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1. Non-Linear Op-Amp Applications

Aims

· to investigate non-linear applications of operational amplifiers.

· to continue to encourage good constructional practice and test procedures.

Objectives

Upon successful completion of this experiment, students should

· have explored practical comparator circuits with and without hysteresis.

· have explored a practical precision rectifier circuit.

· have discovered any non-ideal aspects of the behaviour of these circuits

Deliverables

i) Circuit diagrams for all experiments.

ii) Calculations for all parts.

iii) Sketches and screen shots of voltage waveforms for the circuits.

iv) Comments on the results of the experiments.

v) Conclusions, reflecting on what you have learned from the experiment.

If you need to write this experiment up

· Compare your observations with what you expect from theory and the datasheet.

· Use the measured saturation voltage to calculate component values and comment on any differences from the values you used.

Components and Equipment

One 741 operational amplifier

One small signal silicon diode (1N4148)

One circuit breadboard

Additional resistors, wires etc. as required

Standard bench test equipment

Cables: three-way power cable and three BNC-to-4mm plug cables

Multisim software

Experiment

1) The Operational Amplifier with no feedback

In this part of the lab, you will investigate the performance of a 741 op amp driven into saturation.

a. Before the lab

(i) Find a data sheet for the 741 op amp on the internet and use it to identify the functions of the pins numbered in Figure 1. You should sketch the layout of the op amp in your lab book with the pins labelled so that you can connect it later.

(ii) The datasheet will also contain the expected output voltage swing and the slew rate, make a note of these in your lab book. Use this slew rate to calculate the minimum time that the output will take to switch from a positive to a negative saturation voltage. You can assume the saturation voltages are ±14 V for now.

(iii) In the lab you will connect a 6V pk-pk sine wave input to the op-amp with frequencies of 50 Hz, 500 Hz and 5 kHz. Calculate the period of one cycle for these signals. At what frequency will the output be most affected by the slew rate?

b. During the Lab

Log into the PC and switch on the Virtualbench. When the computer is ready, launch NI Virtual Bench. When it opens find Setup in the top left corner and select “Default”. This will reset the Virtualbench to factory settings and erase those of the last user. However, you should make sure that the power supply and signal generator are switched off before you start.

Using just the op-amp with ±15 V supplies, connect the non-inverting input to 0 V and the inverting input to the signal-generator (hence forming an inverting comparator). Sketch your circuit in your lab book.

Test the circuit using sine wave inputs of 6 V pk-pk magnitude and frequencies of 50 Hz, 500 Hz and 5 kHz. Sketch the resulting input and output waveforms and either save screen shots or export the data from the oscilloscope. Using the cursors measure the actual saturation voltages for the op-amp. Using the highest input frequency, again using the cursors, measure the time taken for the output voltage to switch from negative to positive. Hence calculate the slew rate of the op-amp and compare it to the value given on the datasheet. Save a screen shot of the switching time measurement with the cursors.

Table 1 Available resistor values (W)
51	1.0 k	10 k	100 k
100	1.2 k	12 k	120 k
120	1.5 k	15 k	150 k
150	1.8 k	18 k	180 k
180	2.0 k	22 k	220 k
220	2.2 k	27 k	270 k
270	2.7 k	33 k	330 k
330	3.0 k	39 k	390 k
390	3.3 k	47 k	470 k
470	3.9 k	56 k	560 k
560	4.7 k	68 k	680 k
680	5.6 k	82 k	820 k
820	6.8 k	91 k	1000k
	8.2 k
	9.1 k

2) The Inverting Schmitt trigger

We will now introduce hysteresis into the inverting comparator, enabling it to switch at threshold values of ±1V.

a. Before the Lab

To achieve the requirement, we will implement a Schmitt trigger. As the power supplies are ±15 V and thresholds are ±1V (both symmetric with respect to 0), we are to design a symmetric inverting Schmitt trigger.

(i) Sketch the circuit for a symmetric inverting Schmitt trigger.

(ii) Calculate the required resistance (this is the same case as week4 tutorial Q1). You may use ±14V as the saturation voltages. However, it would be good to also keep the expression with saturation voltages as variables, as it is possible to observe different saturation values in your actual lab.

(iii) Using the list of available resistors in Table 1, choose appropriate resistor values to give thresholds of ±1V. Remembering that all resistors should be between 1kW and 1MW. If necessary, you can use resistors in series, but this should be avoided if possible.

b. During the Lab

Construct the Schmitt trigger circuit and repeat the tests carried out in part 1) for 50Hz only. Sketch and save screen shots of the following:

· Inverting terminal waveform (Channel A) and the output waveform (Channel B);

· Non-inverting terminal waveform (Channel A) and the output waveform (Channel B).

Are they what you expect? Comment on the results. Read out the threshold voltages when the output waveform switches. How do they compare with the design specification?

Now from the drop-down menu at the top right corner of the oscilloscopr panel, change ‘Time Mode’ from Normal to XY. Connect the INPUT to Channel 1 and OUTPUT to Channel 2. The upper graph on the front panel will display channel 1 and channel 2 of the oscilloscope (plotted against time). The lower graph shows channel 1 plotted against channel 2 (called XY mode). Observe the hysteresis curve. Compare the actual threshold values to the design values. You may need to adjust the ‘Acquisition Time’ slider or click ‘Auto’ in order to display the plots properly. Save the screen shot with the measurement of the threshold values.

