What are the Important Multivibrator Circuits for Pulse Generation?

Multi-vibrator circuits refer to the special type of electronic circuits used for generating pulse signals. These pulse signals can be rectangular or square wave signals. They generally produce output in two states: high or low. A specific characteristic of multi-vibrators is the use of passive elements like resistor and capacitor to determine the output state.

Types of Multi-Vibrators

a. Monostable Multi-vibrator: A monostable multivibrator is the type of multivibrator circuit whose output is in only one stable state. It is also known as one-shot multivibrator. In a monostable multivibrator, the output pulse duration is determined by the RC time constant and is given as: 1.11*R*C

b. A Stable Multi-vibrator: A stable vibrator is a circuit with an oscillating output. It doesn’t need any external triggering, and it has got no stable state. It is a type of regenerative oscillator.

c. Bistable Multi-vibrator: A bistable vibrator is a circuit with two stable states: high and low. Generally a switch is required for toggling between the high and low state of the output.

Three Types of Multi-vibrator Circuits

1. Using Transistors

a. Monostable Multi-vibrator

In the above circuit, in absence of any external trigger signal, the base of the transistor T1 is at the ground level and the collector is at a higher potential. Therefore, the transistor is cut off. However, the base of the transistor T2 gets positive voltage supply from the VCC through a resistor, and the transistor T2 is driven to saturation. And, as the output pin is connected to the ground through the T2, it is at logic low level.

When a trigger signal is applied to the base of the transistor T1, it starts conducting as its base current increases. As the transistor conducts, its collector voltage decreases. At the same time, the capacitor C2’s voltage starts discharging through the T1. This causes the potential at the base terminal of the T2 to decrease and eventually the T2 is cut off.  Since the output pin is now directly connected to a positive supply through resistor: Vout is at logic high level.

After sometime, when the capacitor is discharged completely, it starts charging up through the resistor. The potential at the base terminal of transistor T2 starts increasing gradually and eventually the T2 is driven to conduction. Thus, the output is again at a logic low level or the circuit is back to its stable state.

b. Bistable Multivibrator 

The above circuit is a bistable multivibrator circuit with two outputs, defining the two stable states of the circuit.

Initially, when the switch is at the position A, the base of transistor T1 is at the ground potential, and therefore, it is cut off. At the same time, the base of transistor T2 is at a comparatively higher potential, it starts conducting. This causes output pin 1 to be directly connected to the ground, and the Vout1 to be at logic low level.  The output pin2 at the collector of T1 is connected directly to the Vcc, and the Vout2 is at logic high level.

Now, when the switch is at position B, the transistor actions are reversed (T1 is conducting and T2 is cut off) and the output states are reversed.

c. Astable Multivibrator

The above circuit is an oscillator circuit. Suppose, initially the transistor T1 is in conduction and T2 is in cut off.  The output 2 is at logic level, and the output 1 is at logic low level. As the capacitor c2 starts charging up through R4, the potential at the base of T2 starts increasing gradually until T2 starts conducting. This decreases its collector potential and gradually the potential at the base of T1 starts decreasing until it is completely cut off.

Now, as C1 charges through R1, the potential at the base of the transistor T1 starts increasing and eventually it is driven to conduction, and the whole process repeats. Thus, the output is constantly repeating or oscillating.

Apart from using BJTs, other types of transistors are also used in multi-vibrator circuits.

2. Using Logic Gates

a. Mono-Stable Multi-Vibrator

Initially the potential across the resistor is at ground level. This implies a low logic signal to the input of the NOT gate. Thus, the output is at logic high level.

As both the inputs of NAND gate are at logic high levels, the output is at logic low level, and the circuit output remains in its stable state.

