--- title: "Astable multivibrator for square wave generation" subject: "Electronics Devices and Circuit" chapter: 5 key: "semester_3/electronics_devices_and_circuit:astable-multivibrator-square-wave" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/astable-multivibrator-square-wave site: BCT Engineering --- # Astable multivibrator for square wave generation Electronics Devices and Circuit · Chapter 5: Signal Generator and Waveform-Shaping Circuits - Asked in: 2080 Bhadra, 4 marks - Video: [Astable Multivibrator circuit explained in detail | Using BJT](https://www.youtube.com/watch?v=ROJNpp5GI-Q) — HK CircuitEdu - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/astable-multivibrator-square-wave - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-5 ## Question How can we use astable multivibrator for the generation of square wave? ## Short answer An astable multivibrator switches continuously between two quasi-stable states determined by RC time constants, generating a two-level square wave without input. For symmetrical square wave with equal R and C, frequency f = 0.721/(RC). ## Answer An astable multivibrator has no stable state. It switches continuously between its two quasi-stable states, with the time spent in each state set by an RC time constant. Since the output is a two-level waveform, it gives a **square wave** (rectangular wave) without any input. #### Transistor astable multivibrator ```text +VCC | +---------+--------+--------+---------+ | | | | [RC1] [R1] [R2] [RC2] | | | | (x1) (y1) (y2) (x2) | | | | C | | C |--- B ---+ Q1 Q2 +--- B ---| E E | | GND GND C1 joins x1 (collector of Q1) to y2 (base of Q2). C2 joins x2 (collector of Q2) to y1 (base of Q1). ``` R1 connects VCC to the base of Q1, and R2 connects VCC to the base of Q2. The square-wave output is taken from x1 or x2. #### Working 1. One transistor is ON (saturated) and the other OFF. Say Q1 is ON, so its collector is near 0 V, and Q2 is OFF, with its collector at VCC. 2. When Q1 turned ON, its collector fell by about VCC, and C1 passed this drop to the base of Q2, taking it to about −VCC. C1 now charges through R2, and the base of Q2 rises exponentially from about −VCC towards +VCC. When it reaches about 0.6 V, Q2 starts to conduct. 3. Q2 turns ON; its collector falls, and C2 passes this drop to the base of Q1, turning Q1 OFF (regenerative action). 4. Now the roles are reversed: C2 charges through R1 until Q1 turns ON again. 5. The cycle repeats indefinitely. #### Waveforms and frequency ```text vC2 (collector of Q2) VCC +-----+ +-----+ +-----+ | | | | | | 0 --+ +-----+ +-----+ +--- |<-T1>|<-T2>| ``` Each half-period is the time for the base of the OFF transistor to rise from −VCC to about 0 V: - T1 (Q1 ON, Q2 OFF) = 0.693 R2 C1 - T2 (Q2 ON, Q1 OFF) = 0.693 R1 C2 T = T1 + T2 = 0.693 (R1 C2 + R2 C1) For a symmetrical square wave, R1 = R2 = R and C1 = C2 = C: T = 1.386 RC, **f = 1/(1.386 RC) = 0.721/(RC)** The square wave is taken from either collector. Unequal values of R1C2 and R2C1 give an asymmetrical rectangular wave. The same action can be achieved with a 555 timer or an op-amp (relaxation oscillator). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does an astable multivibrator produce a square wave? A. It switches continuously between two states, giving a two-level output, with times set by RC B. It is a linear amplifier C. It needs a sine input D. It has one stable state Answer: A. No stable state means continuous switching between two levels. (Key point: Why square wave) 2. When the OFF transistor's base reaches about 0.6 V, what happens? A. It conducts and its collector drop turns the other transistor OFF via coupling capacitor B. Both turn off C. Both stay on D. The supply is disconnected Answer: A. The regenerative action flips the states. (Key point: Regenerative switching) 3. What is the time T1 for which Q2 stays OFF? A. 0.693 R2 C1 B. 1.386 R C C. R2 C1 D. 0.693 R1 C1 Answer: A. T1 = 0.693 R2 C1 and T2 = 0.693 R1 C2. (Key point: ON time) 4. For R1 = R2 = R and C1 = C2 = C, what is f? A. 1/(1.386 RC) = 0.721/RC B. 1/(0.693 RC) C. 1.44/RC D. 1/(2πRC) Answer: A. T = 1.386 RC so f = 0.721/RC. 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