Multivibrator

A multivibrator is an electronic circuit used to implement a variety of simple two-state[1][2][3] devices such as relaxation oscillators, timers, latches and flip-flops.

[6] A variety of active devices can be used to implement multivibrators that produce similar harmonic-rich wave forms; these include transistors, neon lamps, tunnel diodes and others.

For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers.

This technique was used in early electronic organs, to keep notes of different octaves accurately in tune.

Since it produced a square wave, in contrast to the sine wave generated by most other oscillator circuits of the time, its output contained many harmonics above the fundamental frequency, which could be used for calibrating high frequency radio circuits.

The circuit has two astable (unstable) states that change alternatively with maximum transition rate because of the "accelerating" positive feedback.

The circuit operation is based on the fact that the forward-biased base-emitter junction of the switched-on bipolar transistor can provide a path for the capacitor restoration.

The resistance R3 is chosen small enough to keep Q1 (not deeply) saturated after C2 is fully charged.

Q2 begins conducting and this starts the avalanche-like positive feedback process as follows.

The forward-biased Q2 base-emitter junction fixes the voltage of C1 right-hand plate at 0.6 V and does not allow it to continue rising toward +V.

The duration of state 1 (low output) will be related to the time constant R2C1 as it depends on the charging of C1, and the duration of state 2 (high output) will be related to the time constant R3C2 as it depends on the charging of C2.

In the charging capacitor equation above, substituting: results in: Solving for t results in: For this circuit to work, VCC>>VBE_Q1 (for example: VCC=5 V, VBE_Q1=0.6 V), therefore the equation can be simplified to: The period of each half of the multivibrator is therefore given by t = ln(2)RC.

The output voltage of the switched-on transistor Q1 changes rapidly from high to low since this low-resistive output is loaded by a high impedance load (the series connected capacitor C1 and the high-resistive base resistor R2).

The output voltage of the switched-off transistor Q1 changes exponentially from low to high since this relatively high resistive output is loaded by a low impedance load (capacitor C1).

To approach the needed square waveform, the collector resistors have to be low in resistance.

The base resistors have to be low enough to make the transistors saturate in the end of the restoration (RB < β.RC).

In practice, oscillation always occurs for practical values of R and C. However, if the circuit is temporarily held with both bases high, for longer than it takes for both capacitors to charge fully, then the circuit will remain in this stable state, with both bases at 0.60 V, both collectors at 0 V, and both capacitors charged backwards to −0.60 V. This can occur at startup without external intervention, if R and C are both very small.

A single pair of active devices can be used to divide a reference by a large ratio, however, the stability of the technique is poor owing to the variability of the power supply and the circuit elements.

Chains of bistable flip-flops provide more predictable division, at the cost of more active elements.

[13] While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the Veb breakdown voltage, typically around 5-10 volts for general purpose silicon transistors.

When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state.

The time period monostable multivibrator remains in unstable state is given by t = ln(2)R2C1.

If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable.

The circuit is useful for generating single output pulse of adjustable time duration in response to a triggering signal.

The width of the output pulse depends only on external components connected to the op-amp.

The diode D1 clamps the capacitor to 0.7 V. The voltage at the non-inverting terminal through the potential divider will be + βVsat.

The diode will now get reverse biased and the capacitor starts charging exponentially to -Vsat through R. The voltage at the non-inverting terminal through the potential divider will be - βVsat.

The capacitor discharges through resistor R and charges again to 0.7 V. The pulse width T of a monostable multivibrator is calculated as follows: The general solution for a low pass RC circuit is where

This latch circuit is similar to an astable multivibrator, except that there is no charge or discharge time, due to the absence of capacitors.

This results in more than half +V volts being applied to R4 causing current into the base of Q1, thus keeping it on.