Lab 9

Blogsheet week 9

1.  Measure the resistance of the speaker. 

       It keeps fluctuating between 7.9 and 8.2.

2. Build the following circuit using a function generator setting the amplitude to 5V (0V offset). What happens when you change the frequency? (video)

Fig 1. Test setup for the speaker.

Fill out the following table.

Frequency (kHz)	Observation
1	Steady squeal
2	Lower pitched tone
3	Higher pitched whine
4	Even more annoying
5	Most annoying

3.  Add one resistor to the circuit in series with the speaker (first 47 Ω, then 820 Ω). Measure the voltage across the speaker. Briefly explain your observations.

Voltage with the 47 Ohm resistor:  375 mV(rms) (1 kHz)

Voltage with the 820 Ohm resistor:  54 mV(rms) (1 kHz)

We noticed immediately that the pitch of the tone is lower as we make the resistor bigger.  The volume also decreases the higher the resistance.

Fill the following table.

Resistor Value (Ω)	Oscilliscope Output (Vrms)	Observation
47	0.396	Low pitched hum
820	0.054	A quiet low pitched hum

4.     Build the following circuit. Add a resistor in series to the speaker to have an equivalent resistance of 100 Ω. Note that this circuit is a high pass filter. Set the amplitude of the input signal to 8 V. Change the frequency from low to high to observe the speaker sound. You should not hear anything at the beginning and start hearing the sound after a certain frequency. Use 22 nF for the capacitor.

Fig. 2 Test setup for high pass filter

a.       Explain the operation.  (video)

b.      Fill out the following table by adding enough (10-15 data points) frequency measurements. Vout is measured with the DMM, thus it will be rms value.

Frequency (Hz)	Vout (Vrms)	Vout(rms) / Vin(rms)
1000	0.0854	0.0106
42000	2.64	0.33
56000	3.04	0.38
83000	3.46	0.433
100000	3.58	0.448
160000	3.87	0.484
233000	4.05	0.506
308000	4.13	0.516
377000	4.22	0.528
410000	4.27	0.534
500000	4.46	0.558
801000	5.77	0.721
901000	6.6	0.825

c.       Draw Vout/Vin with respect to frequency using Excel.

Vout/Vin with frequency as the x-axis and the vout/vin as the y-axis

d.      What is the cut off frequency by looking at the plot in b?
          901 kHz

5.     Design the circuit in 4 to act as a low pass filter and show its operation. Where would you put the speaker? Repeat 4a-g using the new designed circuit (e, f, and g are for blogI).

        For the low pass filter you would connect the speaker across the capacitor much like you connect the oscilloscope. 

a.  Explain the operation (video)

b.  Fill out the following table by adding enough (10-15 data points) frequency measurements.  Vout is measured with the DMM, thus it will be the rms value.

Frequency (Hz)	Vout (rms)	Vout (rms) / Vin (rms)
679	5.9	0.738
1000	5.99	0.749
6000	5.8	0.725
15000	5.6	0.7
23000	5.3	0.663
46000	4.3	0.538
52000	4.09	0.511
57000	3.89	0.486
66000	3.57	0.446
77000	3.22	0.403
85000	2.98	0.373
92000	2.8	0.35
121000	2.26	0.283
151000	1.86	0.233

c.  Draw Vout/Vin with respect to frequency using excel.

Vout/Vin charted with frequency as the x axis and vout/vin as the y axis

d.  What is the cut off frequency by looking at the plot in b?

5.6 kHz

6.       Construct the following circuit and test the speaker with headsets. Connect the amplifier output directly to the headphone jack (without the potentiometer). Load is the headphone jack in the schematic. “Speculate” the operation of the circuit with a video.