Transcript Instrumentation amplifier

ENTC 3320
Instrumentation Amplfiers
Improving the input resistance of
amplifiers
Add buffer amplifiers to the inputs
Rin = infinity at both V1 and V2
Instrumentation Amplifier
vo1
v1
I 
I
v1  v2
R1
I
v2
vo 2
The Buffer Amplifier
vo1  vo 2
v1  v2
v1  v2

R2  v1  v2 
R2
R1
R1
 R2
R2 
 v1  v2   1  
R1 
 R1
 2 R2 
 v1  v2 1 

R
1 

The Difference Amplifier

Using superposition, the output is due to
an inverting amplifier and non-inverting
amplifier.
The Difference Amplifier

The inverting amplifier produces

 R4 
voi  vo1  
 R3 
The non-inverting amplifier produces
 R4   R4 
von  vo 2 
 1  
 R3  R4   R3 

The output is the sum of the outputs
produced by each input.
 R4   R4 
 R4 
vout  voi  von  vo 2 
 1    vo1  
 R3  R4   R3 
 R3 
 R4   R3 R4 
 R4 
 vo 2 
     vo1  
 R3  R4   R3 R3 
 R3 
 R4   R3  R4 
 R4 
vout  vo 2 

  vo1  
 R3  R4   R3 
 R3 
 R4 
 R4 
 R4 
 vo 2    vo1    vo 2  vo1  
 R3 
 R3 
 R3 
 R2   R4 
 R2   R4 
 v1  v2 1      v2  v1 1    
 R1   R3 
 R1   R3 

For R2=R3=R4,
vout
 R2 
 v2  v1 1  
 R1 
• Quite often, R1 is a variable resistor that is
used to set the gain and is denoted as Rg.