Astable multivibrator for square wave generation

An astable multivibrator has no stable state. It switches continuously between its two quasi-stable states, with the time spent in each state set by an RC time constant. Since the output is a two-level waveform, it gives a square wave (rectangular wave) without any input.

Transistor astable multivibrator

                        +VCC
                          |
       +---------+--------+--------+---------+
       |         |                 |         |
     [RC1]     [R1]              [R2]      [RC2]
       |         |                 |         |
      (x1)      (y1)              (y2)      (x2)
       |         |                 |         |
       C         |                 |         C
       |--- B ---+  Q1        Q2   +--- B ---|
       E                                     E
       |                                     |
      GND                                   GND

  C1 joins x1 (collector of Q1) to y2 (base of Q2).
  C2 joins x2 (collector of Q2) to y1 (base of Q1).

R1 connects VCC to the base of Q1, and R2 connects VCC to the base of Q2. The square-wave output is taken from x1 or x2.

Working

1. One transistor is ON (saturated) and the other OFF. Say Q1 is ON, so its collector is near 0 V, and Q2 is OFF, with its collector at VCC.
2. When Q1 turned ON, its collector fell by about VCC, and C1 passed this drop to the base of Q2, taking it to about −VCC. C1 now charges through R2, and the base of Q2 rises exponentially from about −VCC towards +VCC. When it reaches about 0.6 V, Q2 starts to conduct.
3. Q2 turns ON; its collector falls, and C2 passes this drop to the base of Q1, turning Q1 OFF (regenerative action).
4. Now the roles are reversed: C2 charges through R1 until Q1 turns ON again.
5. The cycle repeats indefinitely.

Waveforms and frequency

 vC2 (collector of Q2)
  VCC  +-----+     +-----+     +-----+
       |     |     |     |     |     |
  0  --+     +-----+     +-----+     +---
       |<-T1>|<-T2>|

Each half-period is the time for the base of the OFF transistor to rise from −VCC to about 0 V:

• T1 (Q1 ON, Q2 OFF) = 0.693 R2 C1
• T2 (Q2 ON, Q1 OFF) = 0.693 R1 C2

T = T1 + T2 = 0.693 (R1 C2 + R2 C1)

For a symmetrical square wave, R1 = R2 = R and C1 = C2 = C: T = 1.386 RC, f = 1/(1.386 RC) = 0.721/(RC)

The square wave is taken from either collector. Unequal values of R1C2 and R2C1 give an asymmetrical rectangular wave. The same action can be achieved with a 555 timer or an op-amp (relaxation oscillator).