Note: i Answer the questions below as instructed. Which of the following feedback is used to produce oscillations? Positive feedback b. Negative feedback c. Positive and Negative feedback d. Nonregenerative feedback. Answer the following True or False question 2. Oscillator requires external input signal for its operation. An oscillator consists of an amplifier and a feedback network.
Now, let us see which basic components are required to obtain oscillations. To start the oscillation with the constant amplitude, positive feedback is not the only sufficient condition. Oscillator circuit must satisfy the following two conditions known as Barkhausen conditions:. Fill in the blank. A circuit should satisfy …….. To start oscillation, the total phase shift of an oscillator is a.
High b. Low c. We have stated that oscillators do not require any external input. This means an oscillator's output feeds its own input and it must satisfy the two essential conditions to start oscillations. Here, you might be eager to know, from where the starting voltage comes. All of you are aware that every resistor has certain free electrons.
At room temperature these free electrons move randomly and generate a noise voltage across the resistor due to collisions.
This voltage is also known as thermal noise voltage. When power supply is turned on, for the first time this small ac noise voltage gets amplified and appears at the output terminal. This amplified output is applied to feedback circuit and output of feedback circuit is fed back to the amplifier as an input. It is again amplified by amplifier and fed back to input through feedback circuit. This process is repeated and at one particular frequency, circuit satisfies the necessary conditions to start oscillation.
Using proper feedback components, it is possible to select the particular frequency. The voltage that starts an oscillator is caused by a. Ripple from the power supply b. Noise voltage in resistors c. The input signal from a generator d. Positive feedback. Write a short note on the following question in about words. Discuss how oscillations are initiated at first.
The name sinusoidal oscillator itself indicates the meaning that this oscillator produces sine wave output. For any type of circuit to behave as an oscillator, first it must satisfy the necessary and sufficient condition which is mentioned in the previous section.
Depending upon the variation in the output waveform amplitude, there are two types of oscillations. Damped oscillations : Oscillations, whose amplitude goes on decreasing or increasing continuously with time, are called damped oscillations. If amplitude of oscillations is decreasing continuously, it is known as underdamped. Whereas if amplitude of oscillations is increasing continuously, it is known as overdamped. Undamped oscillations : Oscillations, whose amplitude remains constant with time, are called undamped oscillations or sustain oscillations.
In practice, to obtain the sustained oscillations at desired frequency of oscillations, oscillator circuit must satisfy some of the basic requirements such as,. This means, there is output without any external input.
In reality, to get sustained oscillations, at the first time when the circuit is turned on, the loop gain must be slightly greater than one. This will ensure that oscillations build up in the circuit. However, once a suitable level of output voltage is reached, the loop gain must decrease automatically to unity. There are many kinds of things a linear black box can do to a signal, but all must obey the above criterion.
The output produced by a linear black box when given a combination of many different frequency signals will be the sum of the outputs that would be produced for each frequency in isolation. The behavior of many practical circuits is close to that of a one-input one-output linear black box.
Since any wave other than a sine wave is a combination of sine waves at multiple frequencies, for a circuit to oscillate with anything other than a sine wave, there must be multiple frequencies which, if fed in individually, would cause the output to precisely match the input in phase and amplitude.
While it is certainly possible to construct such circuits, most practical circuits will only have one frequency where such behavior will occur. Often, sine waves are the desired output, for example in a radio system where we want to only transmit on a specified frequency band. We actually have to work hard to design an oscillator that produces a pure sine wave output. Generally, by fourier analysis, any repetitive signal can be seen as a sum of harmonically related sine waves. If it oscillates, I would still consider it an oscillator.
Specifically, the op-amp circuit you're probably thinking of is a form of relaxation oscillator. Wikipedia even uses a diagram of an op-amp oscillator as the first illustration on their page on Electronic Oscillators. Edit In reply to your comment, harmonic content is the content at frequencies that are harmonics multiples of the fundamental operating frequency. For example, from a 1 kHz oscillator you will get output at 1 kHz, 2 kHz, 3 kHz, etc.
The oscillator doesn't filter it out, but you can add a filter at the output of your oscillator to try to filter it out. If you want a sine wave output, you design the oscillator carefully to generate as little harmonic content as possible.
But in general, no matter how careful you are, there will be some harmonics in the output. They don't die out over time, they are part of the output as long as the oscillator runs.
Supercat's answer explains some of the reasons why harmonic distortion in the output is unavoidable. Harmonic content is often undesirable, so you try to design your oscillator to not produce it, but there are no perfect components, so you are always stuck with some harmonic distortion in the output.
Theoretically, RC and Wien bridge oscillators generate a single output frequency at a determinable precise point in the spectrum. This precise point in the spectrum will yield a feedback signal that is either:. At other frequencies the phase shift will be different and will not sustain oscillation. A sinewave is a single point in the spectrum therefore a sinewave is produced by these types of oscillator. Craig 1 1. Cambridge City USA. Personalised recommendations. Cite chapter How to cite?
ENW EndNote. Here are a few of the different outputs in more detail to better understand the benefits and tradeoffs of each. This should give you a better idea of which type is best for your application. There are two main output formats that are used. Sine wave — as shown in the graph below creates a continuous analog sinusoidal wave that is denoted by sweeping cycles at the rate of the frequency and amplitude.
Square wave — as shown in the graph below is a digital representation of the oscillator output. Its signal is denoted by degree cycles at the rate of the frequency and amplitude. The outputs we will cover are:. Single Ended Output:. Differential Output:. Sine Wave and Clipped Sine Wave. Sine Wave - is the standard or 'natural' signal output of a crystal or oscillator circuit.
It consists of one fundamental sinusoidal frequency output. The linearity sine wave outputs offer the best phase noise performance of all the outputs.
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