Astable Multivibrator
Aim
To design and simulate an Astable Multivibrator circuit.
Components
Name

EDWin Components Used 
Description 
Number of components
required 
BC107 
BC107A 
Transistor 
2 
RES 
RC05 
Resistor 
4 
CAP 
CASEA600 
Capacitor 
2 
VDC 
VDC 
Dc voltage source 
1 
GND 
SPL0 
Ground 
1 
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.
Operation
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 cutoff. 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 R_{1}C_{1} 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 R_{2}C_{2} towards V_{CC}
. Therefore Q1 is driven to saturation and Q2 to cutoff. 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 cutoff. 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
where V_{C}  the capacitor voltage,
V_{INIT} – the initial
capacitor voltage,
V_{FIN} – the final
capacitor voltage
t – the time
period of charging.
R and C – the resistor
and capacitor through which charging occurs.
The capacitor discharges from –V_{CC} to V_{CC}.
Therefore V_{IN}=(V_{CC}), V_{FIN}=V_{CC},
V_{C@}0V.
Substituting this in equation (2)
Taking natural logarithm,
For a symmetrical astable multivibrator,
Charging and discharging time periods are given by,
From equation(5)
where T is the total time period.
Since the multivibrator is symmetrical
Design
Design Specifications
Manufacturer’s specifications
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.)
Design of C
The total time period T is given by
,(since it is a symmetrical astable
multivibrator.)
From equations (1) and (2)
The free running frequency is given by
Assume the frequency as 100Hz.
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.