Amplifier Summary

Amplifiers are widely used in electronic circuits to magnify an electronic signal without affecting it in any other way.

We often think of amplifiers as sound amplifiers that are found in radios, CD players and the sound systems we use at home. In the tutorial section about these amplifiers, we looked at the amplifier circuit based on a single bipolar transistor, as shown below, but there are several different types of transistor amplifier circuits that we can use.

Typical Single Stage Amplifier Circuit

amplifier summary

Small Signal Amplifiers

  • Small Signal Amplifiers are also known as Voltage Amplifiers.
  • Voltage Amplifiers have 3 main features: Input Resistance, Output Resistance and Gain.
  • The Gain of a small signal amplifier is the amount in which the amplifier "Strengthens" the input signal.
  • Gain is an output rate divided by input, so it has no units, but the most common types of transistor gain are given the symbol (A), voltage gain (Av), Current Gain (Ai) and Power Gain (Ap).
  • The amplifier's power gain can also be expressed as Decibels or simply dB.
  • DC Base Polarization is required to amplify the entire input signal distortion in a Class A type amplifier.
  • DC Polaration adjusts the amplifier's Q-point halfway along the load line.
  • This DC Base polaration means that the amplifier consumes power even if the input signal is not present.
  • The transistor amplifier is not linear and an incorrect polarity setting will produce a large amount of distortion in the output waveform.
  • A very large input signal will produce large amounts of distortion due to cropping, which is also a form of amplitude deterioration.
  • Improper positioning of the Q point on the load line will produce Saturation Trimming or Cutting Cropping.
  • The Common Emitter Amplifier configuration is the most common form of the entire general purpose voltage amplifier circuit using a Bipolar Connection Transistor.
  • The Common Welding Amplifier configuration is the most common form of any general purpose voltage amplifier circuit using a Junction Area Effective Transistor (JFET).

BJT vs. JFET

ParameterCommon Transmitter Amplifier/CCCommon Resource Amplifier/CS
Voltage Gain, ( AV )Medium/HighMedium/High
Current Gain, ( Ai )HighToo High
Power Gain, ( AP )HighToo High
Input Resistance ( Rin )MiddleToo High
Output Resistance, ( Rout )Medium/HighMedium/High
Phase Angle180O180O

Large Signal Amplifiers

  • Large Signal Amplifiers are also known as Power Amplifiers.
  • Power Amplifiers can be divided into different Classes, for example:
  • Class A Amplifiers – the place where the output device transmits for the entire input cycle.
  • Class B Amplifiers – where the output device transmits only 50% of the input cycle.
  • EU Class Amplifiers – where the output device transmits more than 50% of the input cycle, but less than 100%
  • An ideal Power Amplifier loads up to 100% of the available DC power.
  • Class A amplifiers are the most common form of power amplifier, but have a efficiency rating of less than 40%.
  • Class B amplifiers are about 70% more efficient than Class A amplifiers but produce a high amount of distortion.
  • Class B amplifiers consume very little power when there is no input signal.
  • Distillation can be greatly reduced using the "push-pull" output stage configuration.
  • However, simple push-pull Class B Power amplifiers can produce a high level of Cross Distortion due to breakpoint polarizers.
  • Pre-biased resistors or diodes will help eliminate this transitional deterioration.
  • Class B Power Amplifiers can be made using Transformers or Complementary Transistors during the output phase.