The block-diagram of a feedback oscillator is shown in figure 1. The oscillator is made from an amplifier and a feedback network. The feedback network sends some of the system's output back to be re-amplified after a suitable time delay. A and B, are both frequency dependent, so they are considered to be functions of the frequency f.
In general, both the amplifier and the feedback network will alter the magnitude and the phase of the signal. To take this into account it is normal to treat both A and B as complex functions.
As a conclusion, we can say that an initial signal will produce a sustained - repeating signal whose amplitude will not fade away with time, if we arrange that. At this point you may say that we found a necessary condition for oscillation. Well, this is almost the case. Actually, there is another condition:. This condition can be stated in a mathematical expression as follow:.
This is equivalent to say that the feedback signal must be applied in phase with the original input signal. A coupling circuit is also referred as….. Lag circuit b. Lead circuit c. Lead-lag circuit d. Resonant circuit. For better understanding of the Wien bridge oscillator, the same circuit can be redrawn as shown below. Observe the circuit diagram carefully. The lead lag circuit is used as feedback network about which we have already discussed. The wien bridge oscillator circuit consists of two feedbacks, positive as well as negative.
A positive feedback is between output and noninverting terminal and a negative feedback is between the output and inverting terminal of OPAMP. You know that for oscillator, positive feedback is essential. Here, positive feedback is used to produce zero degree phase shift between amplifier and feedback network.
Perhaps you would like to know why negative feedback is required here. To ensure sustained oscillations, the loop gain must be slightly greater than one when circuit is turned on for the first time. Hence, to set this gain negative feedback is essential. In this case, the amplifier gain A must be 3. Similarly, we get the maximum output Vo only if resistive value is equal to reactance value.
Hence, the frequency of oscillation is decided by resistor 'R' and capacitor 'C'. This will start the oscillations. But due to excessive gain, distortion may result. This indicates that some form of gain reduction is required at higher output voltage. Discuss the working of wien bridge oscillators with neat block diagram. Explain the role of feedbacks in Wien Bridge Oscillator.
Give the name of the technique used to reduce the loop gain at higher output voltage. In Wien-bridge oscillator, the gain of amplifier must be…… a.
We get the maximum output Vo only if resistive value is equal to reactance value. Hence, the frequency of oscillation is decided by resistor R and capacitor C. Actually, the oscillation frequency is given by equation no 2. Explain how to select gain and frequency component of Wien bridge oscillator. Compute the component values for Hz frequency of oscillation in the Wien bridge oscillator circuit.
How many resistors to be varied to change the frequency of a Wien bridge oscillator. One resistor b. Two resistors c. Three resistors d. One capacitor. The output voltage lags the input voltage. The output voltage leads to input voltage.
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After reading this chapter, you are expected to learn about: Explain principle of operation of Oscillations with block diagram; Discuss how oscillations are initiated at first; Explain necessary and sufficient condition required to generate the sustain oscillations; Clarify loop gain and phase terms ; Classify oscillator circuits; Explain term lead and lag circuit and its working principle; Discuss the types of RC oscillators with neat block diagram; Explain the role of feedbacks in Wien Bridge Oscillator; Describe how to select gain and frequency component of oscillator; Compute the frequency of oscillation from component values; and Give in detail the technique used to reduce the loop gain at higher output voltage.
Answer the following question below by identifying the correct answers: 1. Nonregenerative feedback Answer the following True or False question 2. Choose the right answer. Positive feedback Write a short note on the following question in about words. Answer the following Match the pairs question. Column A Column B I. Underdamped II. Overdamped III. Damped IV. List out the sinusoidal oscillators with their frequency range. Answer the following question below by identifying the correct answers: 2.
Answer the following question below by identifying the correct answers: 3. The same as the input voltage 4. Answer the following question below by identifying the correct answers: 4. Critical Thinking Question. This will usually be a voltage amplifier and may be biased in class A, B or C. A wave shaping network. This consists of passive components such as filter circuits that are responsible for the shape and frequency of the wave produced. Part of the output signal is fed back to the amplifier input in such a way that the feed back signal is regenerated, re-amplified and fed back again to maintain a constant output signal.
Commonly an oscillator is constructed from an amplifier that has part of its output signal fed back to its input. This is done in such a way as to keep the amplifier producing a signal without the need for any external signal input as shown in Fig. It can also be thought of as a way of converting a DC supply into an AC signal.
This is the condition where a fraction of the amplifier's output signal is fed back to be in phase with the input, and by adding together the feedback and input signals, the amplitude of the input signal is increased. The result of a small amount of positive feedback in amplifiers is higher gain, though at the cost of increased noise and distortion.
If the amount of positive feedback is large enough however, the result is oscillation, where the amplifier circuit produces its own signal.
When an amplifier is operated without feedback it is operating in "open loop" mode. With feedback either positive or negative it is in "closed loop" mode. In ordinary amplifiers negative feedback is used to provide advantages in bandwidth, distortion and noise generation, and in these circuits the closed loop gain of the amplifier is much less than the open loop gain. However when positive feedback is used in an amplifier system the closed loop gain with feedback will be greater than the open loop gain, the amplifier gain is now increased by the feedback.
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