Name: Yoong Jia Wee ID: 011698Title: Wien Bridge OscillatorIntroductionAWien bridge oscillatoris a type ofelectronic oscillatorthat generatessine waves. It can generate a large range offrequencies.The bridge comprises fourresistorsand twocapacitors. The oscillator can also be viewed as a positive gain amplifier combined with abandpass filterthat providespositive feedback.

Objectives Recognize the operation of a Wien Bridge Oscillator. Determine the balance frequency of a Wien network. Determine the attenuation and phase shift of a Wien network at balance frequency. Determine the oscillation frequency of a Wien Bridge Oscillator. Measure the amplitude and phase relationship from waveforms for a Wien Bridge Oscillator. Diagnose faults in a Wien Bridge Oscillator circuit.

Results and discussions1.1 Wien Bridge NetworkThe exercise deals with initial investigation of a simple Wien Bridge circuit and measures of frequency of oscillations and attenuation parameters.

Using the anticipated balance frequency for the Wien network in Circuit #4 (Figure 1.4)employing C10, R8 and C9, R9 can be calculated as follows:

The results for experiment 1.1 are tabulated in Table 1.1Table 1.1:Time PeriodFrequencyVoltage at Socket 4.18Op Amp OutputWien Bridge Attenuation

0.900ms1.14kHz380mVp-p1.12Vp-p0.3393

From Table 1.1, we can know that the experimental frequency matches with the theoretical value. From theory, we know that the attenuation =0.33. The attenuation of the Wien network can be calculated by using the following equation

=0.3393It can be observed that the difference between the theoretical & practical result of both the balance frequency value & the attenuation is very small, this might occur as practically the condition is not ideal.For a Wien bridge network to oscillate with 0 phase difference & a closed loop gain of 1, The two RC Networks must have equal resistors and capacitors so that closed loop gain = gain of the amplifer/3. For 0 phase difference oscillation, the closed loop gain should be = 1, therefore, the amplifier gain should be around 3.1.2 Wien Bridge OscillatorFrom the lab sheet instructions, the voltage VR1 was set to the maximum value which resulted in distortion because the voltage was higher than the operating voltage of operational amplifier. To solve this problem, the voltage was reduced to a level so that the distortion was minimum.Obtaining a distortion free waveform by adjusting the voltage settings was difficult. So we use a p.t.c. resistor which resistance increases with temperature.R5 senses the output voltage. If the output voltage is too high, the extra current flows into R5 increases R5 resistance. Thus the feedback ratio of the op-amp drops, reduces the gain and compensates for the larger output.The time for one cycle off the oscilloscope display is recorded in table 1.2Table 1.2:Time PeriodFrequency

0.96 ms1.106 kHz

Besides, the anticipated frequency was calculated and recorded in Table 1.3Table 1.3Anticipated FrequencyMeasured FrequencyVoltage at Socket 4.18Op Amp OutputWien Bridge Attenuation

96.46 Hz96.52 Hz63.2 Vp-p184 Vp-p0.3435

Theoretically, the attenuation () of the circuit is i.e. 0.3333 and the anticipated frequency of the network is .From table 1.3, we can know that the measured frequency and the experimental Wien Bridge Attenuation is quite close to the theoretical value.1.3 Fault Diagnosis Preliminary InvestigationThis investigation involves the plotting and comparative analysis of the outputs produced at various point of the Wien Bridge Oscillator circuit.The waveform for socket 4.7, 4.6, 4.10, 4.16, 4.17 and 4.18 are plotted on page 10 of the lab sheet.The peak to peak voltage at every socket is recorded in the table below:SocketVpp (V)

4.74.32

4.65.12

4.105.92

4.1613.0

4.177.20

4.187.20

From the plots, we can know that for every socket except for 4.17, the output was in phase with the input. Besides, the output was not distorted.At socket 4.10, the phase difference of the 4.10 and 4.7 was 0.For socket 4.17, we can observe the waveform that the input and the output was not in phase and the input lead the output. The degree at which the input was leading is by:

The amplitude of the waveform at socket 4.17 is 7.20 Vpp.

ConclusionThe experiment of Wien bridge oscillator provided a more in depth understanding of the wien bridge oscillator and Wien bridge network. It can be concluded that for a Wien bridge network to give a stable oscillation the circuit must include a non-inverting amplifier with gain of 3. For a Wien bridge oscillator beside maintain a closed loop gain of 1 and phase shift of 0, a p.t.c resistor also is required. From this experiment we can conclude that a Wien network consists of a series RC network and a parallel RC network.Referenceshttp://en.wikipedia.org/wiki/Wien_bridge_oscillator