BJT DIFFERENTIAL AMPLIFIER TUTORIAL

An Introduction to

Differential Amplifiers

Differential Amplifier is the basic building block of every OP AMP and is the basis of high speed digital logic circuit family, called Emitter Coupled Logic (ECL) and they are commonly used for analog circuits.

REASONS FOR THE WIDE USAGE OF DIFFERENTIAL AMPLIFIERS 

Differential Amplifiers are much less sensitive to noise and interference than single ended transistor Amplifiers.
And
Differential Amplifiers enables us to bias the Amplifiers and to couple Amplifier Stages together without the necessity of coupling capacitors.

Circuit Diagram of Differential Amplifier

DIFFERENTIAL AMPLIFIER FEATURES

• Differential Amplifiers provide HIGH INPUT IMPEDANCE, this is the reason for using them as INPUT STAGE OF OPERATIONAL AMPLIFIERS.
  
  
• Differential amplifiers provides high noise rejection
  
  
• Differential amplifiers provides large immune to interferences.
  
  
• We can couple a large number of amplifier stages together without the need of coupling capacitors.
  
  
• It provides a bias to the amplifiers.
  
  

 Why the name Differential Amplifier?

 The name Differential Amplifier is because of, the Amplifier will find out the Difference between two input sources connected to the base of two transistors and amplify the Difference. hence the name Differential Amplifier.

Configurations Of Differential Amplifier

The following Differential Amplifier Configuration is based on the number of Inputs and the way in which the Output is measured. 

 1. Dual Input Balanced Output Differential Amplifier Configuration 

 2. Dual Input unbalanced Output Differential Amplifier Configuration 

 3. Single Input Balanced Output Differential Amplifier Configuration 

 4. Single Input Unbalanced Output Differential Amplifier Configuration 

When Both the terminals are used it is refered to as Dual Input otherwise Single Input. On the other hand, If the Output is taken such that, across Both the collector terminals it is termed as Balanced Output and if the Output is measured individually from either the two collectors with respect to ground, it is called as Unbalanced Output.

Common Mode Voltage Gain

 Common Mode Voltage Gain, If both the Input signals applied to the bases of transistors is IN PHASE with each other and the Corresponding Gain at the Output is termed as Common Mode Voltage Gain.
For an Ideal Differential Amplifier, the Common Mode Voltage Gain is Zero. The reason is self Explanatory. ie; The Difference Between two Voltage sources, which is in phase with each other is always Zero(i am already mentioned that Difference Amplifier is the another name of Differential Amplifier) so the gain too will be Zero.

Common Mode Rejection Ratio (CMRR)

 It is defined as the ratio of Differential Voltage Gain to Common Mode Voltage Gain Mathematical Representation of CMRR 

CMRR = Ad/Acm

For an Ideal Differential Amplifier, CMRR will be Infinite. ie; Acm will be Zero.