Classification of Power Amplifiers | types of BJT amplifiers

An amplifier can produce an enlarged but exact version of the small input signal at the output of an electronic circuit. Electronic devices like TV, radio, tape recorder, etc. require amplifiers for their operations. These devices do not operate in the same way. Some devices need to amplify the current, some devices require to amplify voltage, etc. Again, in some devices, a part of the input wave is to be amplified. On the basis of those conditions, all the amplifiers are classified into different types. In this article, we are going to discuss the classification of BJT based power Amplifiers.

Classification of Power Amplifiers

Based on the position of Q-point and current flow in output, Power amplifiers are classified into four types –

  • Class-A
  • Class-B
  • Class-AB
  • Class-C

On the basis of the frequency of operation, there are three types of power amplifiers which are –

  • Audio Frequency (AF) Amplifier
  • Radio Frequency (RF) Amplifier
  • Video Frequency Amplifier

Depending on the inter-stage coupling, there are three types of BJT amplifiers –

  • R-C coupled amplifier
  • Transformer Coupled Amplifier
  • Direct coupled Amplifier

A, B, AB and C types of BJT amplifiers

Class-A amplifier

If the position of Q-point is such that the output current flows during the whole cycle of the input signal then the amplifier is called Class-A amplifier. For this type of Amplifier Q-point should be in the middle of the dynamic transfer curve of the Transistor.

Class-B amplifier

If the Q-point is chosen near the cutoff region of the dynamic transfer curve then one can get output current only during half cycle of input signal. In this situation, we call the amplifier a class-B amplifier.

Class-AB amplifier

The amplifier that operates in a manner in between that of class-A and Class-B amplifiers is the class-AB amplifier. In this amplifier, the output current flows for more than half a cycle but less than the complete cycle of the input signal.

Class-C amplifier

For a class-C amplifier, one can get the output current for less than half a cycle of the input signal. We have to choose the Q-point accordingly.

Classification of power amplifiers based on Frequency response

Depending on the frequency of operation we can classify the amplifiers as following three types –

Audio frequency (AF) amplifier

Audio frequency amplifier can amplify the input signal of the frequency range from 20 Hz to 20 kHz. AF amplifies weak signals from phonograph, magnetic tape heads, microphones, etc. sound systems.

Radio Frequency (RF) amplifier

A radiofrequency amplifier can amplify only a narrow band of frequency from a few kHz to a few MHz. This frequency range is within the radio frequency range. As the RF amplifier is tuned to amplify only a narrow band of frequency it is called a tuned RF amplifier or narrow band amplifier. This type of amplifier has wide use in radio receivers.

Video frequency amplifier

A video frequency amplifier is used to amplify the signals of a broad range of frequencies. This range is from a few Hz to several MHz. Therefore we call this type of amplifier a wide-band amplifier or a broad-band amplifier.

BJT based Coupled amplifiers

Based on the type of coupling between several stages of the amplifier we have the following types of amplifiers –

RC Coupled amplifier

We need to couple the multistage of amplifiers if the gain in a single-stage amplifier is not sufficient. The most common way of coupling is through an R-C network. The coupling capacitor provides DC isolation between the coupled stages. But there is a limitation on usable frequencies.

Transformer coupled amplifier

The transformer coupling is also useful for the purpose of coupling the amplifiers. It reduces the loss of power in the collector circuit. That means it can provide almost the same power at the input of the second stage that of the output of the first stage. But the problem with the transformer is that it is bulky and costly and it is not available in IC form. It also has poor frequency response.

Direct coupled amplifier

The coupled amplifier in which two stages are connected directly without any use of R-C network or Transformer is called the direct coupled amplifier. It has a wide range of applications in photocells, thermocouples, OP-AMPs, ICs, etc. to amplify low-frequency signals or dc signals. Its main advantage is that it can amplify dc signals too.

Types of BJT Amplifiers as Current, Voltage and Power amplifier

A current amplifier is an amplifier that amplifies the current at the output. A current amplifier must have very low input impedance and very high output impedance. The voltage amplifier amplifies the voltage and the power amplifier boosts up the power at the output. A voltage amplifier should have a higher input impedance and lower output impedance. CE mode BJT amplifier is a voltage and power amplifier. CC mode BJT amplifier is an example of a current amplifier.

These are the classification of power amplifiers. If you have any doubt on this topic you can ask me in the comment section.

Thank you!

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