Q3Analog Circuits

An Astable Multivibrator is a free-running oscillator that has no stable states. It continuously switches back and forth between two unstable states, generating a continuous square wave output without the need for an external trigger.

The circuit consists of two cross-coupled CE amplifier stages (Transistors $Q_1$ and $Q_2$). The output (collector) of $Q_1$ is coupled to the input (base) of $Q_2$ via a capacitor $C_1$, and the collector of $Q_2$ is coupled to the base of $Q_1$ via $C_2$.

1. Due to slight component mismatches, when power is turned on, one transistor (e.g., $Q_1$) turns ON slightly faster than the other ($Q_2$). 2. As $Q_1$ turns fully ON (saturates), its collector voltage drops to nearly 0V. This voltage drop is transmitted through $C_1$ to the base of $Q_2$, driving $Q_2$ into cut-off (OFF). 3. $Q_2$ remains OFF while $C_1$ charges through a base resistor. Once $C_1$ charges to ~0.7V, $Q_2$ becomes forward-biased and turns ON. 4. When $Q_2$ turns ON, its collector drops to 0V. This drop is transmitted via $C_2$ to the base of $Q_1$, instantly turning $Q_1$ OFF. 5. The cycle repeats continuously. The frequency of the square wave is determined by the RC time constants of the charging capacitors: $f=\frac{1}{1.38RC}$ (assuming symmetric components).