Classification of Amplifiers

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Amplifiers may be classified or Classification of Amplifiers in several ways as described below:

According to the Frequency Range | Classification of Amplifiers

Thus amplifiers may be classified as below:

  1. DC (Direct coupled) Amplifiers from zero frequency (dc) onwards.
  2. Audio Frequency Amplifiers 20 Hz to 20 KHz
  3. Video Frequency Amplifiers up-to a few MHz.
  4. Radio Frequency (RF) Amplifiers from a few KHz to hundreds of MHz.
  5. Ultra-high Frequency (UHF) and Microwave Amplifier up-to hundred or thousands of MHz.

According to the method of operation | Classification of Amplifiers

The position of the zero signal (quiescent) operation point and the extent of the characteristic curves being used determine the category of operation. Thus amplifiers (using either BJT or FET) may be classified as class A, class AB, class B or class C amplifiers as per following definition.

block diagram of operational amplifier Classification of Amplifiers

  1. Class A Amplifier A Class A amplifier is one in which the operating point and the input signal magnitude are so selected that the output circuit current (collector current or drain current) flows all the time (for the full cycle of the a.c. signal). A class A amplifier operates essentially over the linear portion of characteristic curves of the device.
  2. Class B Amplifier A class B amplifier is one in which the operating point is placed at an extreme end of its characteristic curve with the result that zero-signal current (or zero signal voltage) is almost zero. Hence zero signal output power is very small. With a sinusoidal input signal voltage, amplification takes place for only half the cycle. Thus, if the output circuit quiescent current is zero, on application of signal, the output current remains zero for half the cycle and flows for the remaining half cycle.
  3. Class AB Amplifier A class AB amplifier is one in which operation lies in between class A and class B operation. Thus, in a class AB amplifier, output current flows for more than half but less than the complete cycle of input sinusoidal signal.
  4. Class C Amplifier In a class C amplifier, the operating point is so chosen that the output current flows for less than one-half of the input sinusoidal signal cycle.

According to the Type of Load Impedance | Classification of Amplifiers

The load impedance may be an untuned circuit or a tuned circuit. Accordingly, amplifiers may be classified as: (a) untuned amplifiers and (b) tuned amplifiers.

  1. Untuned Amplifier An untuned amplifier uses either a pure resistance or a complex impedance as the load impedance. Untuned amplifiers may be further subdivided into two categories: (i) audio frequency amplifiers and (ii) video frequency amplifiers.
  2. Audio Frequency Amplifiers These are used for amplifying frequencies in the audio frequency range i.e. up-to 20 KHz. When we speak or play a sound instrument, sound frequencies so generated extended up-to about 20kc/s and corresponding electrical signal at the output of microphone also extends up-to about 20 KHz. Amplifiers used for amplifying frequencies in audio frequency range (20 KHz) are, therefore, called audio frequency amplifiers.
  3. Video Frequency Amplifiers Vision signal such as produced at the output of a TV camera extended up-to several MHz. Amplifiers designed for amplifying these signals of frequency extending up-to several MHz are, therefore, called video amplifiers or pulse amplifiers or wideband amplifiers.
  4. Tuned Amplifiers A tuned amplifier uses one or more parallel tuned L-C circuits as the load impedance. Tuned amplifiers are used for amplification of signals consisting of either a signal radio frequency (> 30 KHz) or a narrow band of frequency band of frequencies in the R.F. range. Tuned amplifiers are often referred to as radio frequency (R.F.) amplifiers.

According to the Number of Stage and Method of Coupling

Thus amplifiers may be classified as (a) Single stage amplifiers and (b) multistage amplifiers. Multistage amplifiers may be further classified into following main categories depending on the method of coupling: (i) direct coupled (dc) amplifiers (ii) R.C. coupled amplifiers and (iii) transformer coupled amplifiers.

  • According to the Primary Function Depending on the primary function, amplifiers may be classified:
  1. Small Signal Amplifiers These provide linear amplification with minimum distortion.
  2. Large Signal (Power) Amplifier These are designed to provide large power output but at the same time permit somewhat higher distortion.

General Remarks

  1. Untuned small signal amplifiers use class A operation. Class AB, B or C operation can not be used because of large distortion.
  2. Untuned large signal (power) amplifier may be use class A operation but conversion efficiency is small. More commonly untuned power amplifier use class AB or class B operations using push-pull circuit. Class AB or B operations provide higher conversion efficiency. But push-pull circuit is essential to keep distortion low. Single ended class AB or class B untuned amplifiers are never used. Further untuned amplifier can not use class C operation because the distortion is so large that even push-pull circuit cannot handle it.
  3. Class C operation is used only in tuned (radio frequency) amplifiers.
  4. Several important waves shaping functions may be performed using AB or B overdriven amplifiers.

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