If left to itself, the bistable multivibrator will stay in this position for ever. However, if an external pulse is applied to the circuit in such a way that Q 1 is cut-off and Q 2 is turned on, the circuit will stay in the new position. Another trigger pulse is then required to switch the circuit back to its original state.
In other words a multivibrator which has both the state stable is called a bistable multivibrator. It is also called flip-flop, trigger circuit or binary. The output pulse is obtained when, and why a driving triggering pulse is applied to the input. A full cycle of output is produced for every two triggering pulses of correct polarity and amplitude. Figure a shows the circuit of a bistable multivibrator using two NPN transistors. Similarly, the output of a transistor Q 1 is coupled to the base of transistor Q 2 through a resistor R 1.
The capacitors C 2 and C 1 are known as speed up capacitors. Their function is to increase the speed of the circuit in making abrupt transition from one stable state to another stable state. The transistors used in these circuits will limit the switching time that will be seen on the output pulse. Bipolar transistors used in these circuits will normally have a slow rise time on the output compared to other transistor architecture and families, so they are not ideal for highly precise timing applications.
The construction of a bistable multivibrator circuit depends on how the circuit is triggered. With symmetric triggering, a single input pulse is used to provide regenerative feedback in the circuit. This type of circuit is shown below. Synchronous triggering with a bistable multivibrator. The difference between these two circuits lies in the use of two input pulses asymmetric vs.
With asymmetric triggering, two input pulses are required for triggering, which can be configured to provide two outputs in opposite states. This type of circuit is effectively a Schmitt trigger. Asynchronous triggering with a bistable multivibrator. Perhaps the timer is the most popular IC for building multivibrator circuits. They can be set to run in free-running oscillation mode astable multivibrator or in triggered mode bistable multivibrator by adding a timer IC to some simple passives.
The older TTL timer has been with us for a long time, but newer logic families and transistor manufacturing processes have provided components for building faster multivibrator circuits. The timer can be run as a bistable multivibrator. The standard timer has a limited free-running astable oscillation of about kHz, thanks to the use of bipolar transistors as the active switching elements.
Getting beyond kHz free-running oscillator and below 1 ns rise time takes much faster components. The output waveforms at the collectors of Q 1 and Q 2 along with the trigger inputs given at the bases of Q W and Q 2 are shown in the following figures. Bistable Multivibrators are used in applications such as pulse generation and digital operations like counting and storing of binary information.
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 Q 1 will be OFF as the base is grounded. 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 Q 2 is grounded turning it to OFF state. This is the other stable state. 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 is done to eliminate loading effects.
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. The loading in bistable multivibrator varies according to the application. This causes variation in the saturation level of the transistor.
These are widely used in the circuits of latches and counters. It is also used in frequency divider circuits and in storage devices. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Leave a Reply Cancel reply Your email address will not be published.
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