Wednesday, May 25, 2011

The Differential Input and Differential Output

For getting balanced differential output  we use this circuit. In this circuit  two source is present, so the superposition theory is applied to get the output. In this circuit both the inverting and non-inverting terminal is working.It rejects the common-mode voltages, so  it is very useful in noisy environments.

A  differential input and differential output amplifier using two identical Op amp. It is     most commonly used as a preamplifier and driving push-pull  arrangement. The differential input and output  are inphase or the same polarity provided  Vin  =Vx – Vy and
V=Vox – Voy
When we want to find out the 1st op-amps output VOX , we will use the  superposition theory.
When we get  VX  is active , VY  is  inactive then ,
In non inverting terminal 
V1= (1+ )VX
When we get  Vy   is  active,  Vx   is  inactive then,
In  inverting terminal,
V1 = - Vy
 So, Vox = V1+V1
                =(1+ )V Vy
Fig: The circuit diagram of the differential input and output amplifier. 

When we want to find out the 2nd  op-amps output VOy ,we will use superposition theory.
                      When we get  VX  is active , VY  is  inactive then
                      In  inverting terminal ,
                      V2= - Vx
      Again, when  we get  Vy  is  active ,  Vx  is  inactive then
        In non inverting terminal we get, 
V2= (1+ )Vy
So, Voy = V2+V2
                                      =(1+ )Vy - Vx
So the output result   
Vo = Vox  –  Voy 
= (1+ )V-  Vy  –[(1+ )Vy - Vx ]
                                      = (1+ ) ( VX - Vy ) +( VX - Vy )
                                       = ( VX - Vy ) (1+ )

To design a input and differential output amplifier,  taking a  differential output of at least 3.7V and the  differential  input Vin  =10V.
                        We know,
 Vo = ( VX - Vy ) (1+ )
            Or, 3.7 = (0.1) (1+ )
 Or, 37 = (1+ )
            Or, 36 = 
            Or, Rf  = 18 R1
                                    Let, R1 = 100Ώ, then Rf  = 1.8 KΏ . 

                   Fig: The designing circuit diagram of the differential input and output amplifier. 

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