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© National Instruments Corporation 4-1 Introduction to NI ELVIS

4 Op Amp Filters

Figure 4-1. Frequency Characteristics of a BandPass Filter

Adding a few capacitors and resistors to the basic operational amplifier (op amp) circuit can yield many interesting analog circuits such as active filters, integrators, and differentiators. Filters are used to pass specific frequency bands, integrators are used in proportional control, and differentiators are used in noise suppression and waveform generation circuits.

Goal This lab uses the NI ELVIS II suite of instruments to measure the characteristics of lowpass, highpass, and bandpass filters. Simulate these filters using Multisim with the measured component values. In the lab challenge at the end of this chapter, Multisim is used to design a second order active filter.

Lab 4 Op Amp Filters

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Required Soft Front Panels (SFPs) • Digital multimeter (DMM[:, ])

• Function generator (FGEN)

• Oscilloscope (Scope)

• Impedance analyzer (Imped)

• Bode analyzer (Bode)

Required Components • 10 k: resistor, R1, (brown, black, orange)

• 100 k: resistor, Rf , (brown, black, yellow)

• 1 PF capacitor, C1 • 0.01 PF capacitor, Cf • 741 op amp

Lab 4 Op Amp Filters

© National Instruments Corporation 4-3 Introduction to NI ELVIS

Exercise 4-1 Measuring the Circuit Component Values Complete the following steps to measure the values of the individual components:

1. Launch NI ELVIS II.

2. Select the DMM icon from the Instrument Measurement strip.

3. Select DMM[:] to measure the resistors.

4. Select DMM[ ] to measure the capacitors.

5. Fill in the following information.

R1 ___________ :�(10 k: nominal)

Rf ___________ :�(100 k: nominal)

C1 ___________ PF (1 Pf nominal)

Cf ___________ PF (0.01 Pf nominal)

6. Close the DMM.

End of Exercise 4-1

Lab 4 Op Amp Filters

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Exercise 4-2 Frequency Response of the Basic Op Amp Circuit Complete the following steps to build and perform measurements on an op amp.

1. On the workstation protoboard, build a simple 741 inverting op amp circuit with a gain of 10 as shown in Figure 4-2.

Figure 4-2. Schematic Diagram of a 741 Inverting Op Amp Circuit with a Gain of 10

The circuit looks like Figure 4-2 on the NI ELVIS II protoboard.

Figure 4-3. 741 Inverting Op Amp Circuit with a Gain of 10 on an NI ELVIS protoboard

Lab 4 Op Amp Filters

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Exercise 4-4 Highpass Filter A low frequency cutoff point, fL, for a simple RC series circuit is given by the equation:

2SfL = 1/(RC)

where fL is measured in hertz. The low-frequency cutoff point is the frequency where the gain (dB) has fallen by –3 dB. This (–3 dB) point occurs when the impedance of the capacitor equals that of the resistor.

1. Add a 1 PF capacitor, Cl, in series with the 1 k: input resistor, R1, in the op amp circuit as shown in Figure 4-6.

Figure 4-6. Highpass Op Amp Filter Circuit Design

The highpass op amp filter equation has a low-frequency cutoff point, fL, where the gain has fallen to –3 dB. In other words, when Xc = R:

2SfL = 1/ (R1C1)

Lab 4 Op Amp Filters

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Figure 4-7 shows this circuit on an NI ELVIS protoboard.

Figure 4-7. Highpass Op Amp Filter on NI ELVIS protoboard

2. Run a second Bode plot using the same scan parameters as in Exercise 4-3.

3. Observe that the low-frequency response is attenuated while the high-frequency response is similar to the basic op amp Bode plot.

Figure 4-8. Bode Measurement of Highpass Op Amp circuit

Lab 4 Op Amp Filters

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4. Use the cursor function to find the low-frequency cutoff point, that is, the frequency at which the amplitude has fallen by –3 dB or the phase change is 45 degrees.

5. Compare your results with the following theoretical predication:

�SfL = 1/ (R1C1)

End of Exercise 4-4

Lab 4 Op Amp Filters

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Exercise 4-5 Lowpass Filter The high-frequency roll-off in the op amp circuit is due to the internal capacitance of the 741 chip being in parallel with the feedback resistor, Rf. If you add an external capacitor, Cf, in parallel with the feedback resistor, Rf, you can reduce the upper frequency cutoff point. It turns out that you can predict this new cutoff point from the following equation:

2SfU = 1/(Rf Cf)

Complete the following steps to perform an additional frequency measurement on the op amp circuit:

1. Short the input capacitor (do not remove it because you will use it in Exercise 4-6).

2. Add the feedback capacitor, Cf, (0.01 Pf) in parallel with the 100 k: feedback resistor.

Figure 4-9. Lowpass Op Amp Filter Circuit Design

Lab 4 Op Amp Filters

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3. Run a third Bode plot using the same scan parameters.

Figure 4-10. Bode Measurement of Lowpass Op Amp circuit

Figure 4-10 shows that the high-frequency response is attenuated much more than the basic op amp response.

4. Use the cursor function to find the high-frequency cutoff point, that is, the frequency at which the amplitude has fallen by –3 dB or the phase change is 45 degrees.

5. Compare your results with the following theoretical prediction:

�SfU = 1/ (Rf Cf)

Note Note the 90-degree phase change from the very low-frequency range to the upper-frequency range. This is as expected for a single-pole RC filter stage.

End of Exercise 4-5

Lab 4 Op Amp Filters

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Exercise 4-6 Bandpass Filter If you allow both an input capacitor and a feedback capacitor in the op amp circuit, the response curve has both a low-cutoff frequency, fL, and a high-cutoff frequency, fU. The frequency range (fU – f L) is called the bandwidth. For example, a good stereo amplifier has a bandwidth of at least 20,000 Hz.

Figure 4-11 shows a bandpass filter on an NI ELVIS II protoboard.

Figure 4-11. Bandpass Op Amp circuit on NI ELVIS protoboard

1. Remove the short on C1.

Figure 4-12. Bandpass Op Amp Filter Circuit Design

Lab 4 Op Amp Filters

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3. Run a third Bode plot using the same scan parameters.

Figure 4-10. Bode Measurement of Lowpass Op Amp circuit

Figure 4-10 shows that the high-frequency response is attenuated much more than the basic op amp response.

4. Use the cursor function to find the high-frequency cutoff point, that is, the frequency at which the amplitude has fallen by –3 dB or the phase change is 45 degrees.

5. Compare your results with the following theoretical prediction:

�SfU = 1/ (Rf Cf)

Note Note the 90-degree phase change from the very low-frequency range to the upper-frequency range. This is as expected for a single-pole RC filter stage.

End of Exercise 4-5