Classification of BJT Amplifier

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

Classification of BJT based Amplifier

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

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

On the basis of frequency of operation, amplifiers are divided into three parts –

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

Depending on the inter-stage coupling amplifiers have three types –

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

Class-A, B, AB and C BJT amplifier

Class-A amplifier

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

Class-B amplifier

If the Q-point is chosen near the cut off region of dynamic transfer curve then one can get output current only during half cycle of input signal. At 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 that half cycle but less than complete cycle of input signal.

Class-C amplifier

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

Classification of BJT amplifier 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 system.

Radio Frequency (RF) amplifier

Radio frequency 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 tuned RF amplifier or narrow band amplifier. This type of amplifier has wide use in radio receivers.

Video frequency amplifier

Video frequency amplifier is used to amplify the signals of a broad range of frequency. 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.

Coupled amplifiers

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

RC Coupled amplifier

We need to couple the multistage of amplifiers if the gain in single stage amplifier is insufficient. The most common way of coupling is through a R-C network. The coupling capacitor provides DC isolation between the coupled stages. But there is a limitation on usable frequencies.

Transformer coupled amplifier

In the purpose of coupling of amplifiers transformer coupling is also useful. It reduces the loss of power in collector circuit. That means it can provide almost the same power at the input of second stage that of at output of first stage. But the problem with 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 is which two stages are connected directly without any use of R-C network or Transformer, is called the direct couple amplifier. It has wide range of applications in photocell, thermocouple, OP-AMPs, ICs etc. to amplify low frequency signals or dc signals. Its main advantage is that it can amplify dc signal too.

Classification of BJT Amplifier as Current, Voltage and Power amplifier

Current amplifier is the amplifier that amplifies the current at output. A current amplifier must have very low input impedance and very high output impedance. Voltage amplifier amplifies the voltage and the power amplifier boosts up the power at the output. A voltage amplifier should have 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 current amplifier.

That’s all from the post “classification of BJT amplifier”. If you have any doubt you may ask in comment section.

Thank you!

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