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Op-Amp-Based Wien Bridge Oscillator
The circuit was designed based on the functionality of an op-amp to create a Wien bridge oscillator to generate sine waves in the frequency range of 15 Hz to 150 kHz that is channeled using four switch stages.
- 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 feedback
- Wien 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 source
- Operational Amplifier (Op-Amp) – a DC coupled high gain electronic voltage amplifier with differential inputs and usually a single output
- LF351 – a wide bandwidth single JFET operational amplifier with high slew rate, operation without latch up, compensated internal frequency, high input impedance JFET input stage, protection for output short circuit, low offset current and input bias, wide common-mode and differential voltage range, low power consumption, and adjustable internal input voltage
A sine wave output signal can be produced by a sinusoidal oscillator. Ideal output signal has constant amplitude without variation in frequency. The level by which this ideal output can be reached is dependent on the aspects such as amplitude ability, frequency stability, amplifier characteristics, and amplifier operation. Low audio frequency to ultrahigh radio and microwave frequency signals are produced by sine wave generators. These low frequency generators contain networks for determining the frequency which consist of capacitors and resistors. These are often used in the audio frequency range and normally referred to as RC oscillators. Likewise, other types of sine wave generator may be used for the same purpose but differs in the frequency range where they operate. LC oscillators are used for higher radio frequencies and crystal-controlled oscillators are used for middle radio frequency range. There are two states that are present in a sinusoidal oscillator, a unity closed loop gain and positive or regenerative feedback. To maintain oscillations, the regenerative signals must have enough power to offset the circuit losses.
In a Wien feedback circuit, the open loop gain of the amplifier must be above 3 to compensate for the losses. The gain in this circuit is maintained by an op-amp made from FET type. The high speed JFET single input op-amp LF351 is utilized because it is low cost with an internally trimmed input offset voltage. It maintains a large gain bandwidth while requiring a low supply current. Additionally, very low input bias and offset currents are well matched by high voltage JFET input devices. The parallel combination of resistor and capacitor, in series with a serial RC network, is the design of Wien network. The op-amp output produces the regenerative feedback that is fed to the serial RC input. The Wien bridge sine wave oscillator operates by returning the positive feedback oscillation output to the input. They are the often used circuit because they consist of few parts.
To prevent the uncontrolled oscillation from increasing, stabilization by any means is required. There are several ways of stabilizing the amplitude of the oscillation. The thermistor with a negative temperature coefficient (NTC) can be connected in the series leg of the feedback loop. It is a directly heated glass encapsulated bead type having a response time, although not so fast to cause low frequency distortion, it is fast enough to be usable. Another method is the use of an incandescent lamp or bulb which has positive temperature coefficient on the filament and is connected in parallel leg of the feedback loop. It can be a small lamp that can function as a PTC thermistor. Although it requires more power, it is more preferred than a thermistor. A light dependent resistor (LDR) can be used to produce excellent results due to its high voltage limit prior to distortion. Although it requires more circuitry than the thermistor and lamp, the voltage across it is minimized and the outcome is worth the try. The field effect transistor (FET) can function as a voltage controlled resistor (VCR). The voltage level must be maintained below 1V, since it has limited peak voltage. Doing this will maintain a distortion within highly regarded limits which is suitable for the output of the oscillator. Lastly, voltage controlled amplifier (VCA) can be used but the limited maximum voltage and complex circuitry make it repellent for simple circuits. One of the most important factors in stabilizing the circuit is that it must be slow enough to avoid the shape of low frequency waveforms from being changed. This would lead to significant distortion at low frequencies and the waveform bounce will be produced from the slow response time.
In this circuit, the bulb used has a rating of 12 V at 60 mA. The process of producing the feedback starts with the increase of filament resistance as the bulb heats up. The overall gain of the amplifier decreases at this stage while the output signal is fed back to the input. The same thing happens the other way around when the gain is increased. The filament resistance will drop and the output amplitude at the bulb will also be decreased. This sequence will provide stable amplitude. The distortion is minimized by the 1K ohm preset resistor. The group of 10k ohm potentiometer controls the frequency with a 10:1 range while the circuit is utilizing split power supplies. The switching of frequency range is done with a 4 position, 2 pole rotary switch. The use of capacitors attached directly to the switch is to minimize stray capacitance which is the unwanted effect that produces leakage of signals between circuits. These signals are known as crosstalk that can be a limiting factor to properly function at high frequencies.
The sine wave generator is an exceptional instrument for producing waves with wave drivers and speakers. It permits variation in the amplitude of the sine wave and works on frequency range of 1 to 800 Hz. A fundamental frequency for a particular configuration can also be learned by the generator. Spectators can examine, with the sine wave hopping from one resonant frequency to the next, the quantum nature of standing wave patterns.
The LF351 op-amp may be used in applications such as high speed integrators, sample and hold circuits, fast D/A converters, and other circuits requiring wide bandwidth, high slew rate, high input impedance, low noise, low input bias current and low input offset voltage drift. For maximum current supply, the LF351 is strongly preferred but for devices with critical requirements, the LF356 is used.