Astable Multivibrator

 Aim

  To design and simulate an Astable Multivibrator circuit.

  Components

  Theory

  Astable Multivibrator is a two stage switching circuit in which the output of the first stage is fed to the input of the second stage and vice versa. The outputs of both the stages are complementary. This free running multivibrator generates square wave without any externaltriggering pulse. The circuit has two stable states and switches back and forth from one state to another, remaining in each state for a time depending upon thedischarging of the capacitive circuit.

  The multivibrator is one form of relaxation oscillator, the frequency of which may be controlled by external synchronizing pulses.

  In our experiment we are using transistor, as the amplifying device and also it is a collector coupled multivibrator.

  Figure shows the basic symmetrical astable multivibrator in which components in one half of a cycle of the circuit are identical to their counterpart in the other half. Square wave output can be obtained from the collector point of Q1 or Q2.

  When supply voltage, V_CC is applied, one transistor will conduct more than the other due to some circuit imbalance. Initially let us assume that Q1 is conducting and Q2 is cut-off. Then V_C1, the output of Q1 is equal to V_CESAT which is approximately zero and V_C2 is equal to V_CC. At this instant C1 charges exponentially with the time constant R₁C₁ towards the supply voltage through R1 and correspondingly V_B2 also increases exponentially towards V_CC. When V_B2 crosses the coupling voltage Q2 starts conducting and V_C2 falls to V_CESAT. Also V_B1 falls due to capacitive coupling between collector of Q2 and base of Q1, thereby driving Q1 into OFF state. The rise in voltage V_C1 is coupled through C1 to the base of Q2 causing a small overshoot in voltage V_B2. Thus Q1 is OFF and Q2 is ON. At this instant the voltage levels are: V_B1 is negative, V_C1=V_CC, V_B2=V_BESAT and V_C2=V_CESAT.

  When Q1 is OFF and Q2 is ON the voltage V_B1 increases exponentially with a time constant R₂C₂ towards V_CC . Therefore Q1 is driven to saturation and Q2 to cut-off. Now the voltage levels are:

  V_B1=V_BESAT, V_C1=V_CESAT, V_B2 is negative and V_C2=V_CC.

  From the above it is clear that when Q2 is ON the falling voltage V_C2 permits the discharging of capacitor C2 which inturn drives Q1 into cut-off. The rising voltage of V_C1 is fed back to the base of Q2 tending to turn it ON. This process is regenerative.

Derivation of time period

  The charging equation for a capacitor is given by

  Capacitor voltage,

  Hence

Taking natural logarithm,

For a symmetrical astable multivibrator,

Charging and discharging time periods are given by,

Since the multivibrator is symmetrical

Design

 Applying KVL for the collector side of Q2.

, (since it is a symmetrical astable multivibrator)

Applying KVL for the base loop of the circuit

, (since it is a symmetrical astable multivibrator.)

,(since it is a symmetrical astable multivibrator.)

Procedure

EDWinXP-> Schematic Editor:The circuit diagram is drawn by loading components from the library. Wiring and proper net assignment has been made. The values are assigned for relevant components.

EDWinXP-> Mixed Mode Simulator: The circuit is preprocessed. The desired test points and waveform markers are placed. The Transient Analysis parameters have been set. The Transient Analysis is executed and output observed in Waveform Viewer.

Result

   The output waveform may be observed in the waveform viewer.