Here instrumentation amplifier is designed and tested using LM324 operational amplifier with single supply voltage of +5V. The LM324 is a quad amplifier which means it has four independent operational amplifier. Instrumentation amplifier can be built using three op-amp(operational amplifier). So three op-amps of the LM324 integrated circuit can be used to build an instrumentation amplifier. Another advantage of LM324 is that it supports single power supply so we will make use of single supply voltage in this example design of instrumentation amplifier.
In previous post we explained what is instrumentation amplifier & types of instrumentation circuits. We showed how the basic difference amplifier evolved into instrumentation amplifier with high common mode rejection ratio(CMRR) and high input impedance and low output impedance among other qualities required to be a good instrumentation amplifier. Formula for gain of instrumentation amplifier and application example of instrumentation amplifier was also provided. Basically instrumentation amplifier are used for amplification of low level signal with noise. In most of the instrumentation amplifier design dual supply is used as shown in the tutorial How to build LM324 Instrumentation Amplifier & Test It. But often times we wish to use single power supply, an attempt which is illustrated here.
The following video shows testing the LM324 instrumentation amplifier with PC oscilloscope.
The following is the schematic diagram of single supply based LM324 instrumentation amplifier.
In the above circuit diagram, the U1A, U1B and U1C are the three op-amps within single LM324 IC. The differential input signal at Vin1 and Vin2 are ac-coupled into the first two op-amps. All the three op-amps are biased to mid-voltage 2.5V(between positive supply voltage 5V and ground) using voltage divider biasing method(see How to bias non-inverting op-amp amplifiers). Thus the instrumentation amplifier is biased so that it can be operated with 5V single supply. However for the first two op-amps the decoupling capacitor at the feedback loop is missing. On experiment, placing no decoupling capacitor or placing the decoupling capacitor for the first op-omp only, or the second op-amp only or placing decoupling capacitors for both op-amps had no effect on the output signal. So the decoupling capacitors were removed. The potentiometer R5 of 10KOhm is used to vary the gain.
The third op-amp is a differential amplifier which needs also to be biased(here with R12 and R13). It was noticed that without high feedback resistor(R2 and R3) for the third op-amp the gain was very low. That is the output signal amplitude is lower than the input signal for highest gain possible with the 10KOhm potentiometer R5. That is why high gain with R2=R3=100KOhm and R1=R4=1KOhm were used. The gain of instrumentation amplifier is,
\[A = \frac{V_{out}}{V_{in2}-V_{in1}} = \frac{R_{2}}{R_{1}}(1+\frac{2 R_{f}}{R_{5}})\]
where, \(R_{f1} = R_{f2} = R_{f}\)
\(R_{2} = R_{3}\), \(R_{1} = R_{4}\)
The following shows LM324 instrumentation amplifier with single supply on breadboard following the above circuit diagram.
The above instrumentation amplifier is tested using Labview based PC oscilloscope and function generator. This requires to setup the audio speaker output and mic/line in input on the PC with the instrumentation amplifier input and output. How to set-up the test was explained in previous tutorial How to Build BJT amplifier and test with Soundcard based PC Oscilloscope.
The connections between LM324 instrumentation amplifier and PC audio port for signal input and output is shown below.
The following shows the input signal and LM324 instrumentation amplifier output signal on oscilloscope. Green is input signal and red is output signal.
The following shows the input signal in spectrum analyzer.
The following shows the output signal in spectrum analyzer.
In this way we can design LM324 based instrumentation amplifier with single supply of +5V. We have tested the instrumentation amplifier with PC oscilloscope.
If you have matlab/simulink you can also use Matlab Simulink as oscilloscope and spectrum analyzer to test the amplifier. The following are some example,
- LM324 op-amp Integrator testing with matlab simulink oscilloscope
- LM358 Op-Amp Integrator Test with Matlab Simulink Oscilloscope
- Build LM324 inverting amplifier and test with Matlab/Simulink Oscilloscope
We can also use Proteus software to test the instrumentation amplifier as illustrated in the following tutorials.
- How to build LM324 Instrumentation Amplifier & Test It
- LM741 non-inverting amplifier with dual supply from single supply
