Curriculum and Labs for Engineering Education

cancel
Showing results for 
Search instead for 
Did you mean: 

myDAQ mini-lab: Op Amp II -- Terminal currents

Course Linkage: Linear Circuit Analysis >> Operational Amplifiers >> Terminal Behavior

Measurement Techniques: ELVISmx DMM (voltmeter and ammeter)

 

<hr width=”75%”>

 

Introduction

Overview: An operational amplifier (op amp) is a type of voltage amplifier, a device that senses its input signal voltage without drawing any current from the signal source, increases (amplifies) the voltage signal and delivers whatever current is necessary to the load to sustain the amplified voltage. Creating an output voltage with non-zero load current without drawing any current from the signal source requires energy; consequently the op amp requires connections to a power supply. The op amp draws current from the power supply and directs the current to the load.

 

Objectives: In this mini-lab you will:

  • Measure the terminal currents of an op amp under no-load and loaded conditions
  • Chart the flow of current through the op amp and power supplies
  • Calculate the power required by an op amp under no-load and loaded conditions

 

Equipment

 

  • NI myDAQ
  • Breadboard
  • Connecting wire
  • Alligator clip test leads
  • Texas Instruments (TI) TL072 op amp or similar device
  • Resistors, ¼-W 5% carbon film: 1.0K (two)
  • LEDs (light emitting diodes) (two)

 

 

NOTE: Any dual-supply op amp is suitable for this project. Be sure to use the correct pin numbers, though, because these vary from one device to the next. Refer to the data sheet for your device (search www.datasheetcatalog.com) or ask your instructor for assistance.

 

Deliverables

 

  • Submit your work in the form of a homework set problem or lab notebook entry according to the requirements of your instructor
  • Submit your work for each underlined boldface item, and clearly label the item with its section letter and task number

 

A. Experience the op amp terminal currents by measurement

If you have just completed Op Amps I – Terminal Voltages and your circuit set up has not changed:

  • Omit Steps 2 through 10 and use your previously recorded terminal voltages
  • Disconnect the jumper wire that connects the op amp output to the resistor and LED circuit; leave the resistor and LED circuit in place, though; they will be needed later in this mini-lab.

 

  1. Draw the circuit of Figure 1, a generic op amp connected to its power supply and input signal voltage sources. The op amp terminal currents defined on this diagram follow the standard convention for electronic devices: a positive value for a current indicates that the current enters the device.
    figure 1 -- op amp terminal current definitions.png
  2. Draw the circuit of Figure 2, the TI TL072 op amp (or your specific op amp, if different) connected to myDAQ.
    fig 2 -- mydaq circuit.png
  3. Construct the circuit of Figure 2 with myDAQ and a breadboard; see Figure 3 for the recommended layout and Figure 4 for the myDAQ connections.
    fig 3 -- op amp terminal voltages - bb layout.pngfig 4 -- mydaq connections.png
  4. Connect the DMM COM (black) probe permanently to ground to save effort. Use probe clips if you have them available, or coil a wire around the probe tip with one end of the wire long enough to connect into the breadboard.
    TIP: Wrap the wire around the probe tip to form a coil, remove the coil and bend it slightly, and place it back on the coil (bending the coil ensures a snug fit):
    probe clip coiled wire.png
  5. Download and run “Set AO Voltage.vi” linked at the bottom of this document. Use this VI to operate the analog outputs AO0 and AO1 as variable voltage sources applied to the ‘+’ and ‘-‘ inputs of the op amp.
  6. Start the ELVISmx DMM and configure it as a DC voltmeter with autoranging.
  7. Measure and record the myDAQ +15V supply voltage; this is labeled as VCCp on the circuit of Figures 2. Touch the red DMM probe directly to the op amp pin to ensure that the device connects properly to the power supply.
  8. Measure and record the myDAQ -15V supply voltage; this is labeled as VCCn on the circuit of Figure 2.
  9. Measure and record the signal directly at the ‘+’ pin of the op amp; this is labeled as Vp on the circuit of Figure 2. Vary the AO0 slider of “Set AO Voltage” to confirm that the applied voltage does indeed appear at the pin.
  10. Measure and record the signal directly at the ‘-‘ pin of the op amp (this is labeled Vn) and vary the AO1 slider.
  11. Remove the DMM probes from the circuit.
  12. Reconnect the DMM probes to the “myDAQ_DMM_A” and “myDAQ_DMM_COM” jacks.
  13. Use DMM probe clips if you have them available, or coil a wire around the probe tip with one end of the wire long enough to connect into the breadboard; see Step 4.
  14. Change the ELVISmx DMM to a DC ammeter with the 20 mA fixed range.
  15. Connect the probe clips to measure the op amp positive power supply terminal current ICCp as shown in Figure 5. Note that you must remove the existing wire that connects the op amp to the power supply and replace it with the ammeter. Also note that the ammeter reports a positive value when the current enters the red “A” jack and exits the black “COM” jack. Figure 6 shows the recommended breadboard layout and Figure 7 shows the myDAQ connections.
    fig 5 -- measure ICCp.pngfig 6 -- measure ICCp bb layout.pngfig 7 -- myDAQ connections for current.png
  16. Measure and record the current ICCp for three input voltage conditions: Vp >> Vn (Vp much greater than Vn), Vp << Vn (Vp much less than Vn), and Vp = Vn.
  17. Draw a modified version of Figure 5 to show how you would measure the current ICCn that enters the op amp negative supply terminal (see Figure 1 for the definition of ICCn).
  18. Connect the ammeter to measure ICCn (remember to replace the wire for the positive power supply); measure and record ICCn for the three input voltage conditions you used earlier.
  19. Measure and record Ip for the three input voltage conditions.
  20. Measure and record In for the three input voltage conditions.
  21. State (but do not attempt to measure!) the value of Io, the current entering the op amp output terminal for the three input voltage conditions. Hint: Nothing is connected to this terminal!
  22. Create a data table to present all of your measured currents for this section.

