DIFFERENTIAL AMPLIFIER
Differential amplifier is a building block of IC amplifiers. It is direct coupled amplifier . { this means one end of the amplifier is connected to another by wire , unlike connected by resistors or transformers}. As the name tells us that " it amplifies the difference between the input signals . Operational amplifiers are made by using differential amplifiers ,so differential amplifiers are called as Building block of operational amplifiers.
The basic differential amplifier consists of two identical transistors Q1 ,Q2 , collector resistors RC1 & RC2 , emitter resistor RE . inputs Vi1 and Vi2 are given to the base of each transistors and outputs Vo1 ,Vo2 are taken out from collectors . Two power supplies vcc &vee are provided for the amplifiers .
MODES OF DIFFERENTIAL AMPLIFIES
- Double ended input & double ended output.
- Single ended input & single ended output.
- double ended input & single ended output .
- single ended input & double ended output.
WORKING OF DUAL INPUT BALANCED OUTPUT (1) DIFFERENTIAL AMPLIFIERS
working of differential amplifiers is analyzed by superposition theorem { considering one input is present and other input is grounded }
- considering Vi1 , grounding Vi2 :
here Q1 transistor acts as CE amplifiers , because input Vi1 is applied at base , amplified output Vo1 is taken at collector , hence Vo1 is out of phase with input Vi1 .
here Q2 transistor acts as CB amplifier because input is applied at emitter ,at collector a amplified output Vo2 is appears same as input
2. considering Vi2 , grounding Vi1 :
here Vi2 is the input applied at base hence transistor Q2 acts as CE amplifier , the amplified output Vo2 is out of phase with input Vi2 taken at collector.
Vi2 is the input to Q1 transistor applied at emitter hence Q1 acts as CB amplifiers , vo2 is the output obtained collector which is in phase with applied input .
When both the inputs is applied ,a output is measured between collectors
A= Vo/(Vi1-Vi2).
COMMON MODE OPERATION :
When the same input is applied to both the inputs of a differential amplifiers , the resulting operation is called common mode operation . usually DE refuses to amplify common mode signals .
If we consider dual input single output DE , then
voltage gain A = Rc/-2Re
DIFFERENTIAL MODE OPERATION :
When the two opposite polarity signals are applied to the two input of a differential amplifier , the resulting operation is called differential mode operation .
voltage gain is A=Rc/2re'
COMMON MODE REJECTION RATIO
The ratio of differential mode gain to common mode gain is called is called as CMRR .
The main objective of the differential amplifier is to amplify differential signals and rejects common signals such as noise . CMRR represents the ability of the differential amplifiers . Ideally CMRR is infinity .It is expressed in db.
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