Explain the working principle of an astable multivibrator using transistors or op-amps.
1 Answer
An astable multivibrator, also known as a free-running multivibrator or oscillator, is a type of electronic circuit that generates a continuous square wave output without the need for an external trigger signal. It is commonly used in applications such as clock generators, signal generators, and timing circuits. The two most common configurations for astable multivibrators are using transistors or operational amplifiers (op-amps). I'll explain the working principles of both:
Astable Multivibrator Using Transistors:
The circuit typically consists of two transistors, each configured in a switching arrangement. Let's assume we use NPN transistors in this explanation.
Components required:
Two NPN transistors (Q1 and Q2)
Four resistors (R1, R2, R3, R4)
Two capacitors (C1 and C2)
Working principle:
Initial State: Assume Q1 is off (non-conducting) and Q2 is on (conducting). The voltage at the collector of Q2 (Vc2) is low, and the capacitor C1 is discharged.
Charging Phase: As Q2 is conducting, C1 starts charging through R1 and R2. This charging action continues until C1's voltage reaches a certain threshold (around 2/3 of the power supply voltage Vcc).
Switching States: Once C1 reaches the threshold voltage, Q2 turns off (non-conducting) rapidly, and Q1 turns on (conducting). Now, the roles are reversed, and C2 starts discharging through R3 and R4.
Discharging Phase: C2's voltage decreases until it reaches a lower threshold (around 1/3 of Vcc). At this point, Q1 turns off, and Q2 turns on, completing the cycle.
Cycle Repeats: The circuit continually oscillates between charging and discharging phases, generating a continuous square wave output at the collectors of Q1 and Q2.
The oscillation frequency (f) of the astable multivibrator can be calculated using the formula:
f = 1.44 / ((R1 + 2 * R2) * C1)
Astable Multivibrator Using Op-Amps:
This configuration uses two op-amps, configured as comparators with positive feedback.
Components required:
Two op-amps (IC1 and IC2)
Four resistors (R1, R2, R3, R4)
Two capacitors (C1 and C2)
Working principle:
Initial State: Assume the output of IC1 is high and the output of IC2 is low. Capacitor C1 is discharged.
Charging Phase: The capacitor C1 starts charging through R1 and R2.
Switching States: When the voltage across C1 exceeds the reference voltage at the non-inverting input of IC2 (set by the voltage divider R3 and R4), the output of IC2 switches to a high state.
Discharging Phase: Capacitor C2 starts discharging through R3 and R4.
Cycle Repeats: The circuit continually oscillates between charging and discharging phases, generating a continuous square wave output.
The oscillation frequency (f) of the astable multivibrator can be calculated using the formula:
f = 1.44 / ((R1 + 2 * R2) * C1)
Both configurations operate based on the principle of positive feedback, where the output signal feeds back to the input to maintain the oscillations. The capacitor charging and discharging cycles cause the output to switch between high and low states, creating the square wave output.
Astable Multivibrator Using Transistors:
The circuit typically consists of two transistors, each configured in a switching arrangement. Let's assume we use NPN transistors in this explanation.
Components required:
Two NPN transistors (Q1 and Q2)
Four resistors (R1, R2, R3, R4)
Two capacitors (C1 and C2)
Working principle:
Initial State: Assume Q1 is off (non-conducting) and Q2 is on (conducting). The voltage at the collector of Q2 (Vc2) is low, and the capacitor C1 is discharged.
Charging Phase: As Q2 is conducting, C1 starts charging through R1 and R2. This charging action continues until C1's voltage reaches a certain threshold (around 2/3 of the power supply voltage Vcc).
Switching States: Once C1 reaches the threshold voltage, Q2 turns off (non-conducting) rapidly, and Q1 turns on (conducting). Now, the roles are reversed, and C2 starts discharging through R3 and R4.
Discharging Phase: C2's voltage decreases until it reaches a lower threshold (around 1/3 of Vcc). At this point, Q1 turns off, and Q2 turns on, completing the cycle.
Cycle Repeats: The circuit continually oscillates between charging and discharging phases, generating a continuous square wave output at the collectors of Q1 and Q2.
The oscillation frequency (f) of the astable multivibrator can be calculated using the formula:
f = 1.44 / ((R1 + 2 * R2) * C1)
Astable Multivibrator Using Op-Amps:
This configuration uses two op-amps, configured as comparators with positive feedback.
Components required:
Two op-amps (IC1 and IC2)
Four resistors (R1, R2, R3, R4)
Two capacitors (C1 and C2)
Working principle:
Initial State: Assume the output of IC1 is high and the output of IC2 is low. Capacitor C1 is discharged.
Charging Phase: The capacitor C1 starts charging through R1 and R2.
Switching States: When the voltage across C1 exceeds the reference voltage at the non-inverting input of IC2 (set by the voltage divider R3 and R4), the output of IC2 switches to a high state.
Discharging Phase: Capacitor C2 starts discharging through R3 and R4.
Cycle Repeats: The circuit continually oscillates between charging and discharging phases, generating a continuous square wave output.
The oscillation frequency (f) of the astable multivibrator can be calculated using the formula:
f = 1.44 / ((R1 + 2 * R2) * C1)
Both configurations operate based on the principle of positive feedback, where the output signal feeds back to the input to maintain the oscillations. The capacitor charging and discharging cycles cause the output to switch between high and low states, creating the square wave output.