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Home arrow How To arrow Digital electronics arrow Bistable & Astable Multivibrator circuits
Bistable & Astable Multivibrator circuits PDF Print E-mail
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Bistable & Astable Multivibrator circuits

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. The most common form is the astable or oscillating type, which generates a square wave - the high level of harmonics in its output is what gives the multivibrator its common name.

There are three types of multivibrator circuit:

  • astable, in which the circuit is not stable in either state - it continuously oscillates from one state to the other. Another name for this type of circuit is relaxation oscillator.
  • monostable, in which one of the states is stable, but the other is not - the circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. A common application is in eliminating switch bounce.
  • bistable, in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. Such a circuit is important as the fundamental building block of a register or memory device. This circuit is also known as a flip-flop. A similar circuit is a Schmitt trigger.

In its simplest form the multivibrator circuit consists of two cross-coupled transistors. Using resistor-capacitor networks within the circuit to define the time periods of the unstable states, the various types may be implemented. Multivibrators find applications in a variety of systems where square waves or timed intervals are required, but the simple circuits tend to be fairly inaccurate, so are rarely used where precision is required. An integrated circuit multivibrator, the 555, is very common in electronics. It uses a more sophisticated design to overcome some of the precision issues with the simpler circuits.

Astable Multivibrator circuit

Image:Transistor Multivibrator.svg

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

Suggested values:

  • R1, R4 = 10K
  • R2, R3 = 150K
  • C1, C2 = 10μF
  • Q1, Q2 = 2N3904 or simlar switching transistor

Basic mode of operation

The circuit has two states:

State 1:

  • Q1 switched on
  • Collector of Q1 at 0V
  • C1 charging through R2 (and Q1)
  • Voltage at base of Q2 is the voltage across C1. This is initially low, but rising as C1 charges.
  • Q2 switched off (assuming base voltage < 0.6V)
  • C2 is discharging through R3 and R4 and then charging through R4 and base-emitter of Q1 in the opposite direction, till almost the supply voltage level.
  • Output voltage high (although a little lower than the supply voltage because of the C2 discharge current through R4)
  • This state is self-maintaining until the voltage at base of Q2 reaches 0.6V, at which point Q2 switches on, and the circuit goes into the following state.

State 2

  • Q2 switched on
  • Collector of Q2 (output voltage) goes from +V to 0V
  • This step change on C2 causes a negative going pulse on the base of Q1, which rapidly switches it off.
  • Q1 switched off, its collector rises to about +V.
  • C1 discharging through R1 and R2
  • C2 charging through R3 from -V through 0v to +0.6v (this might be considered a discharge rather than a charge)
  • Voltage at base of Q1 is the voltage across C2. This is initially low, but rising as C2 charges.
  • This state is self-maintaining until the voltage at base of Q1 reaches 0.6V, at which point Q1 switches on, and the circuit flips back into state 1.

Initial power-up

When the circuit is first powered up, neither transistor will be switched on. However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on. This will quickly put the circuit into one of the above states, and oscillation will ensue

Period of oscillation

Very roughly, the duration of state 1 (high output) will be related to the time constant R2.C1 as it depends on the charging of C1, and the duration of state 2 (low output) will be related to the time constant R3.C2 as it depends on the charging of C2 — and these time constants need not be the same, so a custom duty cycle can be achieved.

However, the duration of each state also depends on the initial state of charge of the capacitor in question, and this in turn will depend on the amount of discharge during the previous state, which will also depend on the resistors used during discharge (R1 and R4) and also on the duration of the previous state, etc. The result is that when first powered up, the period will be quite long as the capacitors are initially fully discharged, but the period will quickly shorten and stabilise.

The period will also depend on any current drawn from the output.

Because of all these inaccuracies, more sophisticated timer ICs are commonly used in practice, as described above.

Monostable Multivibrator circuit


Need to expand this section. Similar to the astable but only one capacitor, hence one of the states is stable. Need to add an input to the above circuit.

Output is from the Q2 collector. With R1, R2, R3, R4 = 10K, C1=220μF, period is about 0.5 seconds.

Bistable Multivibrator circuit


Suggested values:

  • R1, R2 = 1K
  • R3, R4 = 10K

Need to expand this section. Similar to the astable/monostable but no capacitor, hence both of the states are stable. Need to add both inputs (set and reset) to the above circuit.

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