Instrumentation Amplifier Circuit

Introduction

Instrumentation amplifier is a type of differential amplifier which is equipped with an input buffer amplifier, which eliminates the need for input impedance matching, making this amplifier suitable for use in measurements. Other characteristics of this amplifier are low DC offset, low noise, high open loop gain, high CMRR, and very high input impedance. This amplifier is used where the accuracy and stability of the circuit is required.

The circuit in Figure 1 is the first step, which is the buffer stage of the instrumentation amplifier. Using the assumptions for the ideal op-amp, the reinforcement of this first stage is

The next stage is the second stage, which is the amplification stage of the instrumentation amplifier which is built using a standard differential amplifier circuit. Using the assumptions for the ideal op-amp, the reinforcement of this second stage is

Furthermore, by combining the circuit in both stages, a complete instrumentation amplifier circuit is obtained. The total strengthening of this circuit is

The thing to be done

• Understand how the buffer amplifier circuit works
• Understand how the differential amplifier circuit works
• Understand how the instrumentation amplifier circuit works

Understand howthe buffer amplifier circuitworks

Arrange the circuit as shown in figure 1 with a value of R1 = R3 = 5K, R2 with a potentiometer 1K, and an op-amp + - 15 V. voltage source. Connect input v1 with a sinusoidal signal generator of 20 mVpp 100 Hz and input v2 to GND. Obtained vo1 and vo2 output as shown below:

In order to get 20 times the reinforcement obtained R2 = 189 Ohm.

This is different from what should be in the calculations obtained from the formula, which are:

Maybe this is because the transistor is not too good or the value of resistance that is not too precise.

Understand howthe differential amplifier circuitworks

Arrange the circuit as shown in Figure 2 with values R4 = 3K9 and R5 = 130. Connect iput vo1 to the signal generator with a sinusoidal signal of 20 mVpp 100Hz and vo2 input to GND. Obtained reinforcement as follows:

In the differential amplifier the output is 560 mVpp or 28 times gain.These results differ greatly from the calculations that should:

Maybe this is because the transistor is not too good or the value of resistance that is not too precise.

Then provide common mode input of 10 Vpp 100 Hz. Obtained output:

From the Common Mode output it is obtained a good CMRR value due to the small Common Mode reinforcement.

Understand how the instrumentation amplifier circuit works

Connect the first stage circuit output to the second stage input. Connect v1 to the signal generator with a 20 mVpp 100 Hz sinusoidal signal and V2 input to GND. Obtained vo output as follows:

From the picture above we get the combined gain of the buffer amplifier with a differential output of 19.6 V or a gain of 980 times.This value is far from calculation or measurement. If the calculation:

With the reinforcement of the measurement:

This value is far from both ways. Perhaps this is due to a transistor that is not very good, a value of resistance that is not too precise, or a shift in the variable resistor causes the value of R2 to change.

Conclusion

In the Instrumentation Amplification Circuit the outputs of the buffer amplifier, differential amplifier and instrumentation amplifier will be observed, resulting in a gain that differs significantly from the measurement. But to strengthen it has been seen how to work.