EE 321 Op-Amps 1 Fall 2007EE321 LabOperational Amplifiers, Part 1 — Frequency CharacteristicsThe purpose of this lab is to examine the frequency response of the operational amplifier. Itsfinite frequency response will be investigated in a simple inverting operational amplifier circuit.The operational amplifier and the inverting amplifier will be found to behave like a single timeconstant low pass filter.Inverting Amplifiers1. Construct an inverting amplifier (designed as part of the pre lab) having an input resistanceof 1 KΩ and a gain of about 5. Use the 741 op-amp with ±15 Volt supply voltages. Employgood breadboard technique by using the long bus lines on your protoboard for each of thesupply voltages and ground.2. Apply a 1 kHz sine wave to the input. Display both the input and output on the scope.(a) Measure the gain and compare with the theoretical gain.(b) Confirm that the op-amp inverts the input by displaying both input and output on thescope (sketch waveforms).(c) Vary the DC level of the input sine wave (offset) to confirm that the amplifier respondsto DC inputs.(d) Increase the input amplitude to determine the DC output voltage levels at which theoutput ‘saturates’.These test can be quickly done to make sure your amplifier is working correctly.Slew Rate3. Change the input to a s quare wave. At 100 Hz adjust the output to be 20 V p-p with nosaturation or DC level. Now increase the signal frequency to about 100 kHz. What hashappened to the output? The output of the op amp can not change faster than a certainrate, called the ‘slew rate’. (See Figure 2.29 of Sedra & Smith.) Measure the slew rate of this741 op amp (the slope in Volts/µs). Compare with the value given in the op amp specs.4. Change the input back to a sine wave. Decrease the frequency until the 20 V p-p sinusoidaloutput can be obtained without distortion. Find the frequency fMwhere the distortion justbegins (don’t try to get too close). This is related to the slew rate of the op amp as measuredin the preceding part. SR=ωMVomax, where Vomaxis the amplitude of the output voltage.(p. 100 of Sedra and Smith). Check this relationship.5. Repeat either one of these slew rate measurements for the 411 op amp. Compare this resultto the data sheet.1EE 321 Op-Amps 1 Fall 2007Open Loop Gain - The gain of the op amp itself.6. The finite open-loop gain of an op amp in the inverting configuration may be determined bymeasuring the small voltage at the (-) input of the op-amp.(a) Re-construct the 741 inverting amplifier and increase the gain to 10. Set the outputvoltage to be 20 V p-p sine wave at a frequency of 1 kHz. Sketch the circuit. Measurethe voltage at the inverting input of the op-amp with the X1 probe.(b) What open loop gain does this imply (it should be near 1000)?(c) Repeat at frequencies of 10 kHz and 100 kHz. If necessary decrease the output if theslew rate causes distortion.(d) How does the op-amp gain appear to vary with frequency?(e) Plot |A|(dB) vs. frequency on log scale, find where |A| is 0 dB.7. At 10 kHz note the phase shift between Voutand V−. What is its approximate value, andwhy is it present?Frequency Response of the Inverting Amplifier with Gain 108. Use the same circuit as used in 6) and 7). To make sure there won’t be any distortion duringthis measurement, set the output voltage to be near 0.1 V p-p at a frequency of 1 MHz.If necessary, decrease the signal amplitude, so that the slew rate does not cause distortion.Measure the cutoff frequency f0of the circuit by adjusting the frequency until the gain is.707 of its low frequency value (measure the output at 100 Hz, then increase the frequencyuntil the output drops by a factor of 0.707).Why is the cutoff frequency f0also called the ‘half power’ frequency or the 3dB frequency(f3dB)?Find ft, the unity-gain bandwidth, using Eq (2.27) in Sedra and Smith (f0is the same asf3dB). Compare your ftto that in the data sheet.9. Find the unity gain bandwidth ftfor the 411 using the same method.10. Use a different circuit to determine the unity gain bandwidth ftof the 741 op amp. Connectit as a unity gain follower and measuring the 3 dB (half power) frequency. Use the X10 probefor measurements above 1 MHz. Keep the output amplitude small to ensure that the slewrate is not affecting the s ignal amplitude. Compare with specs.NOTE – The breadboard does not work at high enough frequencies to be able to use thismethod for the 411.Comparison of op amps.11. From the data obtained so far, which type of op amp, the 741 or 411, is better and why?Which is closer to an ideal op-amp? Which has a more informative spec sheet?Extra credit12. Given the measured value of ft, what 3 dB frequency would you expect for the 411 con-nected to have a non-inverting gain of 10 and 100? Test one of these predictions by actualmeasurement.2EE 321 Op-Amps 1 Fall 2007Pre-LabRead the lab before answering these questions.1. Design an inverting amplifier having an input resistance of 1 KΩ and a gain of about 5. Checkthe 741 spec sheet for the pin connections to the op-amp package, and label the pin numberson your circuit diagram.2. Sketch the distortion to a square wave caused by slew rate. How can this wave form be usedto find the value of the slew rate?3. Sketch the distortion to a sine wave caused by s lew rate. How can the sine wave be used tofind the value of the slew rate? Use a different method than was used for the square wave.4. Find the slew rate for the 411 and 741 from their data sheets.5. What is the difference between the open-loop gain of the op-amp and the gain of the invertingamplifier built from it?6. What is the phase shift of a LP filter at a frequency much higher than the cutoff frequency?7. How are ftand f0related? Define both frequencies. How does the relationship differ forinverting and non inverting amplifiers?8. Find ftfor the 411 and 741 from their data