3) Schmitt trigger with asymmetric thresholds (simulation based)

In the lab, it is recommended that you complete the experimental task-4 on rectifier first before completing the simulation-based task-3 and task-5.

We will now modify the Schmitt trigger so that it switches at threshold voltages not centred on 0 Volts. This is useful if we want the Schmitt trigger to switch when the input crosses a particular value, but we don’t want noise to cause it to switch back again. We will choose threshold levels of +2 V and +1 V.

a1. Before the Lab (complete the calculations before you go to the lab. This is the same as week 4 tutorial Q2)

(i) From your course notes, draw the circuit diagram for an asymmetric Schmitt trigger in which the threshold voltage is fixed using an arbitrary reference voltage.

(ii) Determine the reference voltage V_ref and the relationship between the resistor connected to the output R₂ and the resistor connected to the reference R₁. Remembering that all resistors need to be between 1kW and 1MW, choose a value for R₂, from the available resistors listed in Table 1, to give you a working value for R₁.

(iii) Since the V_ref value you have obtained is unlikely to be available, following the course notes, redesign the circuit so that V_ref and R₁ are replaced by the power supply voltage connected of +15V connected to resistors r₁ and r₂. Your working value of R₁ will enable you to calculate the values of r₁ and r₂. If necessary, you can use resistors in series, but this should be avoided if possible. Sketch your final design.

a2. It is recommended that you also complete this section a2 before you go to the lab. This will ensure that you complete the lab in time.

Now, construct your redesigned circuit in Multisim.

You can find the 741 op-amp from Place>>Component>>Group/Analog>>Opamp>>741. Make sure you know the functions of the pins. The 741 pin numbers in the simulator should be the same as those from the data sheet. You will also need:

· AC voltage source: Place>>Component>>Group/Source>>Signal_voltage_sources >>AC_voltage

· DC power supply: Place>>Component>>Group/Source>>Power_sources>>DC_power

· Earth: Place>>Component>>Group/Source>>Power_sources>>Ground

Of course, there are other ways to locate these components such as from the toolbars. Wire the components up carefully and make sure they all join. Figure 2 shows an example (not a Schmitt trigger). Use ±15 V supplies. Find the Oscilloscope from the vertical toolbar bar down the right-hand side of the programme window.

Figure 2

If you want, try to complete the following task before the lab too.

a. During the Lab

You should have at least attempted building the circuit in Multisim. Now complete the task. Repeat the tests carried out in part 1) for 50Hz only. Sketch and save screen shots of the following:

· Inverting terminal waveform (Channel A) and the output waveform (Channel B);

· Non-inverting terminal waveform (Channel A) and the output waveform (Channel B).

Are they expected? Explain the results.

Now in the Oscilloscope, change the display mode from Y/T (time domain) to B/A as shown in Figure 3. The oscilloscope will show the ouput (Channel B) as a funciton of the input (Channel A). You should see the hysterises curve. Observe the hysteresis curve. Compare the actual threshold values with the design values. Save the screen shot with the measurement of the threshold values.

Figure 3

4) Half wave precision rectifier

Now we will explore the performance of a precision rectifier circuit.

a. Before the Lab

Sketch the precision rectifier circuit from the notes in your lab book. When the input goes from negative to positive, the output of the op-amp, (V₁ in the notes) increases rapidly from the negative saturation value, but the diode will not conduct until V₁ reaches around 0.5 - 0.7 V. Using the slew rate from the datasheet calculate how long this switching process should take.

b. During the Lab

Construct a half-wave precision rectifier circuit using a 1 kW load. Test the circuit again using sine wave inputs of 6 V pk-pk magnitude and frequencies of 50 Hz, 500 Hz and 5 kHz. Sketch the resulting input and output waveforms as well as the op-amp output (V₁ in the notes) waveform and comment on the results. Save screen shots or export the data.  Measure the difference between V₁ and the output voltage for positive voltages using the cursors.

For a frequency of 5 kHz, use the cursors to measure the time that V₁ takes to switch from the negative saturation value to the point at which it is following the input voltage. Compare this time value to the one calculated before the lab using the slew rate.

5) Oscillator (simulation based)

Finally, design an oscillator to oscillate at 100 Hz using Multisim.

a. Before the Lab

Sketch the oscillator circuit in your lab book. Choose appropriate components.

b. During the Lab

Construct an oscillator using appropriate component values and the same power suppliers. Save your design. Sketch the resulting waveforms from the output of the op-amp as well as from the capacitor terminal. Save screen shots or export the data. Measure the period of the waveform and calculate the frequency. Compare this with the specification.

Well done!

This is the end of Lab 1. Switch off the Virtualbench. Before logging out of the PC, make sure you have saved all data and Multisim files securely so that you may retrieve them for your lab report if the relevant task is selected.

Now clean up your desk and put the cables back to where you found them.

Thank you!