Operational Amplifier (Op-Amp) Explained

What is the Operational Amplifier?

As its name suggests, the Operational Amplifier is one type of amplifier. The basic function of any amplifier is to amplify the input signal. But apart from amplifying the signal, it is also possible to perform different arithmetic operations using op-amp.

In the early days, when digital computers were not evolved, then op-amp was used to perform the different arithmetic operations like addition, subtraction, integration, and differentiation.

That’s the reason it is known as the Operational Amplifier. (An amplifier, which can perform different (arithmetic) operations)

The Circuit Symbol of Op-Amp

Fig. 1 The circuit symbol of an Operational Amplifier

As shown in the figure, the op-amp consists of two inputs, one output, and two power supplies. (positive and negative power supplies).

Some operational amplifiers work on the single power supply. (Such op-amps are known as single supply op-amp)

In the op-amp circuit symbol, the input terminal marked as positive is known as a non-inverting input terminal, and the terminal marked with a negative sign is known as the inverting input terminal. (as shown below in the figure)

V+ is the positive biasing voltage

V- is the negative biasing voltage

Fig.2 Inverting and non-inverting input terminals of the op-amp

Operation of Op-Amp in the open-loop configuration:

In the open-loop configuration, the op-amp is operated without any kind of feedback.

In the open-loop configuration, the op-amp amplifies the difference between the two input terminals (Between inverting and non-inverting inputs)

As shown in the fig.1, if V1 and V2 are the inputs at non-inverting and the inverting input terminals, then the output of the op-amp

Vo = Aol x (V1 – V2)

where Aol – open-loop gain of the op-amp

As shown in Fig.3, if the input is applied at the non-inverting input terminal (V1) and another input terminal is grounded then the output can be given as

Vo = Aol x V1

Fig. 3 Operation of the op-amp in open-loop configuration with the input applied at the non-inverting input terminal

Similarly, as shown in Fig. 4 when the input is applied only at the inverting input terminal and non-inverting input is grounded then the output can be given as

Vo = – Aol x V2

Fig. 4 Operation of the op-amp in open-loop configuration with the input applied at the inverting input terminal

If the differential input Vd is applied between the non-inverting and the inverting input terminal (as shown in fig. 5), then output in the open-loop configuration can be given as

Vo = Aol x Vd

Fig. 5 Operation of the op-amp in the open-loop configuration with differential input

Typically, the open-loop gain of the op-amp is very high. (In the range of 10^5 to 10^6).

Even for very small differential input, the output of the op-amp will get saturated.

Example:

if Vd = 1mV and Aol = 10^5, then Vo = 100V. (Theoretically)

But the output of the op-amp will be limited to positive and the negative saturation voltage (± Vsat).

For one op-amp, if the saturation voltages are ±12V, then for the above example, the output will be restricted to 12V,

And even for Vd = 5mV, Vo = 12V.

So, in such a case, it is said that the op-amp is operating in the saturation region. Typically the saturation voltage is less than the biasing voltages of the op-amp.

Voltage transfer curve of the op-amp in open loop configuration:

The same phenomenon explained above can also be explained with the help of the Voltage transfer curve of the op-amp.

Fig. 6 shows the voltage-transfer curve of the op-amp in the open-loop configuration.

Fig. 6 Voltage transfer curve of the op-amp in the open-loop configuration

As shown in the figure, the output voltage Vo is represented in the Y-axis and differential input to the op-amp is represented on the X-axis.

It is evident that, in the open-loop configuration, for very small differential input only (typically in μV) the output of the op-amp will operate in the linear range. Beyond that, the op-amp will operate in the saturation region. (Because of the very high gain of the op-amp). In the linear region, the slope of the curve represents the open-loop voltage gain of the op-amp.

In the open-loop configuration, the op-amp can be used as a comparator. Apart from that, with the feedback, the op-amp can be used in various applications.

Here is the few important applications of the op-amp.

Applications of the Operational Amplifier:

Comparator
Active Filters
Oscillators and Multivibrators
Waveshaping and Waveform generating circuits
Analog to Digital Converter (ADC)
Digital to Analog Converter (DAC)
Linear Amplification
To perform the arithmetic operation on the signal (Addition, Subtraction, Multiplication, Integration, Differentiation etc.)

Equivalent Circuit of the Operational Amplifier:

Fig. 7 Equivalent circuit of the operational Amplifier

Fig. 7 shows the equivalent circuit of the op-amp.

Where,

V1, V2 – Non-inverting and inverting input of the op-amp

Vd = V1 – V2

Ri – Input resistance of the op-amp

Ro – Output Resistance of the op-amp

A- Open loop gain of the op-amp

Characteristics of Ideal Op-Amp:

As, mentioned above, the op-amp is a very versatile IC and can be used in various applications. Because of its favorable characteristics, it is used in various applications.

Here is the list of characteristics of the ideal op-amp

Infinite Input Impedance
Zero Output Impedance
Infinite Voltage Gain
Infinite Bandwidth
Infinite Slew Rate
Infinite Common Mode Rejection Ratio (CMRR )
Zero input offset voltage (Output is Zero when input is Zero)

The characteristics of the actual op-amp will be different from the ideal op-amp. To get an idea, the characteristics of the very popular op-amp IC 741 is shown below:

Table 1: Characteristics of op-amp IC 741

To get more information about op-amp, check this video on Operational Amplifier .