Now, suppose a logic low signal is given to one of the inputs of the NAND gate, the other input being at logic high level, the output of the gate is logic 1, i.e., positive voltage. Since there is a potential difference across R, VR1 is at logic high level, and accordingly the output of the NOT gate is logic 0. As this logic low signal is fed back to the input of NAND gate, its output remains at logic 1 and the capacitor voltage starts increasing gradually. This in turn causes the potential drop across the resistor, i.e., VR1 starts decreasing gradually and at one point it goes low, such that a logic low signal is fed to the input of NOT gate, and the output is again at logic high signal. The time period for which the output remains in its stable state is determined by the RC time constant.

b. Astable Multi-vibrator

Initially, when the supply is given, the capacitor is uncharged and a logic low signal is fed to the input of the NOT gate. This causes the output to be at logic high level. As this logic high signal is fed back to the AND gate, its output is at logic 1.  The capacitor starts charging and the input level of the NOT gate increases until it reaches the logic high threshold, and the output is at logic low.

Again, the AND gate output is at logic low (logic low input is being fed back), and the capacitor starts discharging until its potential at input of the NOT gate reaches logic low threshold, and the output is again switched back to the logic high.

This is actually a type of relaxation oscillator circuit.

c. Bistable Multi-vibrator

The simplest form of bistable multi-vibrator is the SR latch, realized by logic gates.

Suppose the initial output is at a logic high level (Set) and the input trigger signal is at a logic low signal (Reset). This causes the output of NAND gate 1 to be at logic high level. As both the inputs of U2 are at logic high level, the output is at logic low level.

Since both the inputs of U3 are at a logic high level, the output is at logic low level, i.e., Reset. The same operation occurs for a logic high signal at the input, and the circuit changes state between 0 and 1. As seen the use of logic gates for multi-vibrators are actually examples of digital logic circuits.

3. Using 555 Timers

555 Timer IC is the most commonly used IC for pulse generation, especially pulse width modulation, for multivibrator circuits.

a. Monostable Multi-vibrator

To connect a 555 timer in monostable mode, a discharge capacitor is connected between the discharge pin 7 and ground. The pulse width of the generated output is determined by the value of the Resistor R between the discharge pin, Vcc and Capacitor C.

If you are aware of the internal circuitry of 555 timer, you must be aware of the fact that a 555 timer works with a transistor, two comparators and a SR flip-flop.

Initially, when the output is at logic low signal, the transistor T is driven to conduction and pin 7 is grounded. Suppose a logic low signal is applied to the trigger input or the input of the comparator, as this voltage is less than 1/3Vcc, the output of the comparator IC goes high, causing the flip-flop to reset such that the output is now at a logic low level.

At the same time, the transistor is switched off and the capacitor starts charging through Vcc. When the capacitor voltage increases beyond 2/3Vcc, the comparator 2 output goes high, causing the SR flip-flop to set. Thus, the output is again at its stable state after a certain time period determined by the values of R and C.

b. Astable Multivibrator

To connect a 555 timer in astable mode, the pins 2 and 6 are shortened and a resistor is connected between pin 6 and 7.

Initially, suppose the output of the SR flip-flop is at a logic low level. This switches off the transistor and the capacitor starts charging to Vcc through Ra and Rb in such a way that, at one time, the input voltage to comparator 2 exceeds the threshold voltage of 2/3Vcc, and the comparator output goes high. This causes the SR flip-flop to set in such a way that the timer output is at logic low.

Now, the transistor is driven to saturation by a logic high signal at its base. The capacitor starts discharging through Rb, and when this capacitor voltage falls below 1/3 Vcc, the output of the comparator C2 is at logic high level. This resets the flip-flop and the timer output is again at logic high level.

c. Bi-stable Multi-vibrator

A 555 timer in bi-stable multi-vibrator doesn’t require the use of any capacitor; rather a SPDT switch is used between ground and pins 2 and 4.

When the switch position is in such a way that the pin 2 is at ground along with pin 6, the output of the comparator 1 is at logic low signal, whereas the output of the comparator 2 is at logic high signal. This resets the SR flip-flop, and the output of the flip flop is logic low. The output of the timer is thus logic high signal.

When the switch position is in such a way that the pin 4, or the reset pin of the flip-flop is grounded, the SR flip-flop is set, and the output is at logic high. The output of the timer is at logic low signal. Thus, depending on the switch position, high and low pulses are obtained.

So, these are the basic multivibrator circuits used for pulse generation. We hope you have got a clear understanding of multi-vibrators.

Here is a simple question for all the readers:

Apart from multi-vibrators, what are the other types of circuits used for pulse generation?