Design three op-amp IA with gain 20

An instrumentation amplifier (IA) is a precision differential amplifier with very high input impedance, high CMRR and an accurately set gain. It is used to amplify small differential signals from transducers in the presence of large common-mode noise.

Features of an instrumentation amplifier

• Very high input impedance (both inputs buffered), so it does not load the source
• Very high CMRR (typically 100 dB or more)
• Gain set by a single resistor RG, adjustable and stable
• Low offset voltage, low drift and low noise
• Low output impedance and high slew rate

Three op-amp IA

 V1 --->(+)\
           A1>---+--Vo1--[R2]--+--[R3]--+
       +->(-)/   |             |        |
       |         R1          (-)\       |
       +---------+              A3>-----+--> Vo
       RG                    (+)/
       +---------+             |
       +->(-)\   R1   Vo2--[R2]+--[R3]--GND
       |   A2>---+--Vo2
 V2 --->(+)/

A1 and A2 form the buffered input stage. A3 is a difference amplifier.

Vo = (R3/R2)(1 + 2R1/RG)(V2 − V1)

Design for gain A = 20

Make the difference stage a unity-gain stage so that all the gain is in the first stage (only RG needs to be adjusted).

1. Choose R2 = R3 = 10 kΩ, so R3/R2 = 1.
2. Then 1 + 2R1/RG = 20, so 2R1/RG = 19.
3. Choose RG = 1 kΩ:

R1 = 19 × RG / 2 = 19 × 1 kΩ / 2 = 9.5 kΩ

Alternatively, fix R1 = 10 kΩ:

RG = 2R1/(A − 1) = 20 kΩ/19 = 1.053 kΩ

Use 1 kΩ fixed plus a 100 Ω trimmer.

Check: A = (10k/10k)(1 + 2 × 9.5k/1k) = 1 × (1 + 19) = 20.

Design values: R1 = 9.5 kΩ (two, matched), RG = 1 kΩ, R2 = R3 = 10 kΩ (four, 0.1 % matched for high CMRR), op-amps for example 741, OP07 or TL084.

The four resistors of A3 must be closely matched (R3/R2 equal on both sides). Mismatch directly reduces CMRR.