UNIVERSITY
DEPARTMENT HACETTEPE OF ELECTRICAL AND ELECTRONICS ENGINEERING
ELE-314 ELECTRONICS LABORATORY III
EXPERIMENT 5
COLLECTOR COUPLED MONOSTABLE/ASTABLE MULTIVIBRATOR
1. PURPOSE:
To analyse the collector coupled monostable and astable multivibrators.
2. THEORY :
There are three types of multivibrators, known as monostable, bistable and
astable multivibrators.
2.1. MONOSTABLE MULTIVIBRATORS
In
this part the monostable multivibrator will be examined. Monostable
multivibrators are mainly used in delaying and extending the duration of a pulse.
If an external trigger is not applied, the monostable is at its stable state.
When it is triggered externally, it switches to a quasi-stable state.
Figure 1. Typical monostable multivibrator with discrete
components. For the circuit shown above, Vcc=10V, assume
VBEsat=0.6V, VCEsat=0.2V, =125.
After a period of time determined by R and C (see fig.1) it returns to its
stable state. As long as a trigger is not applied to the input, the circuit remains
at stable state. A monostable multivibrator is depicted in fig.1.
In stable state the base current of transistor T2 which flows through the
supply and R is sufficient to force T2 into saturation. The collector-emitter
saturation voltage VCEsat of T2 is coupled through Rf to the base of T1 and
hence T1 is OFF. Therefore VC1 = Vcc. This state, i.e. when T1 is OFF and T2 is
SAT, is the stable state and the circuit remains at this state unless an
external trigger is applied. Assume that the circuit is at stable state at t=0- ,
and at t=0+ , an external trigger pulse brings the circuit to the quasi-stable
state.
At
t = 0- ,
VC1 (0-) = Vcc and VB2(0-) = VBE2sat
the capacitor is, VS1(0-) = Vcc - VBE2sat .
and the voltage across
At t = 0+ T1 becomes ON by the impulse applied to the base of T1. This pulse
is amplified and inverted by T1, and coupled to the base of T2 through the
capacitor C. If it is sufficiently negative T2 will be OFF and VC2 will be equal
to Vcc. Since this voltage is coupled through R to B1 (base of T1) T1 will be
saturated. However, this state (T1 is saturated and T2 is off) has a finite
duration. This is how the monostable circuit is used in modern electronic
circuits.
At t = 0+ , VC2 (0+)= Vcc , VC1(0+)=VCE1sat and
VB2(0+)= - [Vcc - VBE2sat - VCE1sat ] .
At this instant B2 is as negative as VB2 (0+ ). The capacitor will charge to Vcc
with a time constant = RC, therefore VB2 will increase exponentially.
VB2(t)= [-Vcc +VBE2sat +VCE1sat -Vcc ] e -t/ +Vcc
(1)
When VB2 is 0.6V, T2 is initially active and then immediately
saturated. This time is represented by Ts and given as,
Ts = ln2 = 0.69 R C
(2)
The circuit stays at stable state until a new trigger pulse is applied.
Everything repeats with the application of a new trigger . Refer to Fig.2.
Ts=ln(2)RC=0.69RC
Figure 2. Monostable multivibrator with periodic input waveform.
2.2. ASTABLE MULTIVIBRATORS:
In these circuits both states are quasi-stable and hence the circuit switches
from one to another state in the absence of an external trigger. In digital
systems these circuits are used to obtain periodic pulse trains. The durations
of two quasi-stable states can be adjusted independently. A collector coupled
astable multivibrator is depicted in fig.3.
VCC=10 V
R2
R1
R3
R4
C2
C1
T1
T2
Figure 3
The circuit is symmetric and continuously changes states. The reason for this
state change is the same as the one described in the monostable multivibrator,
due to the symmetry this phenomenon occurs at both stages of the circuit.
