Why this topic is important for ECET?
Multivibrators are the fundamental building blocks of waveform generators and timing circuits. They are widely used in communication systems, digital electronics, oscillators, timers, and pulse generation. ECET often tests questions on frequency, time period, design parameters, and applications, so mastering this topic gives you a competitive edge.
📘 Concept Notes
A multivibrator is an electronic circuit used to generate square, rectangular, or pulse waveforms.
They are built using transistors or op-amps/ICs like 555 timer.
Types of Multivibrators
- Astable Multivibrator
- Has no stable state.
- Continuously oscillates between HIGH and LOW → generates a free-running square wave.
- Used in oscillators, clock pulses.
Example: A 555 timer in astable mode → generates pulses automatically.
- Monostable Multivibrator
- Has one stable state and one quasi-stable state.
- Needs a trigger input → produces a single pulse output.
- Used in timers, delay circuits, digital counters.
Example: In traffic signals, monostable generates fixed delay pulse.
- Bistable Multivibrator
- Has two stable states.
- Requires two trigger inputs to switch between states.
- Used as latches, flip-flops, and memory storage elements.
Practical Applications
- Pulse generation
- Clock signals for digital systems
- Frequency division
- Time delay circuits
- Memory elements in sequential circuits
⚙️ Formulas
- Astable Multivibrator using 555 Timer
Time period:
Frequency:
Duty cycle:
- Monostable Multivibrator (555 Timer)
Pulse width:
- Bistable Multivibrator
No direct frequency formula → defined by input trigger switching.
🔟 10 MCQs
Q1. Which multivibrator has no stable states?
a) Monostable
b) Bistable
c) Astable
d) None
Q2. In monostable mode, 555 timer produces:
a) Continuous square wave
b) One pulse per trigger
c) Two pulses per trigger
d) No output
Q3. For an astable 555 timer, if R1 = 1 kΩ, R2 = 2 kΩ, C = 1 μF, calculate frequency.
Q4. Duty cycle of astable multivibrator using R1 = 1 kΩ and R2 = 2 kΩ is:
a) 33%
b) 50%
c) 66%
d) 75%
Q5. A bistable multivibrator is equivalent to:
a) Flip-flop
b) Oscillator
c) Amplifier
d) Clipper
Q6. In a monostable multivibrator, if R = 10 kΩ and C = 100 nF, find pulse width.
Q7. Which circuit is used to generate clock signals in digital systems?
a) Monostable
b) Astable
c) Bistable
d) Differential amplifier
Q8. A 555 astable circuit generates 1 kHz output. If C = 100 nF, approximate value of (R1 + 2R2) is:
a) 7.2 kΩ
b) 14.4 kΩ
c) 72 kΩ
d) 144 kΩ
Q9. Multivibrators are examples of:
a) Feedback oscillators
b) Positive feedback circuits
c) Negative feedback circuits
d) Linear amplifiers
Q10. The duty cycle of a perfect square wave is:
a) 25%
b) 50%
c) 75%
d) 100%
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | c |
| Q2 | b |
| Q3 | 240 Hz |
| Q4 | c |
| Q5 | a |
| Q6 | 1.1 ms |
| Q7 | b |
| Q8 | b |
| Q9 | b |
| Q10 | b |
🧠 Explanations
- Q1: Astable has no stable state → oscillates continuously → (c).
- Q2: Monostable gives one output pulse for each trigger → (b).
- Q3:
. - Q4: Duty cycle =
. - Q5: Bistable works like flip-flop → (a).
- Q6:
. - Q7: Clock generators use astable → (b).
- Q8:
. - Q9: Multivibrators use positive feedback to sustain states → (b).
- Q10: Square wave duty cycle is always 50% → (b).
🎯 Motivation / Why Practice Matters
In ECET 2026, multivibrators are a high-weightage topic because they connect analog circuits (devices) with digital electronics.
- Practicing ensures speed in solving time-period and frequency problems.
- Builds confidence for higher-level topics like flip-flops, counters, and timers.
- Many ECET MCQs are formula-based → accuracy is key for scoring full marks.
📲 CTA
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