器。为了展示其工作，音乐是在手机中播放的，该手机在麦克风附近保持，并且在连接到电路的扬声器上会产生相同的音乐。
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.
Fig. 2: Circuit Diagram of Microphone Couple
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.
Fig. 3: Circuit Diagram of First Stage Amplifier
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;其中IC是输出收集器电流，而Ib是输入碱电流
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.
电压电阻Rb将Vb在基地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.
SECOND STAGE AMPLIFIER
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.
图4： Circuit Diagram of Second Stage Amplifier
The loudspeaker used here is a very common 8 ohm loudspeaker which can operate with a maximum power of 0.5W.