Assume that the circuit has changed at state t=0, and T1 is SAT and T2 is
OFF. Then
VC1(0-) = Vcc ;
VB2(0-) = VBE2SAT ;
and the voltage VS1 on the capacitor C1
VC1(0+) = VCE1SAT
VS1(0- ) = Vcc - VBE2SAT
VC1
VCC
VCE1SAT
VB2
0
TS
TS+ TR
t
VBE2SAT
t
S = R1C1
VC2
TS = 0.69R1C1
TR = 0.69R3C2
VCC
2 = R4C2
VCE2SAT
t
VB1
VBE1SAT
R = R3C2
t
Figure 4
The voltage at B2 (base of T2) is VCE1SAT - VS1(0-) at t = 0+. Hence T2 is OFF.
The voltage VB2 increases with a time constant S = R1C1 and T2 eventually
becomes SAT. The elapsed time TS is 0.69 R1C1. Meanwhile, since T2 is OFF,
VC2 = Vcc and since T1 is SAT, VC1 = VCE1SAT. In the time interval 0 < t < TS
only VB2 is changing. When VBE2(t) reaches 0.6V, T2 becomes initially ACTIVE
and immediately SAT. The voltages at instant TS are :
VC2(TS-) = Vcc ; VB1(TS-) = VBE1SAT ;
and the voltage VS2 on the capacitor C2
VC2(TS+) = VCE2SAT
VS2(TS-) = Vcc - VBE1SAT
Since VB1(TS+) = VCE2SAT - VS2(TS-) < 0, T1 is now OFF.
VB1 increases with a time constant R = R3C2, and when VB1 = 0.6V, the circuit
changes state. VB1 reaches 0.6V in a time interval TR = 0.69R3C2.
When the transistors are saturated their collector voltages immediately
decrease to VCESAT. However, when they are OFF, the collector voltages do
not immediately rise to Vcc because
the collector voltages increase with a
certain time constant as shown in fig.4 and fig.5.
VCC
R3
VCC
R4
C2
R2
T2
VBE1SAT
R1
C1
T1
VBE2SAT
Figure 5a. T1 in SAT, T2 at cut-off Figure 5b. T1 at cut-off, T2 in SAT
When T2 is OFF, C2 charges through R4 and VC2 reaches Vcc with a time
constant 2=R4C2.
When T1 is OFF, C1 charges through R2 and VC1 reaches Vcc with a time
constant 1=R2C1.
In order to obtain sharp rising pulses at the collectors, the collectors of the
OFF transistors have to be isolated from the corresponding capacitors. This
can be achieved by using diodes and resistors as shown in fig.6.
VCC = 10 V
R2
T1
R5
R1
R3
R6
R4
C2
C1
T2
Figure 6
When the transistors are SAT currents through R5 and R6 force the diodes to
be ON, and hence the collectors are connected to the capacitors. However,
when the transistors become OFF, the diodes are reverse biased and they are
OFF, hence, the collectors are isolated from the capacitors. The collector
voltages, therefore, immediately rise to Vcc. The capacitors now charge
through R5 and R6 , and their voltages can reach to steady state values during
the pulse duration if the following inequalities are satisfied.
R5C1 << R3C2
R6C2 << R1C1
3.1. PRELIMINARY WORK
3.1.1. Draw the waveform of VC2 (t), VB2 (t), VC1(t) for a 500 Hz square
wave input in fig.1 (take R1= 15K, R2 = 1K, R = Rf = 47K, C = 10nF).
3.1.2. How can we change the pulse duration at the output ?
3.1.3. What is the method of choosing R1 and R3 if the resistors R2 and R4
are fixed in fig.3? (Note that saturation of the transistors depends on the
values of R1 and R3)
3.1.4. If R2 =R4 =2.2K and R1 =R3 =47K, what must the values of C1
and
C2 be so that TS =0.16msec and TR =0.33msec?
3.1.5. With the values of C1 and C2 you determined above, sketch VB2(t),
VC2(t), VC1(t) and VB1(t) indicating the times and amplitudes.
3.1.6. Discuss how you can modify the ratio TR / TS .
3.1.7. Do the Pspice simulations of step 3.1.1 for three different frequencies of
square wave 500Hz, 1kHz and 2kHz.
3.1.8. Do the Pspice simulation by setting up the circuit shown in fig. 3 and use
component values given in step 3.1.4.
3.1.9. Do the Pspice simulation by setting up the circuit shown in fig. 6. Use
component values given in step 3.1.4. and R5 =R6 =2.2K.