Tutorial 7 A - Introduction to Operational Amplifiers

This tutorial is in several parts.  The first part deals with the basics of operational amplifiers.  The other parts deals with how operational amplifiers are used in circuits.  It is quite a long topic.

Learning Objectives

• recall the characteristics of an ideal op-amp and be aware how these may be different for a typical op-amp;

• know, understand and use the difference between inverting and non-inverting inputs;

• understand the power supply requirements and output voltage swing limitations of real op-amps leading to saturation;

• describe, understand and explain the use of an op-amp in a comparator circuit.

The operational amplifier (op-amp) was originally developed for analogue computing (our PCs are digital computers) and, when introduced, were complex and expensive components.  The full circuit diagram of the op-amp is shown here. Graphic by Daniel Braun, Wikimedia Commons

Now they are in integrated circuit form and are very cheap, about 50 p (0.6 Euros).  They are not very spectacular, but are extremely useful. Operational amplifiers require a dual power supply, which means having a central 0 volts rail, and a + 15 V rail and a – 15 V rail.  The full circuit diagram is shown below, but generally we will ignore the power supply. Notice that the op-amp has two inputs and one output.  It amplifies the difference between the inverting input and non-inverting input. Be careful not to confuse the symbol with a non-inverting gate.

The ideal op-amp should have the following characteristics:

• Infinite open loop gain.

• Infinite input impedance so that no current is drawn (Impedance will covered later)

• Zero output impedance so that maximum current can be transferred to the load.

• Very wide bandwidth.

 Question 1 Answer the interactive question In practice the maximum open loop gain is 200 000.  Beyond that limit the amplifier goes into saturation which means that the voltage cannot go any higher.  The voltage is limited, of course, by the supply voltages.  In practice the limits are rather lower than this, about 1.5 to 2 V below the value of the supply.

We can work out the input voltage that will give this swing.  Suppose the supply voltage was 15 V.  The maximum output voltage would be 15 – 1.5 = 13.5 V

So using a gain of 200 000:

Input voltage = ± 13.5 ÷ 200 000 = ± 67.5 mV

Therefore the input voltage can only swing through 135 mV in total to go from a negative saturation to a positive saturation.

The graph shows how the op-amp behaves: Notice the following:

• The op-amp amplifies the difference between the two input voltages

• When the difference exceeds the limits X and Y, the output is saturated.

• In between the limits the graph is linear, so there is little distortion in these regions. The op-amp cannot increase the power by having a very high gain.

 Question 2 Answer the interactive question Using the Op-Amp as a Voltage Comparator

The op-amp being a differential amplifier is very useful as a voltage comparator.  It will give a high positive or negative voltage, depending on which voltage is higher:

• If the non-inverting input is higher, the output voltage will be positive

• If the inverting input is higher, the output voltage will be negative.

• The voltage difference must be greater than 135 mV if saturation is to occur.  This is not difficult to achieve.

The voltage comparator gives a digital output from an analogue input.

The diagram below shows the voltage comparator used as a light operated switch. The diagram shows the voltage comparator used as a light operated switch.  This is how it works:

• The reference voltage is adjusted with the variable resistor so that it is equal to the voltage V2.

• When light falls on the LDR, V2 goes up because the resistance of the LDR goes down.

• V2 will be bigger than the reference voltage, so that the op-amp goes into saturation.

• The LED lights up, showing a high output.

• The reverse-biased diode protects the LED for when there is a large negative voltage.

 Question 3 The LDR in this circuit has a light level resistance characteristic as shown in the graph below: (a) The potentiometer is set half-way down its track.  Calculate the voltage at X and Y. (b) Work out the resistance of the LDR at which the output of the op-amp just changes. (c) What is the light level at which this happens? (Resistance in kilohms is on the vertical axis). Summary

Ideal Op-amp

• Infinite input impedance so that no current is drawn

• Infinite gain

Real Op-amp

• Gain about 100 000

# Using the Op-Amp as a Voltage Comparator

• It will give a high positive or negative voltage, depending on which voltage is higher.

• If the non-inverting input is higher, the output voltage will be positive

• If the inverting input is higher, the output voltage will be negative.

• The voltage comparator gives a digital output from an analogue input.

 Links St Andrews University Notes Electronics Tutorials on the Op-amp Video tutorial Radio-Electronics notes