Expt. No. 7

MULTIVIBRATORS

Objectives: a) To study the operation of astable, monostable and bistable multivibrators

b) To simulate multivibrator circuits using PSICE

Components and equipments required:

1. Oscilloscope (Scope/CRO).2. Function Generators (FG).3. DC power supply.4. Project Breadboard. 5. Resistors.6. Capacitors. 7. SL100 BJTs / BEL100N/BC1078. Connection Wires. 9. Oscilloscope Probes.

Circuit diagram:

a) Astable multivibrator:

Fig.1

b) Monostable multivibrator:

Fig. 2

c) Bistable multivibrator

Fig. 3

Theory:

(Prepare yourself)

Notes contents are about:

Multivibrators

(Should not be more than three pages)

Design:

(Reference: Pulse, Digital and Switching Waveform: Jacob Millman & Herbert Taub: Chapter 10 and 11)

a) Astable multivibrator:

i) Symmetry wave:

Given that

For symmetrical wave

We have

Choose

Then R=?

Base current

Collector current

Condition for oscillation

For that

ii) Asymmetry wave:

Given that

We have

Choose

Then

b) Monostable multivibrator:

Given that and time period of trigger signal Ti=2ms

Choose

Then R=?

Under stable state

To keep Q1 ON in quasi-stable state

For low current to bias Q1 , R1>>RC2

Let

Design for trigger circuit:

Time period Ti of trigger signal vT must be more than TON.

Given that Ti=2ms.

Then

Choose

c) Bistable Multivibrator

Given that

Operation is always symmetry: then

R1>>RC1 and R2 > R1

Let

Pre-lab Assignments:

1. Consider the following circuit.

If Vcc =5V, VBB=10V, RC1=RC2=RC=1kΩ, R1=R2=R=50kΩ, C1=C2=.01µF. Draw the signals at vC1,vC2, vB1 and vB2. Find the frequency of oscillation. If VBB is a sinusoidal wave what is the output?

2. Design a circuit to turn ON an LED for 2 seconds and OFF for 4 seconds continuously.

3.

4. Consider the following circuit.

Design the components for VCC =5V, βmin = 25, TON = 4seconds

5. Draw a circuit to generate a square wave. Design it for VCC =5V, βmin = 25, f=4kHz

6. Simulate the above circuits.

7. Consider the following truth table and construct a circuit using BJT to generate the logic.

Inputs OutputS R Qn+1

0 0 Qn

0 1 01 0 11 1 X

Model Waveforms Astable multivibrator

Symmetry waves

Asymmetry waves

Monostable multivibrator:

Bistable multivibrator:

Viva Questions:

1. Maximum frequency of oscillation of astable multivibrator is limited due to which property?

2. Give the applications of multivibrators.

3. Give the expression for the delay in trailing edge of square wave generated by astable multivibrator.

4. Define duty cycle.

5. Compare three multivibrator circuits.

6. Which circuit is referred as one-shot circuit and why?

7. Which multivibrator operates as SR flip-flop and why?

8. Which multivibrator is called free running multivibrator and why?

9. Triggering a monostable multivibrator during TON , what will happen to the output?