Purpose

To learn the basics of opamps, and their use in common circuit constructions.

• Topics including gain, saturation, and op amp use in in analog circuits.

To learn how IC packages are connected, numbered, and how to find their datasheets.

Problem

AP Engineering Inc. is designing an audio amplifier circuit for a music production company. The company uses a microphone that generates voltages between -5V and +5V to carry the audio signal. They want to boost these signals to between -15V and +15V to match the rest of the audio equipment's required specifications.

It is important that the -15V to +15V signal represents a scaled version of the -5V to +5V signal generated in order to maintain audio quality.

Design & draw the schematic for a non-inverting amplifier with an appropriate gain using only:  one each of 10kΩ and 20kΩ resistors.

• • The Gain for a non-inverting amplifier is A = 1 + (Rf / Ri)

  • Use PSpice/Multisim and simulate the design with inputs of -5V, -2V, 0V, +2V, and +5V.

Your group has been tasked to create the amplifier design using the provided circuit components and perform the appropriate verification and testing before additional functionality is added.

Procedure

Part 1: DC Signals and Opamps

1. Measure the resistance of the two provided resistors and record them next to the appropriate section in the chart.

2. Insert the Op-amp into your protoboard so that the notch (or small dot) is on the left hand side. Push firmly and confirm all leads seat neatly into a hole.

3. The Op-amp must be inserted so that one row of pins is above the centre valley that separates section A-E from F-J and one below the valley. See image below.

1. Using the Proto-Lab, connect the +15V and -15V supplies (known as Rail voltage) as follows:

   • At the top, on the right side you will see ' 1.3~+15V' and '1.3~-15V'. 

   • The knobs just to the left of these act like the adjustable DC supply used in the lab. One controls the + supply and one the - supply.

   • The connections in the smaller horizontal protoboard connect to these voltage. Follow the white line to confirm which row should be used.

   • Be sure to set them to the appropriate voltage by measuring the output relative to any GND with the multimeter as you would the DC supply.  Use +15V and -15V for this lab.

   • The black Banana port is GND.

   • Record the actual values as V rail + and V rail - in the chart.

2. Connect the V+ to pin 7 and V- to pin 4 on the op-amp

3. Connect the GND from the Digital Lab to the DC supply ground.

   • The black alligator clip from your power supply should clip onto the Black Banana port on the Proto-lab (unscrew it to expose the metal post)

   • This will ensure the Grund form the DC supply and the ground (0V reference) from the proto-labs agree.

4. Build Circuit 5.1.

5. Using the DC power supply, connect +1V to VIN .

6.   Measure If & Ii. These are the current in Ri, and Rf.

   • Notice that V0 is the voltage between the ground node and the output pin on the op amp.

7. Measure V0, Ii.

   • Repeat step 10 for 2V, 3V, 4V, and 5V. Record the VO in each case to fill the chart. You do  not have to remeasure the currents.

   • You can use the analog needle in this case so long as you take care to be as accurate as possible.

8. Reset the DC supply to 1V.

9. While measuring V0 slowly turn up the voltage on the supply and note when Vo ceases to increase along with VIN 

10. Make a note of this supply voltage and the voltage output by the opamp in the top of the saturation section of the datasheet.

11. Do not disassemble or alter your circuit.

Part 2: AC Signals and Op amps

1. Power on the oscilloscope and press 'Default Setup'

2. Attach the T-connector to the High or Main port on the function generator.

3. Attach the BNC to BNC cable to the T-connector and to CH1 on the Oscilloscope.

4. Press the measure button to bring up the Vpp and Frequency measurement features of the digital oscilloscope.

5. Press the grey button next to 'CH1 Mean' then press the button next to 'Voltage'.

6. Press the button next to 'CH1' to change it to 'CH2'.

7. Press the button next to 'Type' until Vpp is selected.

8. Press the button next to return twice.

9. The measure window should now show both CH1 and CH2 Vpp to help you take measurements.

10. Set your function generator to Sine mode, 12kHz frequency & 2VPP on CH1.

    • Use the oscilloscope screen to view/measure the values as they are being adjusted.

    • Remember that 2VPP is the same as 1V above and below the zero line.

11. Attach the BNC to alligator cable the other side of the T-connector and to your Opamp circuit in place of the DC supply (VIN and GND).

12. Attach the oscilloscope probe to CH2 on the oscilloscope and clip the black lead to ground. 

13. Clip the oscilloscope probe hook to V0.

14. Press the AUTO button, and then click the vertical position knob in for each channel to bring them back to centre.  These are the small knobs just above the large scale knobs.

15. You can press the measure button again to bring back the values for interpretation.

16. Adjust the vertical scale for each wave so that they both fit on screen at the same scale.

    • To begin 1V/div gives a good result. You will have to adjust these by your own judgement for further tests.

17. Print a screenshot (or use your cell camera) showing both waves with the measurements showing.

18.  Slowly increase the amplitude to 8VPP or 4V above and below the zero line.

    • Adjust the vertical scaling as required to keep the waves in view, but keep them at the same scale.

19. Increase the amplitude up to 12VPP paying attention to any changes in the waves appearance as you do so.

20. Record the CH1 VPP value where you see a squaring off of the top of the wave (clipping/saturation) toward the bottom of the saturation section in the datasheet chart.

21. Take a final screenshot at 12VPP.

Results

1. Present your data in a neat chart similar to the one shown above.

2. Include your two screenshots.

3. What is the relationship between IIN and If  ? 

   • Explain if and how this matches our assumptions about Opamp behavior.

4. What is the relationship between VIN and V0 ?

   • At what point does this relationship fail based on your tests of DC and AC signals.

   • How do you explain this based on expected Opamp behavior?

Questions

1. If AP Engineering were asked to design a similar converter for a microphone that instead gave an input signal of ±1V rather than ±5V what value should be used for Ri to maintain the ±15V output signal.

2.  Define "saturation" as it applies to Opamps, and specify the theoretical max input signal and output signal for our circuit configuration.

3. If the rail voltage were set to ±12V rather than ±15V, what would the output be when measuring V out for an input of ±5V as V input