Astable Multivibrator using OpAmp (Basics, Circuit, Working & Waveforms) Explained

Summary

In this video, the concept of astable multivibrators using operational amplifiers is explained in detail. The presenter covers both symmetric and asymmetric configurations, showing how these circuits generate square waves. The symmetric astable multivibrator produces a 50% duty cycle, with equal 'on' and 'off' times, while the asymmetric variant results in different duty cycles. Key concepts like differential configuration, capacitor charging, and the role of operational amplifiers are explored. The video aims to provide a clear understanding of how these circuits work and how they can be used in various applications.

Takeaways

• 😀 A stable multivibrator has two quasi-stable states, generating a square wave without requiring an external trigger.
• 😀 The output of a stable multivibrator alternates between positive and negative voltage states (V+ and V-).
• 😀 Symmetric astable multivibrators have equal on-time (T_on) and off-time (T_off), resulting in a 50% duty cycle.
• 😀 The duty cycle of a symmetric multivibrator is calculated as T_on / (T_on + T_off), which equals 50% when T_on equals T_off.
• 😀 The operational amplifier (op-amp) in a symmetric astable multivibrator works in a differential configuration, amplifying the voltage difference between its two inputs.
• 😀 The capacitor in a symmetric multivibrator charges and discharges according to the RFC time constant, causing the output to switch between V+ and V-.
• 😀 The voltage across the capacitor (V2) controls the switching behavior of the output, with transitions happening when V2 exceeds or falls below the input voltage (V1).
• 😀 In an asymmetric astable multivibrator, T_on and T_off are not equal, leading to a duty cycle that is not 50%.
• 😀 In asymmetric multivibrators, different time constants for charging the capacitor (based on R1*C and R2*C) result in different T_on and T_off durations.
• 😀 The diodes in an asymmetric multivibrator (D1 and D2) control the direction of capacitor charging, influencing the timing of the output transitions.

Q & A

What is the primary function of an astable multivibrator?

An astable multivibrator is primarily used as a square wave generator, where it automatically switches between two quasi-stable states, producing a continuous square wave output.

What does it mean for a state to be 'quasi-stable' in an astable multivibrator?

In an astable multivibrator, both the positive and negative states are quasi-stable, meaning they are not completely stable and automatically change to the opposite state without external triggers.

What is the difference between symmetric and asymmetric astable multivibrators?

In a symmetric astable multivibrator, the on-time (T_on) and off-time (T_off) are equal, resulting in a 50% duty cycle. In an asymmetric astable multivibrator, T_on and T_off are different, leading to a duty cycle other than 50%.

How does the duty cycle relate to the time periods T_on and T_off in a symmetric astable multivibrator?

In a symmetric astable multivibrator, since T_on and T_off are equal, the duty cycle is calculated as T_on / (T_on + T_off), which equals 50%.

What is the role of the operational amplifier (op-amp) in an astable multivibrator?

The operational amplifier in an astable multivibrator is used in differential configuration, where it compares the voltages at its two input terminals, and based on the difference, it switches the output between two states (V+ and V-).

How does the capacitor affect the operation of an astable multivibrator?

The capacitor in an astable multivibrator charges and discharges during the cycle. The voltage across the capacitor (V2) influences the output by triggering state changes when it exceeds the voltage at the positive input (V1).

What happens when the capacitor voltage exceeds the voltage at the positive input terminal in an astable multivibrator?

When the capacitor voltage (V2) exceeds the voltage at the positive input (V1), the operational amplifier switches the output state, causing the output to transition from V+ to V-, or vice versa.

What is the significance of the time constants in an asymmetric astable multivibrator?

In an asymmetric astable multivibrator, the time constants (based on the resistors and capacitor) for the charging and discharging of the capacitor are different, leading to unequal T_on and T_off times, which results in a duty cycle different from 50%.

How does the feedback configuration affect the voltage at the positive input terminal (V1) in an astable multivibrator?

The voltage at the positive input terminal (V1) is determined by the feedback voltage (V_out), which is scaled based on the resistive voltage divider. This feedback controls the switching behavior of the output voltage.

In an asymmetric astable multivibrator, how does the forward biasing of diodes impact the charging of the capacitor?

In an asymmetric astable multivibrator, when the output is at a positive voltage, the forward biasing of the diode allows current to flow and charge the capacitor in the positive direction. Similarly, when the output is negative, the other diode charges the capacitor in the negative direction.