What is an Instrumentation Amplifier? Circuit Diagram, Advantages, and Applications

An instrumentation amplifier is one kind of IC (integrated circuit), mainly used for amplifying a signal. This amplifier comes under the family of the differential amplifier because it increases the disparity among two inputs. The main function of this amplifier is to diminish surplus noise that is chosen by the circuit. The capacity to refuse noise is familiar to every IC pins which are known as the CMRR (common-mode rejection ratio). The instrumentation amplifier IC is an essential component in the designing of the circuit due to its characteristics like high CMRR, open-loop gain is high, low drift as well as low DC offset, etc.

What is an Instrumentation Amplifier?

An instrumentation amplifier is used to amplify very low-level signals, rejecting noise and interference signals. Examples can be heartbeats, blood pressure, temperature, earthquakes and so on. Therefore, the essential characteristics of a good instrumentation amplifier are as follows.

  • Inputs to the instrumentation amplifiers will have very low signal energy. Therefore the instrumentation amplifier should have high gain and should be accurate.
  • The gain should be easily adjustable using a single control.
  • It must have High Input Impedance and Low Output Impedance to prevent loading.
  • The Instrumentation amplifier should have High CMRR since the transducer output will usually contain common mode signals such as noise when transmitted over long wires.
  • It must also have a High Slew Rate to handle sharp rise times of events and provide a maximum undistorted output voltage swing.

Instrumentation Amplifier using Op Amp

The instrumentation amplifier using op-amp circuit is shown below. The op-amps 1 & 2 are non-inverting amplifiers and op-amp 3 is a difference amplifier. These three op-amps together, form an instrumentation amplifier. Instrumentation amplifier’s final output Vout is the amplified difference of the input signals applied to the input terminals of op-amp 3.Let the outputs of op-amp 1 and op-amp 2 be Vo1 and Vo2 respectively.

Instrumentation Amplifier using Op Amp
Instrumentation Amplifier using Op Amp

Then, Vout = (R3/R2)(Vo1-Vo2)

Look at the input stage of the instrumentation amplifier as shown in the figure below. The instrumentation amplifier derivation is discussed below.

The potential at node A is the input voltage V1. Hence the potential at node B is also V1, from the virtual short concept. Thus, the potential at node G is also V1.

The potential at node D is the input voltage V2. Hence the potential at node C is also V2, from the virtual short. Thus, the potential at node H is also V2.

Input Stage of the Instrumentation Amplifier
Input Stage of the Instrumentation Amplifier

The working of the instrumentation amplifier is, Ideally the current to the input stage op-amps is zero. Therefore the current I through the resistors R1, Rgain, and R1 remain the same.

Applying Ohm’s law between nodes E and F,

I = (Vo1-Vo2)/(R1+Rgain+R1) ……………………….(1)

I = (Vo1-Vo2)/(2R1+Rgain)

Since no current is flowing to the input of the op-amps 1 & 2, the current I between the nodes G and H can be given as,

I = (VG-VH) / Rgain = (V1-V2) / Rgain……………………….(2)

Equating equations 1 and 2,

(Vo1-Vo2)/(2R1+Rgain) = (V1-V2)/Rgain

(Vo1-Vo2) = (2R1+Rgain)(V1-V2)/Rgain ……………………….(3)

The output of the difference amplifier is given as,

Vout = (R3/R2) (Vo1-Vo2)

Therefore, (Vo1 – Vo2) = (R2/R3)Vout

Substituting (Vo1 – Vo2) value in equation 3, we get

(R2/R3)Vout = (2R1+Rgain)(V1-V2)/Rgain

i.e. Vout = (R3/R2){(2R1+Rgain)/Rgain}(V1-V2)

This above equation gives the output voltage of an instrumentation amplifier.

The overall gain of the amplifier is given by the term (R3/R2){(2R1+Rgain)/Rgain}.

The overall voltage gain of an instrumentation amplifier can be controlled by adjusting the value of resistor Rgain.

The common mode signal attenuation for the instrumentation amplifier is provided by the difference amplifier.

Advantages of Instrumentation Amplifier

The advantages of the instrumentation amplifier include the following.

  • The gain of a three op-amp instrumentation amplifier circuit can be easily varied by adjusting the value of only one resistor Rgain.
  • The gain of the amplifier depends only on the external resistors used.
  • The input impedance is very high due to the emitter follower configurations of amplifiers 1 and 2
  • The output impedance of the instrumentation amplifier is very low due to the difference amplifier3.
  • The CMRR of the op-amp 3 is very high and almost all of the common mode signal will be rejected.

Applications of Instrumentation Amplifier

The applications of the instrumentation amplifier include the following.

  • These amplifiers mainly involve where the accuracy of high differential gain is required, strength must be preserved in noisy surroundings, as well as where huge common-mode signals are there. Some of the applications are
  • Instrumentation amplifiers are used in data acquisition from small o/p transducers like thermocouples, strain gauges, measurements of Wheatstone bridge, etc.
  • These amplifiers are used in navigation, medical, radar, etc.
  • These amplifiers are used to enhance the S/N ratio (signal to noise) in audio applications like audio signals with low amplitude.
  • These amplifiers are used for imaging as well as video data acquisition in the conditioning of high-speed signal.
  • These amplifiers are used in RF cable systems for amplification of the high-frequency signal.

Difference between Operational Amplifier and Instrumentation Amplifier

The key differences between the operational amplifier and instrumentation amplifier include the following.

  • An operational amplifier (op-amp) is one kind of an integrated circuit
  • The instrumentation amplifier is one type of differential amplifier
  • Instrumentation amplifier can be built with three operational amplifiers.
  • The differential amplifier can be built with a single operational amplifier.
  • The output voltage of difference amplifier gets affected because of the mismatch resistors
  • Instrumentation amplifier offers gain with a single resistor of its primary phase which does not need a resistor matching.

Thus, this is all about an instrumentation amplifier. From the above information, finally, we can conclude that this is an essential integrated circuit while dealing with low-voltage conditions. The amplifier gain can be changed by changing the resistors in the input side. This amplifier has high input resistance as well as high CMRR. Here is a question for you, what is the main function of an instrumentation amplifier?

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