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1 KHz Frequency Wien Bridge Oscillator

2016-05-15 02:02  
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The circuit was designed to create an electronic oscillator known as Wien Bridge Oscillator which can be used for the creation of low frequency sine wave signals.

Oscillator – an electronic circuit that produces a time varying or repetitive electronic signal, sine wave or square wave, without an external input signal using positive feedbackWien Bridge Oscillator – a type of electronic oscillator that uses an RC high pass filter and an RC low pass filter for setting the frequency of oscillation while generating sine waves without any input sourceLM741 – a general purpose single operational amplifier with features such as offset null, compensated internal frequency, voltage range with high input, good stability of temperature, and protected from short circuitNE5534 – a high performance low noise operational amplifier having a combined excellent AC and DC characteristics and features such as capability of external compensation, capability of offset nulling, wide range of voltage supply, low harmonic distortion, with output voltage swing at peak to peak, high slew rate, 100 dB common mode rejection ratio, high gain of DC Voltage, unity gain bandwidth at 10 MHz, and noise voltage of equivalent input

The basic construction of a Wien bridge oscillator comprises of low frequency amplifiers which are used to produce positive and negative feedback. A feedback is a method of sampling a part of the output signal and mixing it with some obtained part of the signal source. The mixture is then fed to the input of the active feedforward element of the feedback loop. The energy obtained from an external signal source which comes from outside the system, comprises the input to the system as a whole. This input in prone to noise and leakage on its way to and within the system. The feedback element mixes the input with a sample from the output within the system. A well defined loop around which the signal power propagates is the concept behind the feedback creation.

A feedback is said to be positive if the signal causes further increase or amplification in the magnitude of the output signal instead of stabilizing the signal. For this reason, it can be referred to as regenerative feedback. Negative feedback exists when the output of a system operates to oppose changes to the system input, thereby causing less modification while the feedback signal decreases. This is also known as a self-correcting or balancing loop. Negative feedback is typically used to increase the accuracy and stability of a system by correcting unwanted changes while positive feedback causes the amplifier to increase its internal gain by having the same phase as the input signal. Since positive feedback is useful only in exceptional circumstances, it is rarely useful in amplifiers while negative feedback is useful in setting the parameters of an amplifier like the input and output impedance, stability and bandwidth, and voltage gain. Both positive and negative feedback require a feedback loop to operate for its control of the system.

The Wien bridge is shown in the simplified circuit where the positive feedback handles the frequency of oscillations while the negative feedback handles the unity gain of the circuit. To maintain the oscillation without any distortion at the output signal, the gain should be higher which can be possibly handled by the positive feedback. The positive feedback consists of resistor and capacitors that will provide a gain of 3 to the circuit. The RC filter or tuning network together with the amplifier is necessary to achieve the conditions for oscillations. The increasing gain causes the net resistance to be more negative where the amplitude increases. A result of low distortion oscillations occurs when suitable amplitude is reached as the gain is reduced to exactly 3. The distortion will stay minimal as long as the amplifier and RC network are linear. With the aid of a non-linear component like thermistor, a glowing lamp, or other suitable circuit, the adjustment of the gain becomes automatic.

The second figure illustrates an oscillator where the amplifier contains two units in coupling DC, the IC1 and the output. The IC1 acts as an amplifier while the output consists of transistor Q1 and Q2. This allows the oscillator to power loads bigger than 50R. The parallel combination of diodes D1 and D2 with resistor R1 handles the automatic gain modification. The function of placing the resistor R1 in parallel with the diodes is to keep the distortion in low level at about 1% to 5% and reduce the non linearity of combination. Without increasing the distortion, the frequency of oscillation can increase up to 20 KHz which is enough to couple the DC. But based on the values of components used, the circuit will produce a frequency of 1 KHz. It can be varied by changing the values of the resistors and capacitors without using electrolyte capacitors. It is also suggested to have values where R1 is equal to R2 and C1 is equal to C2.

The current that runs through the circuit oscillates between 10 mA to 50 mA on a load with a value of 50R. The circuit can be powered by two 9 V batteries or from a single but stable power supply. The generator can be adjusted with the use of an oscilloscope. In this scenario, the presence of the waveform is being modified by the trimmer TR1 until the waveform produces less distortion. If the waveform is not visible on the oscilloscope, the resistor can be replaced with 47R in connection with the loudspeaker and the output. The loudspeaker is rated with 8 ohms at 0.5 W. The trimmer is adjusted until hearing the presence of oscillation in the loudspeaker.

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Figure 2 

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