Bistable Multivibrator

Definition: A type of multivibrator whose output consists of 2 stable states is known as Bistable Multivibrator. The circuit switches from one stable state to the other when an appropriate trigger pulse is applied.

In a nutshell, in a bistable multivibrator, one stable state can be maintained until a triggering pulse is provided.

It is also known as flip-flop multivibrator because due to applied trigger pulses, multivibrator flip from one stable state. And by applying the next trigger pulse it flops back to its initial stable state.

Circuit and Operation of Bistable Multivibrator

The figure below shows the circuit of a fixed bias bistable multivibrator:

In fixed-bias bistable multivibrators, the device is biased by using two different sources. The circuit comprises of 2 identical transistors namely, Q₁ and Q₂. And the output of each is given as input to the other.

The resistors R₁ and R₂ are the part of the feedback network. Here, the two collector resistances R_C1 and R_C2 are equal.

Let us now move further and understand the working of a bistable multivibrator.

• As the power supply is provided both the transistors come into conducting state. As we know that no two transistors are exactly equivalent and show some variation in their operating principle. Thus, one of the transistors conducts more than the other.

Let us consider that here Q₁ conducts somewhat more than Q₂. Hence for simplicity, we assume that Q₁ is conducting and Q₂ almost negligibly conducting.

As Q₁ is conducting, thus its collector potential start dropping. This low potential is fed to the base of transistor Q₂ through resistor R₂. Due to the applied low voltage, the conducting state of Q₂ drops. This causes the collector potential of transistor Q₂ to increase.

This increased potential is then fed to the base of transistor Q₁ through resistor R₁. This potential puts the transistor in a more forward biased condition. Due to this cumulative action, Q₁ reaches saturation and Q₂ reaches cutoff.

The collector potential of saturated transistor Q₁ becomes nearly 0 and the collector potential of cut-off transistor Q₂ reaches the maximum value of the supply voltage i.e., V_CC.

As we are taking the output from the collector of transistor Q₂ through C_C. Hence, the first stable state achieved is 1.

• Now, consider a negative trigger pulse of high magnitude and short duration is provided to the base of transistor Q₂ through a capacitor C₁. This trigger pulse causes a reduction in the forward biasing provided to the transistor Q₁. Due to this, Q₁ now conducts less than the previous case.

This leads to an increase in the collector potential of transistor Q₁. This increased voltage is then provided to the base of transistor Q₂. That causes Q₂ to conduct more than earlier. Thus, a low voltage is achieved at the collector of transistor Q₂.

This low voltage is fed back to the base of transistor Q₁. This causes it to conduct less thus generating a high potential at the collector of transistor Q₁. This successive cycle drives Q₁ to cutoff and Q₂ to saturation. Thereby, generating a low voltage at the collector of transistor Q₂. So, now 0 is achieved at the output due to saturated transistor Q₂.

Hence, we get 0 as the next stable state of the bistable multivibrator.

In order to have transition from current stable state to the next stable state, another triggering pulse is required to provide.

The output of the bistable multivibrator is shown below:

It is to be noted here that, the maximal collector current is achieved in case of saturation condition. So, in order to limit this current R_C1 and R_C2 must be chosen accordingly.

Also, the value of four resistances R₁, R₂, R₃ and R₄ must be larger than R_C1 and R_C2. This is done to eliminate loading effects.

Loading in Bistable Multivibrator

As bistable multivibrators are used to drive other circuits. Thus, shunting loads must be present at one or both the collectors of the transistors. These loads decrease the collector potential of the cut-off transistor.

Due to this, the voltage at the output will show less swing.

The loading in bistable multivibrator varies according to the application. This causes variation in the saturation level of the transistor.

Advantages of Bistable Multivibrator

• It has the ability to store previous output until no any input trigger is provided.
• The circuit design is not complex.

Disadvantages of Bistable Multivibrator

• Every time in order to have transition from one stable state to another, triggering pulse is required.
• It is somewhat costly than astable and monostable multivibrator.

Applications of Bistable Multivibrator

These are widely used in the circuits of latches and counters. It is also used in frequency divider circuits and in storage devices.