1. Components - The Operational Amplifier
The operational amplifier (op-amp) is an extremely versatile device.
Formed as an integrated circuit op-amps offer virtually infinite voltage
gain and input resistance. The addition of two resistors will produce an
amplifier with a precisely defined gain. Op-amps are Active
components since they provide gain.
The op-amp can be used as a signal amplifier or as a comparator for example to
increase the amplitude of a signal or switch on an LED.
Example Circuit Symbol
+
-
RF
Input signal
(sinewave)
Output signal
(inv-sinewave)
+
-
RI
Gnd (0V)
0V
0V
Input signal
(sinewave)
Output signal
(inv-squarewave)
+
-
RI
Gnd (0V)
0V
0V

2. Manufactures use an alphanumeric code printed on the device as identification e.g.
TL071, TL082, µA741, LM358.
It is necessary to refer to manufacturers data sheets to obtain lead information and
ratings.
Single and dual op-amps are available in 8 pin dual-in-line (DIL) packages and
quad versions are available in 16 pin DIL packages. In each case pin 1 is
always on the top left.
Op-Amp Markings and Packaging
non-inverting input 3
TL
07
1
-ve supply 4
7 +ve supply
Pin 1 marker
6 Output
inverting input 2
1
5
8
Pin layout for typical single op-amp

3. Operational Amplifier Connections
The op-amp has two inputs and one output, the inverting input produces a 180o
phase shift while non-inverting input produces zero phase shift.
It is common to operate op-amps using a dual-rail d.c. supply this allows the output
voltage to swing symmetrically about 0volt level, usually within 2 volts of the
supply rails.
Inverting Input
Non-Inverting Input +
-
+ve supply
(VCC)
Output
72
3
6
4
-ve supply
(VDD)
VCC (+)
VDD (-)
Gnd (0v)

4. Gain function
Closed Loop
Uses negative feedback to control the gain and increase frequency response.
Some (or all) of the output signal is fed back to the input via a resistor (RF).
This mode of operation is used for small signal amplification.
Open Loop
No feedback is provided in open loop the gain is extremely high but frequency
response is very low.
This mode of operation is ideal for comparator circuits.
Open Loop
+
-
Vo
Vi
RF
+
-
RI
Vo
Vi
Closed Loop
100 000
VoltageGain
0 10 103
105
106
f (Hz)
10
100
1000
10 000
open loop
closed loop

5. The Inverting Amplifier
The input signal (Vi )is applied to the inverting (-) input via resistor RI.
Negative feedback is applied to control the gain via resistor RF.
RF
+
-
RI
0V
0V0V Vi Vo
Gain = = -
Vo
Vi
RF
RI
VO = - Vi
RF
RI
The negative sign shows that Vo is negative when Vi is positive, (180° phase shift),
or inverting the signal.

6. The Non-Inverting Amplifier
The input signal (Vi )is applied to the non-inverting (+) input.
A portion of the output is fed back to the inverting input via RF and RI to set
the gain.
The output signal is in-phase (non inverting) with the input signal.
Gain = = 1 +
Vo
Vi
RF
RI
Vo = Vi (1 +
RF
RI
)
RF
+
-
RI
0V
0V0V Vi
Vo

7. The Summing Amplifier
By using multiple inputs the op-amp can provide the sum of ac or dc voltages.
The gain function is the sum of the individual gain for each input.
These circuits are used as “mixers” for audio signals such as microphones,
guitars etc. and can also perform mathematical calculations in analogue
computers.
RF
+
-
R1
0V
V1
Vo
R3
R2
V2
V3
Vo = -
RF
RI
(V1 + V2 + V3)

8. The Comparator
When used in ‘open’ loop mode the op-amp has extremely high gain (typ.100,000)
this is ideal if we want to compare a varying voltage with a fixed reference.
The output (Vo) can then be used to operate an alarm, pump, lamp, power supply
regulation etc.
This type of circuit is ideal for the control of a central heating system, the room
temperature can be compared with a thermostat setting and then used to switch
the pump on or off.
+
-
0V
Vi Vo
0V
0V
Vi
Vo
+
+
-
-
-
-

9. Op-Amp Circuits
Assessment
1. An inverting amplifier circuit has a feedback resistor of 100kΩ and an
input resistor of 5kΩ. Determine the output voltage if the input is 2mV.
2. A control circuit requires a 6 volt trigger signal to operate a relay. If the
sensor output is 100mV determine suitable resistor values for input
and feedback.
3. A non-inverting amplifier has a feedback resistor of 50kΩ and an input
resistor of 2kΩ. If the input voltage is 500mV determine the output
voltage.
4. Explain why the open loop mode of operation is good for the
comparator circuit but useless for audio signals.

10. Inverting Amplifier
(practical activity)
This task is to build and test an inverting amplifier using the 741 op-amp and a
dual rail power supply.
On completing the circuit compare your results with others in the class. You may
also wish to investigate further by observing the effect of changing R2 for a
value of your choice.
circuit diagram
The activity also forms part of your achievement criteria.
R1
10k
VCC +15V
R2 100k
+
-
Gnd 0V
Vi
Vo
R3
10k
VDD -15V
Gnd 0V

11. Regulated Power Supply
The power supply uses an operational amplifier to provide a stable regulated
voltage at its output.
It uses negative feedback to control the output voltage.
VDC 11.2V
ZD1
5.6V
+
-
R1
10k
Gnd 0V
Unregulated DC
Input
R2
10k
R3
10k Regulated DC
Output
Gnd 0V
VDC 13 - 20V

12. Op-amp Comparator
(practical activity)
This task is to build and test a comparator circuit using the 741 op-amp and a dual
rail power supply.
You will gain experience in identifying and handling components, planning circuit
layout and use of test equipment.
circuit diagram
The activity also forms part of your achievement criteria.
R1
10k
VCC +15V
+
-
Gnd 0V
Vi
Vo
VR1
47k
VDD -15V