8- Amplifiers Summary

Amplifiers Summary

Amplifiers are used extensively in electronic circuits to make an electronic signal bigger without affecting it in any other way

Generally we think of Amplifiers as audio amplifiers in the radios, CD players and stereo’s we use around the home. In this amplifier tutorial section we looked at the amplifier circuit based on a single bipolar transistor as shown below, but there are several different kinds of transistor amplifier circuits that we could use.

Typical Single Stage Amplifier Circuit

Small Signal Amplifiers

• Small Signal Amplifiers are also known as Voltage Amplifiers.
• Voltage Amplifiers have 3 main properties, Input Resistance, Output Resistance and Gain.
• The Gain of a small signal amplifier is the amount by which the amplifier “Amplifies” the input signal.
• Gain is a ratio of output divided by input, therefore it has no units but is given the symbol (A) with the most common types of transistor gain being, Voltage Gain (Av), Current Gain (Ai) and Power Gain (Ap)
• The power Gain of the amplifier can also be expressed in Decibels or simply dB.
• In order to amplify all of the input signal distortion free in a Class A type amplifier, DC Base Biasing is required.
• DC Bias sets the Q-point of the amplifier half way along the load line.
• This DC Base biasing means that the amplifier consumes power even if there is no input signal present.
• The transistor amplifier is non-linear and an incorrect bias setting will produce large amounts of distortion to the output waveform.
• Too large an input signal will produce large amounts of distortion due to clipping, which is also a form of amplitude distortion.
• Incorrect positioning of the Q-point on the load line will produce either Saturation Clipping or Cut-off Clipping.
• The Common Emitter Amplifier configuration is the most common form of all the general purpose voltage amplifier circuit using a Bipolar Junction Transistor.
• The Common Source Amplifier configuration is the most common form of all the general purpose voltage amplifier circuit using a Junction Field Effect Transistor.

BJT Amplifier to JFET Amplifier Comparison

Parameter	Common Emitter Amplifier	Common Source Amplifier
Voltage Gain, ( AV )	Medium/High	Medium/High
Current Gain, ( Ai )	High	Very High
Power Gain, ( AP )	High	Very High
Input Resistance, ( Rin )	Medium	Very High
Output Resistance, ( Rout )	Medium/High	Medium/High
Phase Shift	180o	180o

Large Signal Amplifiers

• Large Signal Amplifiers are also known as Power Amplifiers.
• Power Amplifiers can be sub-divided into different Classes, for example:
  • Class A Amplifiers – where the output device conducts for all of the input cycle.
  • Class B Amplifiers – where the output device conducts for only 50% of the input cycle.
  • Class AB Amplifiers – where the output device conducts for more than 50% but less than 100% of the input cycle.
• An ideal Power Amplifier would deliver 100% of the available DC power to the load.
• Class A amplifiers are the most common form of power amplifier but only have an efficiency rating of less than 40%.
• Class B amplifiers are more efficient than Class A amplifiers at around 70% but produce high amounts of distortion.
• Class B amplifiers consume very little power when there is no input signal present.
• By using the “Push-pull” output stage configuration, distortion can be greatly reduced.
• However, simple push-pull Class B Power amplifiers can produce high levels of Crossover Distortion due to their cut-off point biasing.
• Pre-biasing resistors or diodes will help eliminate this crossover distortion.
• Class B Power Amplifiers can be made using Transformers or Complementary Transistors in its output stage.