# PHY C21: Operational Amplifiers

In this chapter, we will take a look at one of the most widely used electronic devices: the operational amplifier (Op-Amp for short)!

• Use of operational amplifiers as comparators
• Properties of ideal op-amps
• Graphs of input vs output voltage

What is an operational amplifier?
It is a type of integrated circuit which:

• consists of transistors, resistors, & capacitors
• can carry out multiple operations such as:
• amplifying voltages
• comparing 2 input voltages & providing a corresponding output voltage
• acting as a switch dependent on the level of input voltage

A complete circuit diagram of an op-amp looks like this:

…but we prefer to simplify it to this:

Before explaining how it works, let’s take a look at its parts & properties:

Anatomy of an op-amp

If these don’t make sense yet, try reading how an op-amp works below first, then return here.

How does an op-amp work?
At its most basic use, an op-amp is a COMPARATOR.

What is a comparator?
A device which delivers an output depending on the difference between 2 inputs. It compares the 2 inputs.

Once an op-amp has a D.C. power supply, here’s how it operates:

What’s this value A0?
Open-Loop Gain
or Amplification of the op-amp.
It is the measure of how many times the output voltage is greater than the difference between inputs.

IF Vout < Vsupply,

## Vout = A0(V+-V–)

What happens if A0(V+-V) exceeds the power supply?
The output voltage becomes SATURATED.
This means that it cannot increase above the power supply. In this case, we cannot calculate A0 using Vout/Vin.

# Here’s a flowchart to summarise the working of an op-amp:

Properties of an IDEAL op-amp
These only apply to op-amps which are PERFECT – real-life ones don’t always have these properties!

# Using an ideal op-amp as a Comparator

Here’s an example of one application:

When T > 25°C:

When T < 25°C:

One last thing: what if the input voltage varies over time?

How does the output voltage of an op-amp vary?
Let’s look at 2 different examples, both with supply voltages of ±6V.

### Non-ideal Op-Amp

For example:

• Red: input voltage (V+ – V)
• Blue: output voltage (Voutput)
• In this case, Vsupply = ±6V

### Ideal Op-Amp

For example:

• Red: input voltage (V+ – V)
• Purple: output voltage (Voutput)
• In this case, Vsupply = ±6V