Operational Amplifier (Op-amp)

How does op-amp work? 

Op-amp is a device with two inputs, positive and negative input, denoted by the + and – signs respectively. The inputs are high impedance. As a result, it doesn’t cause loading. There is one output which is low impedance.

Fig1: Basic op-amp symbol
Op-amp summing block
Fig2: Op-Amp


You are used to seeing Figure 1. You do not see Figure 2 often. It has a summing junction and an amplification block. Control theory textbooks use this diagram often, so we are using this diagram.

The summing block has positive and negative input as in the op-amp. The summing block sums the inputs, but input at the negative terminal is multiplied by -1. As a result, there is a difference between the two inputs, and we call op-amp a differential amplifier. This can be represented by the equation V_{err}=(V+)-(V-) .

Variable “G” is called the open-loop gain. G represents the amount by which the error signal (V_{err}) is amplified. The open-loop gain of the op-amp is very high and is in the order of hundreds of thousands. The typical value of G ranges from about 20000 to 200000. The output voltage can be represented by the equation V_{out}=G \times ((V+)-(V-)).

Now let us take an example where, V+=3volts and V-=2 volts. Take G=100000, and I have a question for you. What will be the value of V_{out}? 100000V?

Well, the output will not be 100000V. Since the sum of the two input voltage is positive, as a result, the output will become as high as possible. If the sum of two voltage had been negative the output would be as negative as possible. This is called the railed output. For +5 \text{volt } V_{cc}, the positive railed output will be 5V, and negative railed output will be 0V. This is just an example, but for the actual practice, you always should see the manufacturer’s datasheet.

This is the most important thing you have to remember if you want to understand op-amp. Using this basic principle, you can analyze the complicated circuit involving the op-amp. In the next post, I will teach you how to apply this principle for positive and negative feedback circuits, as well as complicated circuits involving op-amp.

Points to Remember

  1. The input of the op-amp is high and the output is low impedance.
  2. The output of the op-amp is always railed.
  3. Depending upon the sum of inputs, the output tries to become as positive or as negative as possible.