Which of the following oscillations makes use of both positive and negative feedback ?
Wien Bridge
The Wien-bridge oscillator is built round a bridge with two arms, and each arm carries one kind of feedback — option 4.
| Path | Network | Goes to | Role |
|---|---|---|---|
| Positive | Series and parallel RC | Non-inverting input | Sustains oscillation, sets the frequency |
| Negative | Resistive divider | Inverting input | Fixes the gain at 3, controls amplitude |
The positive path sets the frequency. The lead-lag RC network has zero phase shift at exactly one frequency,
\(f=\dfrac{1}{2\pi RC}\)
and there its attenuation is \(1/3\). Since the amplifier is non-inverting and contributes 0°, the Barkhausen phase condition \(\angle A\beta=0^{\circ}\) is met at that frequency alone.
The negative path sets the amplitude. To satisfy \(|A\beta|=1\) against an attenuation of \(1/3\), the amplifier must have a gain of exactly 3, which the resistive divider on the inverting input provides:
\(A=1+\dfrac{R_{f}}{R_{1}}=3\quad\Rightarrow\quad R_{f}=2R_{1}\)
Why "exactly" is the difficulty, and how it is solved. A gain below 3 lets the oscillation die away; above 3 it grows until the amplifier clips, giving a distorted output. Since no resistor pair holds a ratio that precisely, practical circuits make the negative-feedback arm amplitude dependent — classically a small incandescent lamp, whose resistance rises as it warms, or a thermistor, a JFET used as a voltage-controlled resistor, or back-to-back diodes. Gain then falls automatically as the output grows and settles where \(A=3\) exactly. Hewlett's 1939 use of a lamp in this role was the foundation of Hewlett-Packard's first product.
Why the other three do not qualify. Hartley (tapped inductor), Colpitts (tapped capacitor) and the phase-shift oscillator (three-section RC ladder) all use a single feedback path. The phase-shift circuit's inverting amplifier supplies 180° and its ladder the other 180°, but that is one loop, not two — and its amplitude is controlled only by the amplifier's own saturation, which is why its output is less pure than the Wien bridge's.
Hence, the answer is the Wien bridge oscillator.
Similar Questions
The phase locked loop (PLL) is one of the interesting applications of the lock-in amplifier. Apart from FM stereo decoders, tracking filters, motor speed control, FM demodulators, etc. it has found wide applications in generation of local oscillator frequencies in house-hold TV and FM tuners as automatic frequency control (AFC). Indeed, PLL has emerged as one of the fundamental building blocks in electronics and it is commercially available as a single package. Basically, a PLL is a lock-in amplifier in which the reference signal is provided by its own output, converted to frequency by a voltage controlled oscillator (VCO). When locked to the input frequency the dc output is small but sufficient to drive the VCO to produce a frequency which is equal to that of the signal. In this tracking situation, the input signal and the VCO output are almost in phase quadrature and the lock-in amplifier produces a small dc voltage which is often referred to as error voltage. The moment input signal is fed, the VCO frequency starts changing and the PLL is said to be in the capture mode. The VCO continues to change its frequency until it equals that of the input and stays there ; the PLL is then in the phase-locked state. In this state, if there is any change in the input frequency, the loop automatically tracks it through its repetitive action.
Consider the following statements regarding an RC phase shift oscillator :
i. amplifier gain is positive.
ii. amplifier gain is negative.
iii. phase shift introduced by the feedback network is 180°.
iv. phase shift introduced by the feedback network is 360°.Which is correct ?
Assertion (A) : In applications such as FM and FSK, VCO plays an important role.
Reason (R) : The frequency control is easily possible by varying d.c. voltage.
For a FET based phase shift oscillator, what should be the value of capacitor (C) for oscillator operation at 1 kHz. The resistor (R) in the feedback network is 20 kΩ.
The current amplification factor in radian square of Colpitts oscillator is :
The voltage controlled oscillator is used for :
The PLL is in the free-running state when :
Assertion (A) : A monostable multivibrator can be used to alter the pulse width of a repetitive pulse train.
Reason (R) : Monostable multivibrator has a single stable state.
Select your answer using the codes given below :
In an RC phase shift oscillator the frequency of oscillation is given by
Important Questions from Types of Oscillators
Electronic ohmmeter uses OP-AMP as a/an:
Which of the following statements about the Wien Bridge Oscillator is CORRECT?
Hartley Oscillator is a:
Which of the following is the fixed frequency oscillator?
If R = 51 kΩ and C = 0.001 μF, the resonant frequency of a Wien Bridge oscillator is:
