Astable Multivibrator

Prior to the Lab session:

Study the operation and working principle Astable Multivibrator.
Study the procedure for conducting the experiment in the lab.

Objectives:

To study the operation and observe the wave forms of Astable Multivibrator.
To Design an Astable Multivibrator to generate a square wave of 1KHz frequency using Transistor.

Apparatus:

CRO 0 to 20 MHz (Dual Channel) - 1 No.
Function Generator 1Hz to 1 MHz - 1 No.
Bread board - 1 No.
Resistor (1K $\Omega$ , 10K $\Omega$ ) - 2 Nos. each
Capacitors (0.1µF) - 2 No.’s
Transistor (BC 107) - 2 No.’s
Regulated D.C Power Supply 0 to 30V (dual) - 1 No.
Connecting wires
Bread board

Circuit diagram:

Fig. 1.1 Astable Multivibrator

Theory:

The Astable circuit has two quasi-stable states. Without external triggering signal the Astable configuration will make successive transitions from one quasi-stable state to the other. The Astable circuit is an oscillator. It is also called as free running multivibrator and is used to generate “Square Wave”. Since it does not require triggering signal, fast switching is possible.

Operation:

When the power is applied, due to some imbalance in the circuit, the transistor Q₂ conducts more than Q₁ i.e. current flowing through transistor Q₂ is more than the current flowing in transistor Q₁. The voltage V_C2 drops. This drop is coupled by the capacitor C₁ to the base by Q₁ there by reducing its forward base-emitter voltage and causing Q₁ to conduct less. As the current through Q₁ decreases, V_C1 rises. This rise is coupled by the capacitor C₂ to the base of Q₂. There by increasing its base- emitter forward bias. This Q₂ conducts more and more and Q₁ conducts less and less, each action reinforcing the other. Ultimately Q₂ gets saturated and becomes fully ON and Q₁ becomes OFF. During this time C₁ has been charging towards V_CC exponentially with a time constant T₁= R₁C₁. The polarity of C₁ should be such that it should supply voltage to the base of Q₁. When C₁ gains sufficient voltage, it drives Q₁ ON. Then VC1 decreases and makes Q₂ OFF. V_C2 increases and makes Q₁ fully saturated. During this time C₂ has been charging through V_CC, R₂, C₂ and Q₂ with a time constant T2 = R₂C₂. The polarity of C₂ should be such that it should supply voltage to the base of Q₂. When C₂ gains sufficient voltage, it drives Q₂ On, and the process repeats.

Design Procedure:

The period T is given by

T = T₁ + T₂ = 0.69 (R₁C₁ + R₂C₂)

For symmetrical circuit, with R₁ = R₂ = R & C₁ = C₂ = C

T = 1.38 RC

Let V_CC = 12V; h_fe = 51 (for BC107), V_BESat = 0.7V; V_CESat = 0.3V Let C = 0.1 $\mu$ F & T = 1mSec.

10^-3 = 1.38 x R X 0.1 X 10^-6

R = 7.24K $\Omega$ (Practically choose 10K $\Omega$ ) i.e., R1 and R2 resistors.

Let I_Cmax=10mA

R_C= $\frac{V_{cc} - V_{cesat}}{I _{cmax}} = \frac{12 - 0.3}{0.01}$ = 1.17K $\Omega$ ( 1K $\Omega$ is selected for Rc1 and Rc2)

Procedure:

Make then connections as per the circuit diagram.
Observe the Base Voltage and Collector Voltages of Q1 & Q2 on CRO in DC mode and measure the frequency (f = 1/T).
Trace the waveforms at collector and base as each transistor with the help of dual trace CRO and plot the waveforms.
Verify the practical output frequency with theoretical values f = 1/T, where T = 1.38 RC

Expected Waveforms: