FT009/2
Industrial Electronics Laboratory
Lab No. 1
Objective:
Op Amp Circuits
Instruction Sheet
To observe and measure the behaviour of operational amplifiers used in standard inverting and
non-inverting configurations.
Equipment required:
Audio Signal Generator, 0-1 Volt, 100 Hz
+/- 15 volt power supply unit
Digital Multimeter
Oscilloscope
Breadboard
Test Leads
Background:
Operational amplifiers – op amps – are very useful building blocks for designing analogue circuits to perform a
variety of functions. In this lab, we will
•
Measure the open loop differential voltage gain – AO/L of an op amp
•
Measure the gain of an inverting amplifiers
•
Examine the effect of signal frequency on the gain of an inverting amplifier
The op amp chosen for this lab exercise is the TL081. This is produced in a 8-pin dual in-line package (DIL
package or DIP). The connections to the op-amp as viewed from the top, are specified in the diagram below.
Typical AO/L for this device is about 200,000.
How to measure the Open Loop Voltage Gain of an Op Amp.
Normally we would measure the voltage gain of an amplifier by measuring the input voltage amplitude – Vin –
and the output voltage amplitude Vout. The voltage gain A is then calculated from:
A=
Vout
Vin
In the case of the op-amp, because the open loop voltage gain is so large, it is difficult to apply the very small
differential input voltages (of the order of microvolts) that are required in order to avoid saturating the output of
the amplifier – i.e. such that the op amp output voltage Vout < Vsupply (where Vsupply is the amplitude of the power
supply voltage to the op amp).
We can however measure the op amp open loop gain by building a unity gain buffer amplifier as shown below
and applying Vsource = 5V rms at 100 Hz. Ideally the theory says that the output voltage will also be 5V rms.
However, in practice, if we measure Vout we will find that it is very slightly smaller by an amount Vdiff. By
measuring Vdiff we can calculate the open loop voltage gain AO/L from
AO / L =
Vout
V diff
FT009/2
Industrial Electronics Laboratory
Procedure:
To measure Op Amp Open Loop Gain - AO/L:
1.
Connect the op amp buffer amplifier circuit together on the breadboard. Use colour-coded wire if
possible to connect the op amp circuit to the dc power supply and the input signal source.
•
Ensure that the power supply and the signal source are switched OFF until all wiring has
been completed to avoid possible damage to the chip.
•
Double check your wiring to ensure that the circuit has been correctly assembled before
turning on the power supply and the signal source.
2.
Turn on the dc power supply and set the voltage to about +12 volts and –12 volts for the + and – supply
rails respectively.
3.
Turn on the signal source and set it to provide a 5 volt rms sine wave at 100 Hz – use the DMM to
accurately set the voltage amplitude.
4.
Connect the DMM between the inverting and non-inverting input terminals of the op amp and measure
the ac voltage Vdiff.
5.
Connect the DMM between the op amp output terminal and the zero volt line and measure Vout.
6.
Turn off the signal source first and then the dc power supply.
7.
Calculate the open loop differential voltage gain of the op amp using the values measured at step 4 and 5
above. Write your calculations up in the space provided on the results sheet. Does the calculated value
seem to be close to that expected?
To Measure the Gain of an Inverting Amplifier
8.
9.
Connect the inverting amplifier circuit together on the breadboard. Use colour-coded wire if possible to
connect the op amp circuit to the dc power supply and the input signal source.
•
Ensure that the power supply and the signal source are switched OFF until all wiring has
been completed to avoid possible damage to the chip.
•
Double check your wiring to ensure that the circuit has been correctly assembled before
turning on the power supply and the signal source.
Turn on the dc power supply - the voltage should still be about +12 volts and –12 volts for the + and –
supply rails respectively.
10. Turn on the signal source and set it to provide a 1 volt peak sine wave at 100 Hz – use the oscilloscope
channel 1 to set this voltage amplitude. This is Vin.
11. Connect channel 2 of the oscilloscope between the op amp output terminal and the zero volt line and
measure Vout.
12. Simultaneously displaying channel 1 and channel 2 waveforms on the oscilloscope, sketch the
waveforms to scale on the results sheet provided and comment on the phase relationship between them.
13. Turn off the signal source first and then the dc power supply.
14. Calculate the inverting amplifier voltage gain of the circuit using the values measured at step 10 and 11
above. [Remember A =
Vout
]. Write your calculations up in the space provided on the results sheet.
Vin
Does the calculated value seem to be close to that expected?
15. Repeat steps 10 – 14 with the signal source set to frequencies 1000 Hz, 10,000 Hz and 100,000 Hz and in
each case write up the results on the results sheet. Is the gain the same at all frequencies? How is the gain
affected by the frequency of the signal that is being amplified?
FT009/2
Industrial Electronics Laboratory
Results:
Step 4:
Vdiff =
Step 5:
Vout =
Step 7:
AO / L =
Vout
=
V diff
Step 10:
Vin =
Step 11:
Vout =
Step 12:
Vertical volts/div:
Horizontal time/div:
Step 12:
Step 14:
Comment:
A=
Vout
= ---------- =
Vin
Comment:
=