The operational amplifier (op-amp) is the most widely utilized analog electronic subassembly; it is the basis of instrumentation amplifiers, filters, and myriad analog and digital data processing equipment.

The electrical signals produced by most transducers are at a low voltage and or power level, often it is necessary to amplify them before they are suitable for transmitting, further analog or digital processing, indication, or recording.

There are three types of amplifiers.

Depending on the requirement of system, the amplifiers are selected. They are:

• Inverting and non inverting amplifiers

• Differential amplifiers

• Instrumentation amplifiers

(a.) Inverting and non inverting amplifiers

These types of amplifiers are single embedded amplifiers with one terminal of input grounded. The voltage gain for inverting amplifier can be calculated as,

$\frac{eo}{e1}$=$\frac{-R2}{R1}$

And for non inverting amplifier it is calculated as,

$\frac{eo}{e1}$=1+$\frac{R2}{R1}$

Both inverting and non inverting amplifiers are capable of delivering any desired voltage gain. The input impedance of inverting amplifier non inverting amplifier has an infinite input impedance. The non inverting amplifier has better performance.

(b) Differential Amplifiers:

In this type of amplifiers both the input terminals are floating. There are wide ranges of applications for this amplifier in instrumentation. The differential amplifier is as shown:

By using superposition theorem, the output voltage can be calculated as, eo = $\frac{R4}{R3+R4}$ (1+$\frac{R2}{R1})$e2- $\frac{R2}{R1}$(e1) However, such a differential amplifier with a single op-amp with a single op-amp configuration suffers from the limitation of finite input impedance The performance of the amplifier can be judged on the basis of offset, drift, input impedance, gain & bandwidth.

(c) Instrumentation Amplifier:

Sometimes there is a need to amplify a small differential voltage to a few hundred times its value. A single stage differential amplifier is not capable to perform this job because of several reasons. In such a case input impedance is finite but the achievable gain is limited. Therefore, improved version of amplifier is required. A three op-amp instrumentation amplifier is used in such case. A three op-amp instrumentation amplifier is as shown:

In this amplifier no current will be drawn by the input stage of op-amps. The same current I will flow through R1 and R2.

Therefore,

I =$\frac{(e1-ei1)}{R1}$=$\frac{(ei1-ei2)}{R2}$=$\frac{(ei2-e2)}{R1}$

∴ e1 = ei1 + $\frac{R1}{R2}(ei1-ei2)$

e2 = ei2 - $\frac{R1}{R2}(ei1-ei2)$

The main advantages are:

(i.) High differential gain

(ii.) Infinite input impedance

(iii.) Moderate bandwidth