Explain the operation of an astable multivibrator (oscillator).
1 Answer
An astable multivibrator, also known as an astable oscillator, is an electronic circuit that generates a continuous square wave or rectangular waveform without the need for external triggering signals. It's a type of multivibrator circuit that doesn't have a stable state; hence the term "astable." Instead, it continuously alternates between two unstable states, causing the output voltage to oscillate between high and low levels.
The primary components of an astable multivibrator circuit are two cross-coupled amplifying elements, typically transistors, although other devices like operational amplifiers (op-amps) can also be used. Let's walk through the operation step by step:
Initial State: Assume that one of the transistors is initially in an ON state (saturated) while the other is in an OFF state (cut-off). The transistor in the ON state allows current to flow through a resistor, connected to the collector-emitter path of the transistor, and charges a capacitor.
Charging Phase: The capacitor charges up relatively quickly through the resistor connected to the ON transistor. As the capacitor voltage increases, the voltage at the base of the OFF transistor begins to rise.
Threshold Reached: The voltage at the base of the OFF transistor rises to a point where it becomes sufficient to turn it ON. This transition causes the OFF transistor to enter the active region, allowing current to flow through its collector-emitter path.
Discharging Phase: When the OFF transistor turns ON, it creates a low-impedance path for the previously charged capacitor to discharge. The capacitor discharges through the collector-emitter path of the newly ON transistor and another resistor.
Voltage Drop: As the capacitor discharges, the voltage at the base of the ON transistor starts to decrease. Eventually, the voltage drops to a point where the ON transistor turns OFF.
Restarting the Cycle: With the ON transistor turned OFF, the capacitor charging process begins anew. The cycle then repeats, causing the circuit to continually switch between charging and discharging phases.
This alternating process continues indefinitely, generating a square wave output signal at the junction of the collector of the ON transistor and the emitter of the OFF transistor. The frequency of oscillation is determined by the values of the resistors, capacitors, and the characteristics of the transistors used. The duty cycle of the square wave (the ratio of time the output is high to the total period) is also determined by these component values.
Astable multivibrators find applications in various electronic systems, including timing circuits, clock generators, pulse generators, and tone generation in audio circuits. They're versatile and can be easily tuned by adjusting the component values to achieve the desired frequency and duty cycle for the generated waveform.
The primary components of an astable multivibrator circuit are two cross-coupled amplifying elements, typically transistors, although other devices like operational amplifiers (op-amps) can also be used. Let's walk through the operation step by step:
Initial State: Assume that one of the transistors is initially in an ON state (saturated) while the other is in an OFF state (cut-off). The transistor in the ON state allows current to flow through a resistor, connected to the collector-emitter path of the transistor, and charges a capacitor.
Charging Phase: The capacitor charges up relatively quickly through the resistor connected to the ON transistor. As the capacitor voltage increases, the voltage at the base of the OFF transistor begins to rise.
Threshold Reached: The voltage at the base of the OFF transistor rises to a point where it becomes sufficient to turn it ON. This transition causes the OFF transistor to enter the active region, allowing current to flow through its collector-emitter path.
Discharging Phase: When the OFF transistor turns ON, it creates a low-impedance path for the previously charged capacitor to discharge. The capacitor discharges through the collector-emitter path of the newly ON transistor and another resistor.
Voltage Drop: As the capacitor discharges, the voltage at the base of the ON transistor starts to decrease. Eventually, the voltage drops to a point where the ON transistor turns OFF.
Restarting the Cycle: With the ON transistor turned OFF, the capacitor charging process begins anew. The cycle then repeats, causing the circuit to continually switch between charging and discharging phases.
This alternating process continues indefinitely, generating a square wave output signal at the junction of the collector of the ON transistor and the emitter of the OFF transistor. The frequency of oscillation is determined by the values of the resistors, capacitors, and the characteristics of the transistors used. The duty cycle of the square wave (the ratio of time the output is high to the total period) is also determined by these component values.
Astable multivibrators find applications in various electronic systems, including timing circuits, clock generators, pulse generators, and tone generation in audio circuits. They're versatile and can be easily tuned by adjusting the component values to achieve the desired frequency and duty cycle for the generated waveform.