Bistable Multi Vibrator

Prior to the Lab session:

1. Study the operation and working principle Bistable Multivibrator.
2. Study the procedure for conducting the experiment in the lab.

Objective:

1. To Design a Bistable Multivibrator & observe its response.

Apparatus:

1. CRO 0 to 20 MHz (Dual channel)          -           1No.
2. Function generator 1Hz to 1 MHz          -           1No.
3. Capacitor (0.001F, 0.33 F)               -           2 Nos each.
4. Resistors (1 k, 10k, 100K)           -           2 Nos each.
5. Transistor (BC 107)                              -           2 No. each.
6. Diode (IN4007)                                     -           4 No. each.
7. Regulated Power supply 0 – 30 V(dual ) -           1 No.
8. Connecting wires        
9. Bread board                                        

Circuit diagram:

Theory:

A Bistable  circuit is one which can exist indefinitely  in either  of  two stable  states  and  which can  be  induced  to make an abrupt transition  from one state to the other by means of external excitation. The Bistable circuit is also called as Bistable multivibrator, Eccles Jordon circuit, Trigger circuit, Scale-of-2 toggle circuit, Flip-Flop & Binary.

A Bistable multivibratior is used in a many digital operations such as counting and the storing of binary information. It is also used in the generation and processing of pulse-type waveform. They can be used to control digital circuits and as frequency dividers.

There are two outputs available which are complements of one another. i.e. when one output is high the other is low  and vice versa .

Operation:

When VCC is applied, one transistor will start conducting slightly more than that of the other, because of some differences in the characteristics of a transistor. Let Q₂ be ON and Q₁ be OFF. When Q₂ is ON, The potential  at the collector of Q₂ decreases, which in turn will  decrease the potential  at the  base of Q₁ due to potential  divider action of R₁ and R₂. The potential at the collector of Q₁ increases which in turn further increases the base to emitter voltage at the base of Q₂. The voltage at the collector of Q₂ further decreases, which in turn further reduces the voltage at the base of Q₁. This action will continue till Q₂ becomes fully saturated and Q₁ becomes fully cutoff.

Thus the stable  state of  binary  is such  that  one device  remains  in cut-off and  other device remains  at saturation.  It will be in that state until the triggering pulse is applied to it.  It has two stable states. For every transition of states triggering is required.  At a time only one device will be conducting.

NEED OF COMMUTATING CAPACITORS (SPEED UP CAPACITORS):

It is desired that the transition should take place as soon as the trigger pulse is applied but such is not the case.

When  transistor is in active region it stores charge in its base and when it is in the saturation region it stores even more charge. Hence transistor cannot come out of saturation to cut- off. Until all such charges are removed.  The interval during which conduction transfer one transistor to other is called as the transition

Design Procedure:

-1.2 = (-15R₁ + 0.2R₂) /(R₁ + R₂) ; given R1=10K

R2 = 100K

F_max = (R₁ + R₂)/2C R₁ R₂         R₁ = 10K, R₂ = 100K and C = 0.1µF  

 = (10 + 100) X 10³ / (2 X 0.3 X 10^-6 X 10 X 100 X 10⁶) = 55KHz

Procedure:

1. Make the connections as per the circuit diagram.
2. Apply trigger pulse of 1 KHz 5v (p-p) from function generator.
3. Obtain waveforms at different points such as V_B1, V_B2, V_C1 & V_C2.
4. Trace the waveform at collector and base of each transistor with the help of dual trace CRO. Note the Time relation of waveforms.

Expected Waveforms:

Inference: Bistable Multivibrator is designed; and the waveforms are observed

Viva Questions:

1. What are the other names of Bistable Multivibrator?

Ans: Eccles-Jordan circuit .multi trigger circuit, scale of  two toggle circuit ,flip-flop and binary

2. What are the applications of a Bitable Multivibrator?

Ans: It is used to perform many digital operations such as counting and storing of binary imformation . it is also used in the generation and processing of pulse type waveforms

3. Describe the operation of commutating capacitors?

Ans: The main feature of the commutating capacitors is that they reduces the transition time and increase the switching speed .Hence it s called seed u capacitors .

4. Commutating capacitors are also called as __ or __ .

Ans: Speed –up capacitors  and transpose  capacitors

5. What is the meaning of a stable state in a multi-vibrator?

Ans: Output will be maintains as constant irrespective of changes in parameters of circuit.

6. Mention the names of different kinds of triggering used in the circuit shown?

Ans: Positive edge triggering and negative edge triggering.

7. What are the disadvantages of direct coupled Binary?

Ans:

1. The voltage swing is only a fraction of a volt and hence the binary is susceptible to serious  voltages .
2. Triggering is difficult
3. Since q2 is driven heavily into saturation,the storage time delay will be large and switching speed will be low.

8. The diodes used in a bistable multivibrator to maintain a constant output swing are called __ diodes.

Ans: Collector catching diodes

9. The interval during which conduction transfers from one transistor to another is called the __.

Ans: Transition time

10. What are the coupling elements of a Bi-stable Multivibrator?

Ans: Resistors

Design Projects

1. Design a fixed bias binary employing two n-p-n silicon transistor to operate max frequency of 16KHz, V_CC = V_BB = 10V, I_C(sat) = 5mA, h_fe(min) = 30.
2. Design and verify the bistable multivibrator by using different triggering methods.

Outcomes: After finishing this experiment students are able to design Bistable Multivibrator and able to explain its operation.