Astable, monostable and bistable.
Astable- In which the circuit is not stable in either state. It continually
switches from one state to another. No clock pulse or any other
input is needed in case of astable multivibrator.
Monostable- in this kind of multivibrator, one of the states is stable while
the other is unstable. What happens actually is that we provide a trigger. But after some
time the circuit itself becomes stable. Monostable circuits are used
to create timing pulses of a fixed duration.
Bistable-In bistable circuits, both states are stable. In this state,
flipping between the states takes place by trigger.
Lets throw some more light on these different types of multivibrators.
Astable multivibrator- This is a regenerative circuit which consists of two
amplifying states connected in a positive feedback loop. It is connected by
coupling of capacitors and resistors. Now we will emphasize on its working. For
this we need to take a look of its circuit once. The astable circuit consists of
two switching transistors, a cross-coupled feedback network, and two time delay
capacitors which allows oscillation between the two states with no external
trigger signal to produce the change in state.
Monostable multivibrators have only one state which is stable. It produces a
single output pulse. This output is produced when an external input in the form
of trigger is provided to it. This is also a RC coupled circuit provided with a
time constant. The circuit returns back to its stable state after a certain
period of time which is determined by the time constant. Monostable
multivibrators are also known as one-shot multivibrators. The capacitor first
charges itself with the help of a resistor and then it is discharged and brings
the circuit's state to its stable state. Monostable type has the capability to
produce a very short pulse and long pulse as well. It is to be noted that the time constant can
be varied by changing the values of capacitors. The leading edge rises according
to trigger applied while its declining edge depends on the time-period for which
the trigger is applied.
As we have already covered, one fact is that a bistable has two stable states.
The bistable multivibrator can be switched over from one stable state to the
other by the application of an external trigger pulse thus, it requires two
external trigger pulses before it returns back to its original state. As
bistable multivibrators have two stable states they are more commonly known as
Latches and Flip-flops for use in sequential type circuits.
The discrete Bistable Multivibrator is a two state non-regenerative device
constructed from two cross-coupled transistor switches. In each of the two
states, one of the transistors is cut-off while the other transistor is in
saturation, so as to fulfill the criteria of being stable all the time.
Working: Now we will see how a two transistor astable multivibrator works. We can understand its proper function with the help of the pictures below. For a better understanding lets take an example of any of the multivibrators. I am taking an example of astable multivibrator.
Assume that the transistor connected to a green LED is turned off and its collector voltage is rising towards the power supply; at that time second transistor is turned on. The other side of the capacitor is connected to the base terminal of the second transistor is at 0.6v as transistor T2 is conducting. Thus capacitor has potential difference of 5.4v. The first Capacitor starts its charging in opposite direction through resister R3.
Gowtham RajamanickamPosted Apr 20, 2015, 3:06 PM
Good one,,...