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 :
Both (A) and (R) are true, but (R) is not correct explanation of (A).
Both (A) and (R) are true, but (R) is not the correct explanation of (A) — option (B).
The assertion is true. A monostable multivibrator — the one-shot — responds to any trigger by producing an output pulse of one fixed width, whatever the width of the trigger. Feed it a repetitive pulse train and every incoming pulse is replaced by a pulse of the standard width, so the train is reshaped. This is its most common use, along with pulse regeneration and switch debouncing.
The reason is true as well: a monostable does have exactly one stable state. That is what distinguishes it from its two siblings :
| Multivibrator | Stable states | Behaviour |
|---|---|---|
| Astable | None | Free-runs; a square-wave oscillator |
| Monostable | One | Rests there until triggered, fires one pulse, returns |
| Bistable | Two | Stays in either until told to change — a flip-flop |
Why (R) does not explain (A). The two statements answer different questions :
| Statement | What it accounts for |
|---|---|
| (R) one stable state | Why the circuit returns by itself after firing — why it produces a single pulse rather than latching or oscillating |
| (A) alters pulse width | Why that pulse has a particular width the designer chooses |
The width is set not by the number of stable states but by the RC time constant that governs the quasi-stable interval. For the 555 in monostable mode,
\(T=1.1\,RC\)
and the pulse lasts until the timing capacitor charges to two-thirds of the supply. Change R or C and the output width changes; the count of stable states is untouched. So (R) states a true and relevant property that is not the operative cause — the situation code (B) describes.
The distinction to carry away. Having one stable state explains that the pulse ends; the RC network explains when. A reason can be true, and about the same device, and still not be the reason for the specific claim made.
A practical caution: the one-shot is non-retriggerable in its basic form — a trigger arriving during the output pulse is ignored, so the input rate must stay below \(1/T\). Retriggerable versions restart the timing on each trigger instead, which is what makes them useful as missing-pulse detectors.
Hence, the answer is option (B).
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.
Which of the following oscillations makes use of both positive and negative feedback ?
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 :
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:
