Task 5: Frequency-domain measurements on amplifiers

Background: In Task 4 you measured some important parameters of an amplifier you designed: (1) gain into open circuit, (2) input impedance and (3) output impedance. To do these measurements in Task 4, you acquired time-domain signals and compared them. The advantage of this method is its relative simplicity; the disadvantage in that you only get data from one frequency at a time, namely the input frequency you used for the measurement. Of course, it is possible to use input signals that contain some power at all frequencies within some range (like noise), and then process the measured input and output from a circuit to get information on the transfer function at all of those frequencies. That process is the subject of this task.

In the first part of this task, you will use a demo VI that measures the transfer function of an artificial, digital filter. You should download and decompress [use the WizZip program on the lab computers] the Task5Demo.zip VI now, and put it on your desktop. Here’s its Block Diagram:

Double-click on the Filter VI. Select Transfer function as the View Mode, as shown below:

You will see the Transfer Function of the filter. From this Configuration Panel, you can adjust the cutoff frequencies, topology, and order if your wish. Also, double-click on the two Signal Processing VI’s, and observe that the first one [Signal Processing VI] is set up to measure the transfer function of the filter by taking 100 RMS averages of its input and output signals, and the second one [Signal Processing VI 2] is set up to measure the Power Spectrom of the applied input signal by the same technique. Run the Task5Demo VI and observe how the measured transfer function of the filter converges to the one on the Configuration Panel of the Filter VI as the averaging progresses.

You can use the Task5Demo VI to measure transfer functions of circuits you build in the lab if you replace the Filter VI with the Stimulus-Response VI as shown below:

If you wired your amplifier from Task 4 into this VI, using Vin and Vout for the AI channels AI0 and AI1 respectively, the magnitude and phase plots would be for the transfer function of your amplifier.

Required Notebook Entries for this Task:

Task 4-1: _____ Modify the Task5Demo.vi as shown above and use it Measure the gain into open circuit versus frequency, and the accompanying phase shift, for the amplifier you designed and built in Task4. Place in your notebook a printout of your front panel showing your measurements. Comment on how well this measurement agrees with the one at a single midband frequency from Task 4.

Task 4-2:_______ The transfer function measured with the above modified VI is the complex ratio of Vout/Vin as a function of frequency. The input impedance of your amplifier is just the ratio of Vin/Iin versus frequency. Recall how you measured the input impedance of your amplifier in Task 4, and adapt your circuit and the VI to produce a plot of input impedance vs. frequency for your amplifier. Place in your notebook a printout of your front panel showing your input impedance measurements. Comment on how well this measurement agrees with the one at a single midband frequency from Task 4.

Task 4-3:_______ Investigate using other input signals than the noise input that has been configured as the default for the Waveform Generator. Can you figure out why only the noise input signals give reasonable results for the transfer function. Write a brief explanation in your notebook and include any printouts of front panels you need to back up your explanation.