Pulse Circuits - Bistable Multivibrator

A Bistable Multivibrator has two stable states. The circuit stays in any one of the two stable states. It continues in that state, unless an external trigger pulse is given. This Multivibrator is also known as Flip-flop. This circuit is simply called as Binary.

There are few types in Bistable Multivibrators. They are as shown in the following figure.

Construction of Bistable Multivibrator

Two similar transistors Q₁ and Q₂ with load resistors R_L1 and R_L2 are connected in feedback to one another. The base resistors R₃ and R₄ are joined to a common source –V_BB. The feedback resistors R₁ and R₂ are shunted by capacitors C₁ and C₂ known as Commutating Capacitors. The transistor Q₁ is given a trigger input at the base through the capacitor C₃ and the transistor Q₂ is given a trigger input at its base through the capacitor C₄.

The capacitors C₁ and C₂ are also known as Speed-up Capacitors, as they reduce the transition time, which means the time taken for the transfer of conduction from one transistor to the other.

The following figure shows the circuit diagram of a self-biased Bistable Multivibrator.

Operation of Bistable Multivibrator

When the circuit is switched ON, due to some circuit imbalances as in Astable, one of the transistors, say Q₁ gets switched ON, while the transistor Q₂ gets switched OFF. This is a stable state of the Bistable Multivibrator.

By applying a negative trigger at the base of transistor Q₁ or by applying a positive trigger pulse at the base of transistor Q₂, this stable state is unaltered. So, let us understand this by considering a negative pulse at the base of transistor Q₁. As a result, the collector voltage increases, which forward biases the transistor Q₂. The collector current of Q₂ as applied at the base of Q₁, reverse biases Q₁ and this cumulative action, makes the transistor Q₁ OFF and transistor Q₂ ON. This is another stable state of the Multivibrator.

Now, if this stable state has to be changed again, then either a negative trigger pulse at transistor Q₂ or a positive trigger pulse at transistor Q₁ is applied.

Output Waveforms

The output waveforms at the collectors of Q₁ and Q₂ along with the trigger inputs given at the bases of Q_W and Q₂ are shown in the following figures.

Advantages

The advantages of using a Bistable Multivibrator are as follows −

Stores the previous output unless disturbed.
Circuit design is simple

Disadvantages

The drawbacks of a Bistable Multivibrator are as follows −

Two kinds of trigger pulses are required.
A bit costlier than other Multivibrators.

Applications

Bistable Multivibrators are used in applications such as pulse generation and digital operations like counting and storing of binary information.

Fixed-bias Binary

A fixed-bias binary circuit is similar to an Astable Multivibrator but with a simple SPDT switch. Two transistors are connected in feedback with two resistors, having one collector connected to the base of the other. The figure below shows the circuit diagram of a fixed-bias binary.

To understand the operation, let us consider the switch to be in position 1. Now the transistor Q₁ will be OFF as the base is grounded. The collector voltage at the output terminal V_O1 will be equal to V_CC which turns the transistor Q₂ ON. The output at the terminal V_O2 goes LOW. This is a stable state which can be altered only by an external trigger. The change of switch to position 2, works as a trigger.

When the switch is altered, the base of transistor Q₂ is grounded turning it to OFF state. The collector voltage at V_O2 will be equal to V_CC which is applied to transistor Q₁ to turn it ON. This is the other stable state. The triggering is achieved in this circuit with the help of a SPDT Switch.

There are two main types of triggering given to the binary circuits. They are

Symmetrical Triggering
Asymmetrical Triggering

Schmitt Trigger

Another type of binary circuit which is ought to be discussed is the Emitter Coupled Binary Circuit. This circuit is also called as Schmitt Trigger circuit. This circuit is considered as a special type of its kind for its applications.

The main difference in the construction of this circuit is that the coupling from the output C₂ of the second transistor to the base B1 of the first transistor is missing and that feedback is obtained now through the resistor R_e. This circuit is called as the Regenerative circuit for this has a positive feedback and no Phase inversion. The circuit of Schmitt trigger using BJT is as shown below.

Initially we have Q₁ OFF and Q₂ ON. The voltage applied at the base of Q₂ is V_CC through R_C1 and R₁. So the output voltage will be

$$V_0 = V_{CC} - (I_{C2}R_{c2})$$

As Q₂ is ON, there will be a voltage drop across R_E, which will be (I_C2 + I_B2) R_E. Now this voltage gets applied at the emitter of Q₁. The input voltage is increased and until Q₁ reaches cut-in voltage to turn ON, the output remains LOW. With Q₁ ON, the output will increase as Q₂ is also ON. As the input voltage continues to rise, the voltage at the points C₁ and B₂ continue to fall and E₂ continues to rise. At certain value of the input voltage, Q₂ turns OFF. The output voltage at this point will be V_CC and remains constant though the input voltage is further increased.

As the input voltage rises, the output remains LOW until the input voltage reaches V₁ where

$$V_1 = [V_{CC} - (I_{C2}R_{C2})]$$

The value where the input voltage equals V₁, lets the transistor Q₁ to enter into saturation, is called UTP (Upper Trigger Point). If the voltage is already greater than V₁, then it remains there until the input voltage reaches V₂, which is a low level transition. Hence the value for which input voltage will be V₂ at which Q₂ gets into ON condition, is termed as LTP (Lower Trigger Point).

Output Waveforms

The output waveforms are obtained as shown below.

The Schmitt trigger circuit works as a Comparator and hence compares the input voltage with two different voltage levels called as UTP (Upper Trigger Point) and LTP (Lower Trigger Point). If the input crosses this UTP, it is considered as a HIGH and if it gets below this LTP, it is taken as a LOW. The output will be a binary signal indicating 1 for HIGH and 0 for LOW. Hence an analog signal is converted into a digital signal. If the input is at intermediate value (between HIGH and LOW) then the previous value will be the output.

This concept depends upon the phenomenon called as Hysteresis. The transfer characteristics of electronic circuits exhibit a loop called as Hysteresis. It explains that the output values depends upon both the present and the past values of the input. This prevents unwanted frequency switching in Schmitt trigger circuits

Advantages

The advantages of Schmitt trigger circuit are

Perfect logic levels are maintained.
It helps avoiding Meta-stability.
Preferred over normal comparators for its pulse conditioning.

Disadvantages

The main disadvantages of a Schmitt trigger are

If the input is slow, the output will be slower.
If the input is noisy, the output will be noisier.

Applications of Schmitt trigger

Schmitt trigger circuits are used as Amplitude Comparator and Squaring Circuit. They are also used in Pulse conditioning and sharpening circuits.

These are the Multivibrator circuits using transistors. The same Multivibrators are designed using operational amplifiers and also IC 555 timer circuits, which are discussed in further tutorials.