 

The next video shows some expected results for this section.

 

 

B. Learn the underlying principles

The following video tutorial discusses the terminal currents and power requirements of an op amp. The tutorial also shows how Kirchoff’s Current Law (KCL) applies to an op amp:

 

 

C. Connect the principles to your measurements:

  1. Add the LED indicators to the output of your op amp as shown by the schematic diagram in Figure 8. Figure 9 shows the recommended breadboard layout. LED1 indicates positive saturation and LED2 indicates negative saturation.
    fig 8 -- mydaq circuit with LEDs.pngfig 9 -- mydaq circuit with LEDs - bb layout.png
  2. Measure and record the five op amp terminal currents defined in Figure 1. Use the same input signal voltage ranges (Vp >> Vn, Vp << Vn, and Vp=Vn) for each current.
  3. Create a data table to present all of your measured currents for this section.
  4. Show that KCL applies to the entire op amp for each of the three input signal voltage ranges.

 

D. Build your intuition:

The circuit of Figure 10 shows the power supply voltage sources and input signal voltage sources for the circuit that includes the LED indicators:

fig 10 -- op amp terminal currents with LEDs.png

  1. Draw this circuit and use your measured currents for the Vp >> Vn input condition to complete the current indicators. For example, your measured value for ICCp should be a positive value; write this value next to the current arrow entering the top of the op amp. As another example, your measured value for ICCn should be a negative number. The related current arrow leaves the bottom of the op amp in Figure 10, opposite to the definition you used in Figure 1. Consequently, change the sign of your measured value for ICCn and write it next to the arrow. Complete the two “unarrowed” current indicators to show the direction of positive current flow.
  2. Repeat the previous step for Vp << Vn.
  3. The power delivered by the myDAQ +15V source is the product of your measured values VCCp and ICCp; the power delivered by the myDAQ -15V source is the product of your measured values VCCn and ICCn. Calculate the power for each power supply source for the Vp>>Vn input condition and write these values next to the power supplies
  4. Repeat the previous step for Vp<<Vn.
  5. The op amp “sources” current to the load when positive current leaves the op amp output terminal. Respond: Which source “works hardest” (delivers the most power) when the op amp output sources current to its load?
  6. The op amp “sinks” current from the load when positive current enters the op amp output terminal. Respond: Which source “works hardest” (delivers the most power) when the op amp output sinks current from its load?
  7. Respond: Do the op amp power supplies need to work at all (i.e., supply power to the op amp) when the op amp output is disconnected from a load? Explain your answer.
  8. Respond: Does the op amp ever draw any power from the input signal voltage sources? Explain your answer.

 

For more information