Astable multivibrator circuit

Figure 1: Basic  BJT astable multivibrator

The circuit shows a simple astable circuit, with an output from collector of Q₁, and an inverted output from collector of Q₂.

Basic mode of operation

The circuit keeps 1 transistor switched on and other switched off. Assume that initially, Q₁ is switched on and Q₂ is switched off.

State 1:

• Q₁ holds bottom of R₁ (and left side of C₁) near ground (0 V).
• The right side of C₁ (and base of Q₂) is being charged by R₂ from below ground to0.6 V.

• R₃ is pulling base of Q1 up, but its base-emitter diode prevents voltage fromincreasing above 0.6 .

• R₄ is charging right side of C₂ up to power supply voltage (+V). As R₄ is less than R2, C₂ charges faster than C₁. When base of Q₂ reaches 0.6 V, Q₂ turns on, and following positive feedback loop takes place:

• Q₂ pulls the right side of C₂ abruptly down to near 0 V.
• Because voltage across a capacitor cannot change suddenly, this causes the left sideof C₂ to fall suddenly to almost -V, well below 0 V.

• Q₁ switches off because of the sudden disappearance of its base voltage.

• R₁ and R₂ work to pull both the ends of C₁ toward +V, completing Q₂'s turn on. The process is stopped by B-E diode of Q₂, which will not let right side of C₁ increase very far.

This takes us to State 2, mirror image of the initial state, where Q₁ is switched off and Q₂ is switched on. Then R₁ quickly pulls C₁'s left side toward +V, while R₃ slowly pulls C₂'s left side toward +0.6 V. When C₂'s left side reaches 0.6 V, cycle repeats.

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