Instrument amplifier are special amplifier which allows us to acquire precise measurement of physical quantities such as temperature, pressure, humidity, weight, electrical activity within human or animal etc. The instrument amplifiers are used in all kinds of industries. For example, Instrumentation amplifier applications includes in food factories where precise measurement of flow, temperature, humidity are required. Another example is for example in plastic production where precise temperature must be measured and maintained. Due to precised measurement the product quality is improved.
The instrumentation amplifiers are required because most often the signal from transducers are weak, that is, the amplitude of the signal is low. Transducers are devices that converts one form of energy to another such as temperature sensor which converts temperature to electrical signal, microphone which converts sound energy to electrical signal. The signal acquisition is complicated as the signal of interest from the transducer contains inherent noise and there will be EMI(Electromagnetic Interference) such as from power supplies and other electrical devices around the transducer circuit. Long cables required to transfer signals is another source of noise. Thus low level signal in order of few mV and uV with noise, EMI makes it difficult to obtain clean signal. Therefore amplifier is needed to amplify the signal of interest and reject noise.
Instrumentation amplifier gain
The instrumentation amplifier is basically a difference amplifier and the general expression for voltage gain is as follows,
\[A = \frac{V_{out}}{V_{in2}-V_{in1}}\]
where, \(V_{out}\) is the output of the amplifier and \({V_{in2}-V_{in1}}\) is the differential input which is to be amplified.
There are many variation of difference amplifier circuit to create instrumentation amplifier which is explained below.
Instrumentation amplifier characteristics
The characteristics of an instrumentation amplifier is as follows.
1. Finite, Accurate Stable Gain
2. Easy Gain Adjustment
3. High Input Impedance
4. Low Output Impedance
5. High CMRR
6. Low power consumption
7. Low thermal and time drift
8. High slew rate
Instrumentation amplifier circuits
Basically we can design instrumentation with one, two and three op-amps and thus construct different instrumentation amplifier circuits. The one op-amp based instrumentation amplifier is the difference amplifier and if we add buffer stage at the input we will get modified one op-amp instrumentation amplifier. We can also construct instrumentation with two op-amps and three op-amps. Thus basic types of instrumentation amplifier circuits are as follows.
1. Difference amplifier as instrumentation amplifier
2. Buffered Difference amplifier as modified instrumentation amplifier
3. Two op-amp instrumentation amplifier
4. Three op-amp instrumentation amplifier
1. Difference amplifier as instrumentation amplifier
A difference amplifier amplifier amplifies the differential inputs and thus can be considered as the most basic instrumentation amplifier. The circuit diagram of a difference amplifier is shown below.
The output voltage for the difference amplifier is,
\[V_{out} = - \frac{R_{2}}{R_{1}}V_{in1} + (1+\frac{R_{2}}{R_{1}} )(\frac{R_{4}}{R_{3}+R_{3}}) V_{in2} \]
Selecting resistors values as \(R_{1} = R_{3}\) and \(R_{2} = R_{4}\) we can derive the gain of the difference amplifier as basic instrumentation amplifier as follows,
\[A = \frac{V_{out}}{V_{in2}-V_{in1}} = \frac{R_{2}}{R_{1}}\]
Hence the gain is ratio of R2 and R1 which is same as that of an inverting amplifier.
Limitation:
This difference amplifier can be used as basic instrumentation amplifier but the main limitation is the input impedance. The input impedance of this circuit is very low. Another problem is that the input impedance as seen by the two input is different. When the input Vin1=0 then the input impedance is R3+R4 = R1+R2 whereas when the input Vin2=0 then the input impedance is just R1. Such imbalance in input impedance for different inputs is not suitable for instrumentation amplifier. Another problem is that the circuit external and internal components may not be matched exactly which produces another source of output errors.
2. Buffered Difference amplifier as modified instrumentation amplifier
The limitation of the difference amplifier can be to some extent solved using buffered difference amplifier. Here we just add buffer or voltage follower at the two inputs to create high input impedance. The circuit diagram is below.
Because the gain of the buffers is just unity, it has no effect on the overall gain of the amplifier. Hence the gain of this circuit is same as that of difference amplifier.
\[A = \frac{V_{out}}{V_{in2}-V_{in1}} = \frac{R_{2}}{R_{1}}\]
Limitation and disadvantages:
The buffered difference amplifier solves one problem as instrumentation amplifier which is the required high input impedance. But the disadvantage is that we need two more op-amps. We require to use three op-amps such as LM741 or quad op-amp like LM324.
3. Two op-amp instrumentation amplifier
The limitation of buffered difference amplifier as instrumentation is that it requires two more op-amps but solves the problem of having input impedance. But we can also get high input impedance by adding just one op-amp to the difference amplifier. Thus we get two op-amp based instrumentation amplifier whose circuit diagram is shown below.
In the above circuit, \(R_{5} = R_{1}\) and \(R_{4} = R_{2}\).
We can derive the two op-amp based instrumentation amplifier gain as follows,
\[A = \frac{V_{out}}{V_{in2}-V_{in1}} = 1 + \frac{R_{2}}{R_{1}}+ \frac{2 R_{2}}{R_{3}}\]
Advantages:
The two op-amp instrumentation amplifier gain is easier to adjust using R3 as variable resistor. The CMRR is independent on R3 and thus if the ratio R2/R1 can be made set precisely we can change the gain without degrading the performance of the amplifier.
Limitation:
The main limitation of the two op-amp based instrumentation amplifier is that the resistor values for R1 and R2 must be accurately matched. Another problem is that the it treats the two inputs asymmetrically because the input Vin1 has to go through the first op-amp while the input Vin2 does not. This creates additional delay in propagating the input Vin1. At high frequencies this becomes problematic because at as the frequency is increased the CMRR decreases due to asymmetrical nature.
4. Three op-amp instrumentation amplifier
The most commonly used instrumentation amplifier is based on using three op-amps. The circuit diagram of three op-amp based instrumentation amplifier is shown below.
In the above circuit, \(R_{4} = R_{1}\) and \(R_{3} = R_{2}\).
Here two op-amps in non-inverting configuration are added before the two inputs which forms the input stage. This is like in case of buffered difference amplifier but here we are using these are non-inverting amplifier with adjustable resistor. The third op-amp is used as difference amplifier which forms the output stage.
The overall gain of the three op-amp based instrumentation can be derived to be as,
\[A = \frac{V_{out}}{V_{in2}-V_{in1}} = \frac{R_{2}}{R_{1}}(1+\frac{2 R_{f}}{R_{g}})\]
where, \(R_{f1} = R_{f2} = R_{f}\)
Advantages and features:
In three op-amp based instrumentation amplifier we can adjust the gain easily using the resistor Rg without disturbing the symmetry on the inputs. The input impedance is high because the input impedance of an non-inverting amplifier are high. The output impedance of the circuit is low because the output impedance of the difference amplifier at the output stage is low. The CMRR of the difference amplifier at the output stage is high because most of the common mode signal will be rejected.
See next LM324 Op-Amp Instrumentation amplifier design and test with multisim electronics design and simulation software and How to build LM324 Instrumentation Amplifier & Test It where instrumentation amplifier is build on breadboard and tested with Proteus Electronics Design Software.
