Aim

Apparatus

Block Diagram

Student name

Signature

Date

Batch

To study the behavior of the instrumentation amplifier

EXP-2. INSTRUMENTATION AMPLIFIER

Transducer Instrumentation

amplifier Application

Pin Diagram

Fig2: IC 741

Circuit Diagram

Fig3: Analog Circuit

Stage 1: Differential input/output amplifier

Stage 2: Inverting amplifier

R2

Vo

‐Vee

‐Vee

‐Vee R’

+Vcc

+Vcc

+Vcc

I V1

V2 R’

R

R2

R1

R1

V2

V1

‐

+

+

‐A1A1

‐

+A2A2

Stage 1 Stage 2

Theory

Instrumentation amplifier is a kind of differential amplifier with additional input buffer stages. The addition of input buffer stages makes it easy to match (impedance matching) the amplifier with the preceding stage. Instrumentation are commonly used in industrial test and measurement application. The instrumentation amplifier also has some useful features like low offset voltage, high CMRR (Common mode rejection ratio), high input resistance, high gain etc. The circuit diagram of a typical instrumentation amplifier using op-amp is shown figure2. In the circuit diagram, op-amps labeled A1 and A2 are the input buffers. Anyway the gain of these buffer stages is not unity because of the presence of R’ and R. Op amp labeled A3 is wired as a standard differential amplifier. R2 connected from the output of A3 to its non- inverting input is the feedback resistor. R1 is the input resistor. The voltage gain of the instrumentation amplifier can be expressed by using the equation below.

12

If it’s needed to vary the gain of the instrumentation amplifier then the resistance R is replaced by a suitable potentiometer.

Formulae applied

The voltage gain of the complete circuit

Where Av1 is the voltage gain of stage 1 and Av2 is the stage 2 gain.

12

The overall voltage gain can be controlled by adjustment of R. The common mode signal attenuation for the instrumentation amplifier is that provided by the difference amplifier. This can be maximized by making resistor adjustable. The dc output voltage level can be controlled if is connected to an adjustable bias voltage of being directly grounded.

Sample Calculation

Design an instrumentation amplifier to have an overall voltage gain of 100. The input signal amplitude is varied till the op-amps are saturated (741 op-amps are to be used) and the supply voltage is 12V.

Solution:

Stage 1 design: Let

√ √100

10

Stage 2 design: Designing the resistor values

The output and input voltage is related by the below stated formula

12

Assume 10and1 10. Taking 1 Ω, then 10 Ω .

Taking 10 Ω on solving 4.5 Ω (approximate it to 5k by using two 10k resistors in parallel)

Theoretical Calculation

Procedure

Step1: On conducting the calculation set up the circuit as shown in the figure.

Step2: Measure the output voltage obtained

Step3: Compare with the theoretically obtained value.

Step4: Vary the input subsequently and note down the output voltage. Calculate the gain.

Tabulation

Input voltage( mV) Output voltage(V) Practical gain(Av)

Observations and Remarks

Conclusion