What is an Astable Multivibrator?
An astable multivibrator is a type of electronic circuit that produces a continuous square wave output without any external input. It is also known as a free-running oscillator or a relaxation oscillator because it switches between two unstable states by itself. The astable multivibrator is one of the three types of multivibrators, along with the monostable multivibrator and the bistable multivibrator.
A multivibrator is a circuit that has two output states and can switch between them. It usually consists of two amplifying stages that are cross-coupled by a feedback network. The feedback network can be made of active components, such as transistors, or passive components, such as resistors and capacitors. The feedback network determines the mode of operation and the frequency of oscillation of the multivibrator.
The astable multivibrator has no stable output state, meaning that it does not stay in one state for a fixed period of time. Instead, it alternates between two states at a constant rate, producing a square wave signal. The frequency and duty cycle of the square wave can be adjusted by changing the values of the components in the feedback network.
How does an Astable Multivibrator work?
The basic circuit diagram of an astable multivibrator using two bipolar junction transistors (BJTs) is shown below:
The circuit consists of two identical transistors Q1 and Q2, two capacitors C1 and C2, and four resistors R1, R2, RC1, and RC2. The transistors are connected as common-emitter amplifiers with 100% positive feedback. The capacitors provide the coupling between the collector of one transistor and the base of the other. The resistors control the charging and discharging of the capacitors and the biasing of the transistors.
The operation of the circuit can be explained as follows:
- When the circuit is powered on, one of the transistors will turn on faster than the other due to slight differences in their characteristics. Let us assume that Q2 turns on first and Q1 turns off.
- When Q2 is on, its collector voltage drops to almost zero, and its collector current flows through RC2. This causes the right plate of C2 to be grounded and its left plate to be at zero volts as well (assuming C2 is initially uncharged). This voltage is applied to the base of Q1, keeping it off.
- When Q1 is off, its collector voltage rises to almost Vcc, and its collector current is zero. This causes the left plate of C1 to be at Vcc and its right plate to be at 0.7 volts (assuming C1 is initially uncharged). This voltage is applied to the base of Q2, keeping it on.
- At this point, the output O1 is high (Vcc), and the output O2 is low (0 volts). This is one state of the circuit.
- Next, C1 starts to charge through R1, and C2 starts to charge through R2. As C1 charges, its right plate voltage increases gradually from 0.7 volts to Vcc. As C2 charges, its left plate voltage increases gradually from 0 volts to 0.7 volts.
- When the right plate voltage of C1 reaches Vcc, it has no effect on Q2 as it is already on. However, when the left plate voltage of C2 reaches 0.7 volts, it turns on Q1 by forward biasing its base-emitter junction.
- When Q1 turns on, its collector voltage drops to almost zero, and its collector current flows through RC1. This causes the left plate of C1 to be grounded and its right plate to be at -Vcc (due to charge conservation). This voltage turns off Q2 by reverse biasing its base-emitter junction.
- When Q2 turns off, its collector voltage rises to almost Vcc, and its collector current is zero. This causes the right plate of C2 to be at Vcc and its left plate to be at 0.7 volts (due to charge conservation). This voltage keeps Q1 on.
- At this point, the output O1 is low (0 volts), and the output O2 is high (Vcc). This is the other state of the circuit.
- Next, C2 starts to charge through R2, and C1 starts to charge through R1. As C2 charges, its left plate voltage decreases gradually from 0.7 volts to 0 volts. As C1 charges, its right plate voltage increases gradually from -Vcc to 0.7 volts.
- When the left plate voltage of C2 reaches 0 volts, it has no effect on Q1 as it is already off. However, when the right plate voltage of C1 reaches 0.7 volts, it turns on Q2 by forward biasing its base-emitter junction.
- When Q2 turns on, its collector voltage drops to almost zero, and its collector current flows through RC2. This causes the right plate of C2 to be grounded and its left plate to be at -Vcc (due to charge conservation). This voltage turns off Q1 by reverse biasing its base-emitter junction.
- When Q1 turns off, its collector voltage rises to almost Vcc, and its collector current is zero. This causes the left plate of C1 to be at Vcc and its right plate to be at 0.7 volts (due to charge conservation). This voltage keeps Q2 on.
- At this point, the output O1 is high (Vcc), and the output O2 is low (0 volts). This is the same state as before, completing one cycle of oscillation.
The cycle repeats indefinitely as long as the power supply is connected. The time taken for one cycle is called the period (T) of the oscillation, and the number of cycles per second is called the frequency (f) of the oscillation. The ratio of the time spent in one state to the total time spent in both states is called the duty cycle (D) of the oscillation.
How to calculate the frequency and duty cycle of an Astable Multivibrator?
The frequency and duty cycle of an astable multivibrator depends on the values of the resistors and capacitors in the circuit. The following formulas can be used to calculate them:
- Frequency:
- Duty cycle:
Where:
- t1
- Pin 6: Threshold
- Pin 7: Discharge
- Pin 8: Vcc
The working principle of the 555 timer IC as an astable multivibrator is similar to the transistor-based one. The capacitor C charges and discharges through resistors R1 and R2, while the output oscillates between high and low states. The trigger and threshold inputs (pins 2 and 6) are connected together and to the capacitor C. The control input (pin 5) is bypassed with a capacitor C2 to prevent noise interference. The reset input (pin 4) is connected to Vcc to enable the circuit. The discharge input (pin 7) is connected to the junction of R1 and R2. The output (pin 3) can drive a load of up to 200 mA.
The operation of the circuit can be explained as follows:
- When the circuit is powered on, the capacitor C starts to charge through R1 and R2. As the capacitor voltage increases, it is compared with two reference voltages by two internal comparators inside the 555 timers IC. One comparator compares the capacitor voltage with 2/3 Vcc, and the other compares it with 1/3 Vcc.
- When the capacitor voltage reaches 2/3 Vcc, the output of the first comparator goes high and sets an internal flip-flop. This causes the output (pin 3) to go low and the discharge (pin 7) to go low as well. The low discharge pin shorts R2 to the ground, creating a discharge path for capacitor C.
- When the capacitor voltage drops to 1/3 Vcc, the output of the second comparator goes high and resets the internal flip-flop. This causes the output (pin 3) to go high and the discharge (pin 7) to go high as well. The high discharge pin opens the discharge path for capacitor C, allowing it to charge again through R1 and R2.
- The cycle repeats indefinitely as long as the power supply is connected. The time taken for one cycle is called the period (T) of the oscillation, and the number of cycles per second is called the frequency (f) of the oscillation. The ratio of the time spent in a high state to the total time spent in both states is called the duty cycle (D) of the oscillation.
The frequency and duty cycle of an astable multivibrator using a 555 timer IC can be calculated using the following formulas:
- Frequency:
- Duty cycle:
Conclusion
An astable multivibrator is a circuit that produces a continuous square wave output without any external input. It can be implemented using different types of components, such as transistors, op-amps, or 555 timer ICs. The frequency and duty cycle of the output wave can be adjusted by changing the values of the resistors and capacitors in the feedback network.
Astable multivibrators are widely used in applications such as pulse generation, frequency modulation, signal processing, and timing circuits. They are also useful for creating harmonic-rich waveforms that can be used for sound synthesis, music, and communication. Astable multivibrators are simple, reliable, and easy-to-construct circuits that can perform various functions in electronic systems.
