Ariel Moss EE 254 Project 6 12/6/13 Project 6: Oscillator Circuits The

Ariel Moss EE 254 Project 6 12/6/13 Project 6: Oscillator Circuits

The purpose of this experiment was to design two oscillator circuits: a Wien-Bridge oscillator at 3 kHz oscillation and a Hartley Oscillator using a BJT at 5 kHz oscillation.

The first oscillator designed was a Wien-Bridge oscillator (Figure 1). Before designing the circuit, some background information was collected. An input signal sent to the non-inverting terminal with a parallel and series RC configuration as well as two resistors connected in inverting circuit types to achieve gain. The circuit works because the RC network is connected to the positive feedback part of the amplifier, and has a zero phase shift at a frequency. At the oscillation frequency, the voltages applied to both the inputs will be in phase, which causes the positive and negative feedback to cancel out. This causes the signal to oscillate.

To calculate the time constant given the cutoff frequency, Calculation 1 was used. The capacitor was chosen to be 0.1µF, which left the input, series, and parallel resistors to be 530Ω and the output resistor to be 1.06 kHz. The signal was then simulated in PSPICE, and after adjusting the output resistor to 1.07Ω, the circuit produced a desired simulation with time spacing of .000335 seconds, which was very close to the calculated time spacing. (Figures 2-3 and Calculations 2-3).

1.07k

10Vdc V1 R1 LM741 4 2 V- 1 - OS1 530 6 OUT 0 3 5 + 7 OS2 U1 V+ V2 10Vdc

Cs Rs

0.1u 530

Cp Rp 0.1u 530

Figure 2: Wien-Bridge Oscillator built in PSPICE Ariel Moss EE 254 Project 6 12/6/13

Figure 3: Simulation of circuit

Next the circuit was built and tested. After measuring the frequency of the output, the output resistor had to be adjusted to 3.01 kHz using a decade box, and the parallel and series resistors were lowered to 360Ω to get an output of 3.08 KHz because at 530Ω, the frequency was 2.17 kHz (Figure 4) .

Figure 4: Oscillation of circuit at 3.08 kHz

The other oscillator was a Hartley BJT oscillator with an oscillating frequency of 5 kHz (Figure 5). It uses an LC circuit combined with a transistor for feedback. When the tank circuit (LC circuit) is working, the Ariel Moss EE 254 Project 6 12/6/13 capacitor and two inductors make a resonant circuit because current moves back and forth as the capacitor charges and discharges through the two inductors. The transistor amplifies this oscillation, which is why the oscillation remains steady. The emitter resistor causes the emitter current to be close to the collector current. The two currents also need to be “in phase” with each other to get the necessary positive feedback. The “in-phase” currents are caused when the 180 degree phase shift in the feedback loop adds with the 180 degree phase shift between the emitter and collector currents.

To calculate the necessary inductors and capacitor for the tank circuit, Calculation 4 was used. The capacitor was chosen to be 0.1µF, which left L1+L2 to be 0.010132 H. L1 was chosen as 5.02mH, and L2 was chosen as 5.02mH. The circuit was then simulated in Microcap (Figures 6-7). After simulating, the time spacing was taken to check for frequency. Comparing the time spacing of .000238 seconds to the calculated time spacing of 0.0002 seconds(Calculation 5-6), this circuit should oscillate at approximately 5kHz, but probably a little higher.

0.1u C1

1:3 V1 3 Q1 2:3 4:3 L2 3:2.287 L1 5.02m

5.02m

PlusPlus 200

MinusMinus

Figure 6: simulated Hartley oscillator circuit 691.419u,5.848 774.928u,5.838

1.562m,4.207 1.441m,4.202

Ariel Moss EE 254 Project 6 12/6/13

Micro-Cap 10 Evaluation Version hartley (1).CIR 3.90

6.598m,3.548 6.836m,3.548

3.60

3.30

3.00

2.70

2.40 0.00m 1.60m 3.20m 4.80m 6.40m 8.00m v(1) (V) T (Secs)

Figure 7: simulated output voltage with correct frequency

When building the circuit, a decade box was used in place of the 200 Ohm resistor. After testing the circuit, the resistance was lowered to 150 Ohms in order to reduce the frequency to 5.1 kHz instead of the 5.56 kHz that was produced with the 200 Ohm resistor (Figure 8-9). Ariel Moss EE 254 Project 6 12/6/13

Figure 8: output voltage at 200 Ohms Ariel Moss EE 254 Project 6 12/6/13

Figure 9: output voltage at 150 Ohms

Conclusion

After doing this experiment, the Wien Bridge oscillator was designed correctly, by using an RC network of parallel and series combinations in the non-inverting terminal of an op-amp, and positive feedback in the inverting terminal. The circuit worked as expected; by lowering the value of the resistor R2, the frequency was reduced to 3.08 kHz. The Hartley oscillator also worked; by using an LC circuit combined with positive feedback from a transistor with an emitter resistor for stability. The emitter resistor had to be lowered to get a value of 5.1 kHz, which was still higher than the design frequency of 5 kHz. The only other parameters that could be lowered would be the two inductors or the capacitor, which changes the design parameters.

Ariel Moss EE 254 Project 6 12/6/13