How does an Amplifier Work? (Class-A)

An amplifier is an electronic device used for  increasing the amplitude of electrical signals   they are available in different classes a, b, ab, c, d, and so on In this video we will explore the class a amplifier which uses only one transistor You would know that the transistor can amplify the base current by its gain to the collector current Hence, we will use the transistor to amplify the voltage applied at the base to a higher voltage at the collector. In this circuit the output will always be Vcc. As it is directly connected to the source. Thus we add a resistor between them. The voltage drop across a resistor  is directly proportional to the current flowing through it, and the current through the resistor  is equivalent to the collector's current and the collector current depends on the base current  which depends on the voltage of the input signal thus any change in input voltage will change the output voltage and the low voltage at the input is amplified at the output. When the input voltage is high the current is high hence the voltage drop is high and the output is low, similarly when the input turns low the output goes high The output has a phase shift of 180 degrees. But the waveform looks distorted. Because the gain of the transistor is very high and it is getting saturated. To limit the gain we need to provide the feedback. Hence we add a resistor between the emitter and the ground As you know the voltage drop across a resistor  is directly proportional to the current flowing through it. Thus when the base current increases the collector current also increases The increasing current increases the voltage drop across the resistors. Here as the supply is fixed the voltage at the other end falls this drops the output voltage and here as the ground is fixed the voltage at the other end rises and this increases the output voltage This provides negative feedback for the output limiting the gain of the amplifier. The gain of the amplifier is now more dependent on these two resistors rather than the gain of the transistor. Allowing us to have better control over the total gain of the amplifier. The gain of this amplifier is given by the ratio Rc to Re. At last we add a capacitor to remove  the DC bias and get the amplified input signal You would know that a transistor requires 0.7 volts or above at the base to turn on Thus this circuit cannot amplify voltages below 0.7 volts To make this actually work we need to add a dc bias to the input signal. Such that the input signal is always above 0.7 volts. And to do that we add a voltage divider here. This will provide the required bias to the voltage of the signal Now a complete waveform of the signal can be  obtained at the output, but now the transistor is always on also this dc bias voltage can interfere  with the source of the input signal Hence we add a capacitor here. The capacitor will block dc from any of the directions and allow only the ac signal to pass. The input signal is first biased  such that it's above 0.7 volts, then the signal is amplified by the transistor with the gain provided by the relation between rc and re and at last the dc bias is removed to obtain the amplified signal This is how a class a amplifier works. This is the simplest type of class a amplifier circuit. We can add more components to it for improvements. In this circuit the biasing resistor is connected after  the Rc. When the input voltage increases the base current increases which increases the collector current and the voltage at the output falls This also drops the bias voltage thus the base  current drops collector current drops and the voltage at output rises. This resistor now provides the negative feedback and limits the gain of the amplifier. This circuit has a capacitor across this resistor This increases the ac gain of the amplifier. The capacitor will block the DC thus the amplification of dc will depend on the resistors But for ac the capacitor will act as a short circuit. Its gain will depend on the beta of the transistor which is in hundreds thus you can amplify a very weak signal to strong signal This circuit uses the Darlington transistor  in place of the single transistor   Darlington transistor is two transistors within a single package One small pilot transistor and another larger switching transistor This increases the current handling capacity and gain of the circuit. Also this provides high impedance  at the input and low impedance at the output This circuit has a transformer in place of Rc to  increase the efficiency of the circuit. However the transformer is an inductive device due to its windings in core So using inductive components in amplifier switching circuits is best avoided as any back emfs generated may damage the transistor. A class a amplifier can reproduce full  waveform with very low distortions But as the transistor is biased it's always on. This and some other factors reduce its efficiency to less than 30 percent which is very low Now you know how a class A amplifier works. In the upcoming videos we will explore the remaining  classes of amplifiers. Thank you for watching

Channel: Explorer

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Keywords: amplifier, electronics, class ab, class a, integrated amplifier, powerful ultra bass amplifier, class a amplifier, class a power amplifier, amplifier working, transistor amplifier, transistor amplifier circuit, transistor as an amplifier, single transistor amplifier circuit, circuit diagram, amplifier working principle, mic, speaker, amplification

Id: dKTbrZMscpM

Length: 5min 45sec (345 seconds)

Published: Sun Aug 01 2021