Question

Wein bridge oscillator is a

• A. fixed frequency oscillator
• B. variable frequency oscillator
• C. low gain oscillator
• D. uses both positive and negative feedback

Hint:

Wien bridge oscillator uses a Wien bridge (RC network) as the frequency-determining element providing positive feedback.
The condition for oscillation is that the loop gain \(A\beta=1\) at the resonant frequency, with a total phase shift of \(0^\circ\) or \(360^\circ\).
Negative feedback is commonly used in the amplifier stage to precisely control the gain (usually to slightly greater than 3 initially, then settling at 3) and stabilize the output amplitude.
It is well-suited for generating sinusoidal signals in the audio frequency range and is often designed to be variable.

Answer 1

The Correct Option is D

The Wien bridge oscillator is known for several characteristics:
It is commonly used as an audio frequency oscillator because it can produce sinusoidal waves with low distortion over a range of frequencies.
The frequency of oscillation can be varied by changing the values of the resistors and capacitors in its frequency-selective Wien bridge network. Thus, it is a variable frequency oscillator (option b).
The Wien bridge network itself provides frequency-dependent positive feedback. To stabilize the amplitude of oscillations and ensure sustained oscillation at the desired frequency, an amplifier with controlled gain is used. This often involves a non-linear element or a negative feedback loop for gain stabilization. The Barkhausen criterion requires loop gain \(A\beta=1\). The amplifier provides gain A, the Wien bridge provides feedback factor \(\beta\). The bridge itself can be seen as providing frequency selective positive feedback. The amplifier gain is usually controlled using negative feedback to precisely set the gain to just above what's needed for oscillation to start, then reducing it to maintain stable amplitude.
So, it uses a frequency-selective positive feedback path (the Wien bridge) and often incorporates negative feedback for gain stabilization and amplitude control within the amplifier stage. Considering the options: (a) fixed frequency oscillator: False, it's typically variable. (b) variable frequency oscillator: True. (c) low gain oscillator: The amplifier gain required is typically 3 for oscillation with a standard Wien bridge (\(\beta=1/3\) at resonance). This isn't particularly "low" in an absolute sense, but it's a specific value. This option is less defining than others. (d) uses both positive and negative feedback: True. The Wien bridge provides the frequency-selective positive feedback path for oscillation. Negative feedback is often used in the amplifier section to stabilize the gain and output amplitude. Comparing (b) and (d), both can be considered true. However, the fundamental design often incorporates both types of feedback for practical operation. If only one defining characteristic is chosen, its ability to be a variable frequency audio oscillator is key. But the mechanism involves positive feedback for oscillation and negative feedback for stability. The question asks what it "is". The most encompassing description of its operational principle from the choices that highlights its functional design often points to the feedback mechanisms. Many oscillators are variable frequency. The specific feedback arrangement is quite characteristic. If the checkmark is on (d), it emphasizes the feedback mechanism. \[ \boxed{\text{uses both positive and negative feedback}} \]