# Electric Lab reports 2

Write lab reports.

Follow the Procedures: 1. General theories, 2. statistics analysis, 3. describe  process of exercise, 4 conclusion.

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EET 202

LAB EXPERIMENT INSTRUCTIONS

AC circuit analysis

Lab # 6

Oscillators

Experiment #1: A simple Colpitts oscillator (in MultiSim)

Objective:

After performing this experiment, you will be able to:

1. Construct a Colpitts oscillator in MultiSim using a resonant tank circuit and
an NPN transistor as the gain element.
2. Predict the frequency of a given oscillator design.
3. Design an oscillator which runs at a desired frequency.
4. Determine the frequency required for a particular musical note, and design and construct a Colpitts oscillator in MultiSim to produce that frequency.

Materials Required:

Computer with NI MultiSim installed

Procedure (refer to the attached schematic – Figure 1):

1. Add a 5VDC source (components -> sources -> power sources) and a ground.

1. Construct a voltage divider with two 10k resistors.

1. Place a 2N3904 NPN transistor on the schematic.

1. Connect the base of the transistor (see whiteboard) to the node between the two 10k resistors.

1. Construct a tank circuit with a 47mH inductor and two 10uF capacitors.
(Note the polarity of the caps!)

1. Connect this tank circuit to the base of the transistor via a 10uF cap.
(Note the polarity of this cap.)

1. Connect the node between the two tank capacitors to the emitter of the transistor.

1. Connect a 2.2kΩ resistor between the transistor emitter and Ground.

1. Connect the collector of the transistor to your 5V voltage source.

1. Find a generic 1P1S transformer (basic -> transformers-> 1P1S)

1. Convert this to a 1000:8 transformer by modifying its properties

1. Connect the primary of the transformer across the 2.2kΩ resistor.

1. Connect an 8-ohm resistor across the secondary of the transformer.
This will act as a speaker load.

1. Find a four-channel oscilloscope on the right. Set it up to look at waveforms
from the following locations. (Use colors as suggested):

1. Base of transistor (green)
2. Emitter of transistor (blue)
3. Top of inductor (red)
1. Turn on the simulation power to the circuit, and adjust the ‘scope controls.
Note the polarity relationships between the following pairs of signals,
which have electrolytic capacitors connected between them.
Describe whether relative polarity between these pairs changes or not.
(Is one node always at least as positive as the other node?)

1. Blue and Ground
2. Red and Blue
3. Green and Red
2. Using the formula for the frequency of a Colpitts oscillator, determine the expected frequency of oscillation of this circuit.

1. Using the four-channel oscilloscope (or the two-channel ‘scope, or the virtual Agilent ‘scope – whichever you prefer), measure the actual frequency of oscillation of the circuit.
1. Does this (more or less) agree with your calculations for #17?

1. Stop the simulation. Replace the 47mH inductor with a 100mH inductor.
1. Intuitively, what effect do you expect this to have on the frequency?

1. Calculate the new expected frequency, using the 100mH inductor.

1. Restart the simulation, and measure the new frequency.
1. Does it (more or less) agree with the new frequency you calculated?

1. Go back to using the 47mH inductor. Using Excel or whatever tools you wish, determine what value of C1 you would need to produce the following notes:

(Remember that note frequencies are 440Hz * 2^(N/12),
where N is the number of half-steps from A4, positive or negative)

1. A4 (440Hz)
2. B4 (two musical half-steps above A4)
3. C#4 (four musical half-steps above A4)
4. D4 (five musical half-steps above A4)

1. Choose at least two of these frequencies. Replace C1 with the appropriate capacitor value, and note the resulting frequency as measured by oscilloscope.

Note                C1 value                      Frequency

Experiment #2: A simple Colpitts oscillator (breadboarded circuit)

Objective:

After performing this experiment, you will be able to:

1. Construct a Colpitts oscillator using a resonant tank circuit and an NPN transistor as the gain element.
2. Predict the frequency of a given oscillator design.
3. Design an oscillator which runs at a desired frequency.
4. Determine the frequency required for a particular musical note, and design and construct a Colpitts oscillator to play that note through a speaker.

Materials Required:

Inductor: 47mH.

Resistors:10k (2), 2.2k (1).

Capacitors:10uF (3), as well as others as needed.

Transistor: 2N3904 NPN signal transistor

Transformer: Small, 1kΩ – to –  8Ω impedance matching transformer

Speaker: 8Ω speaker (0.5W or similar)

Procedure:

1. Obtain all the components listed in Table 6-1-1. Measure the listed components and record their measured values in Table 6-1-1. Use listed values for those which could not be measured.

 Listed Value Measured Value L1 47mH C1 10µF C2 10µF C3 10µF R1 10kΩ R2 10kΩ R3 2.2kΩ

Table 6-1-1

1. Place the transistor facing you near the right side of the breadboard. Note the order of the pins: with the flat side facing you and pins downward, pins are E, B, C from left to right.

1. Construct the DC biasing network, using the two 10k resistors connected to the base of the transistor. Have your instructor check your circuit after this step.

1. Near the transistor but not connected to it, construct the tank circuit with two capacitors and one inductor. (Just use one C1 capacitor as shown in the schematic, for now.) NOTE THE POLARITY OF THE CAPACITORS.

1. Connect the top of the tank circuit to the base of the transistor, via a 10uF capacitor. NOTE THE POLARITY OF THIS CAP.

1. Connect the node between C1 and C2 to the emitter of the transistor.

1. Connect a 2.2kΩ resistor from the emitter of the transistor to ground.

1. Connect the collector of the transistor to positive voltage (5VDC).

1. Power up the circuit and check the output (across the 2.2kΩ) with an oscilloscope.
(Remember to connect the ‘scope ground to your circuit’s common ground.)
Take a picture of the signal on the ‘scope (or sketch this for your report.)

1. Once you have verified a good signal with the scope, turn off the power.
Connect up the transformer and speaker as shown in the schematic. (remember that the primary is the center-tapped side; do not use the center tap.

1. When the circuit is built, power it on. You should hear a tone from the speaker.

Experiment 3: A (very simple) electronic synthesizer

11

Figure 6-3-1. A simple electronic synthesizer, using four sets of C2 capacitances.

Procedure:

1. Locate C2 in your design. Note where it is located, so that you can attach wires back to its leads.

1. Move C2 to an empty space on your breadboard, suitable for attaching a switch to connect and disconnect it. (Try to leave room for two or three other capacitor combinations, next to it.)

1. Connect a SPST switch to C2. Connect the switch-capacitor combination back to the point in the circuit where C2 originally was. Remember to make sure the polarity is still the same.

1. Power up the circuit and press the switch. You should hear a tone when the switch is pressed.

1. Experiment with other capacitances, either in parallel or series. Try total capacitance values in the range of about 1uF to 20uF. See if you can find values which make a useful set of tones to allow you to play a simple tune.

1. Connect two or three extra sets of capacitors with switches, similar to
Figure 6-3-1, so that a different tone sounds when each switch is pressed.

Appendix A: Circuit schematic for Colpitts oscillator 