Class A Amplifier Circuit Working and Applications

We have already discussed, the classes and classifications of power amplifiers in our earlier articles. The power amplifier circuits are used to deliver high power to drive the loads likes loudspeakers. The power amplifiers are classified based on their mode of operation that is the portion of the input cycle during which the collector current is expected to flow. On this basis, the power amplifiers are classified as given below. In this article, we will discuss Class A Amplifier in detail.

Generally, the power amplifiers (large signal) are used in the output stages of an audio amplifier system to drive a loudspeaker load. A typical loudspeaker has an impedance of between 4Ω and 8Ω, thus a power amplifier must be able to supply the high peak currents required to drive the low impedance speaker.

Class A Power Amplifier

In Class A amplifier, If the collector current flows all times during the full cycle of the input signal, the power amplifier is known as class A power amplifier. It is less used for higher power output stages, as it has poor efficiency.

The purpose of the class A bias is to make the amplifier relatively free from noise by making the signal waveform out of the region between 0v to 0.6v where the transistor’s input characteristic is non-linear.

Class A amplifier design produces a good linear amplifier, but most of the power produced by the amplifier goes wastage in the form of heat. Since the transistors in Class A amplifier, are forward biased all the time, few current will flow through them even though there is no input signal and this is the main reason for its poor efficiency. The circuit diagram of the direct-coupled class A Power Amplifier is shown in the figure below.

Transformer Coupled Class A Amplifier
Transformer Coupled Class A Amplifier

The above-shown circuit is a directly coupled Class A amplifier. An amplifier where the load is coupled to the output of the transistor using a transformer is called a direct coupled amplifier.

Using transformer coupling technique, the efficiency of an amplifier can be enhanced to a great extent. The coupling transformer provides good impedance matching between the load and output, and it is the main reason behind the improved efficiency.

Generally, the current flows through the collector resistive load, this will cause the wastage of the DC power in it. As a result, this DC power dissipated in the load in a form of heat, and it does not contribute any output AC power.

Hence it is not advisable to pass the current through the output device (ex: loudspeaker) directly.

For this reason, a special arrangement done by using a suitable transformer for coupling the load to the amplifier as given in the above circuit.

The circuit has the potential divider resistors R1 & R2, biasing and emitter bypass resistor Re, used for circuit stabilization. The emitter bypass capacitor CE and emitter resistor Re are connected parallel to prevent AC voltage.

The input capacitor Cin (Coupling Capacitor)  used to couples AC input signal voltage to the base of the transistor and it blocks the DC from the previous stage.

A step-down transformer provided with a suitable turn ratio to couple the high impedance collector to a low impedance load.

Impedance Matching of Class A Amplifier

Impedance matching can be done by making the output impedance of the amplifier equal to the input impedance of the load. This is an important principle for the transfer of maximum power (accordance with the maximum power transfer theorem).

Here the Impedance matching can be achieved by selecting the number of turns of the primary so that its net impedance is equal to the transistor output impedance and selecting the number of turns of the secondary so that its net impedance is equal to the loudspeaker input impedance.

Output Characteristics of Class A Power Amplifier

From the below figure, we can observe that the Q-point is placed exactly at the centre of the AC load line and the transistor conducts for every point in the input waveform. The theoretical maximum efficiency of a Class A power amplifier is 50%.

Class A Power Amplifier Output Characteristics- AC Load line
Class A Power Amplifier Output Characteristics- AC Load line

In practice, with the capacitive coupling and inductive loads (loudspeakers), the efficiency can decrease as low as 25%. This means 75% of power drawn by the amplifier from the supply line is wasted.

The majority of the power wasted is lost in a form of heat on the active elements (transistor). As a result, even a moderately powered Class A power amplifier requires a large power supply and a large Heatsink.

Advantages and Disadvantages of Directly Coupled Class A Amplifier

We use the power amplifiers for various purposes depending upon the constraint. Each and every class power amplifier has its own advantages and disadvantages as per its reliability and efficiency.

Advantages of Class A Amplifier

  • It has high fidelity because of the output exact replica of an input signal.
  • It has improved high-frequency response because the active device is ON full time, i.e. no time is required to turn on the device.
  • There is no crossover distortion because the active device conducts for the entire cycle of the input signal.
  • The single ended configuration can be easily & practically realized in class A amp.

Disadvantages of Class A Amplifier

  • Due to the large power supply and heat sink, class A amplifier is costly and bulky.
  • It has Poor Efficiency.
  • Due to the transformer coupling frequency response is not as good.

Applications of Class A Amplifier

  • The Class A Amplifier more suitable for outdoor musical systems, since the transistor reproduces the entire audio waveform without ever cutting off. As a result, the sound is very clear and more linear, that is, it contains much lower levels of distortion.
  • They are usually very large, heavy and they produce nearly 4-5 watts of heat energy per a watt of output. Therefore, they run very hot and need lots of ventilation. So they are not at all ideal for a car and rarely acceptable in a home.

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