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DIGITAL ELECTRONICS
THEME 9: Multi-vibrators. Multi-vibrators in integral realization.
Multi-vibrators (also called oscillators or pulse train generators) have two unstable
(temporary) states which are changed continuously in time. The result is the pulse train
generated at the output of the circuit.
There are great variety of multi-vibrator circuits: composed with logic gates and RCchains, with Schmidt flip-flop, with crystal resonators, multi-vibrators in integral
realization, with specialized integrated circuits, with operation amplifiers and analogue
comparators.
Milti-vibrator with Schmidt flip-flop.
This multi-vibrator is realized with one logic gate having embedded Schmidt flip-flop and
a RC-chain. At the first moment, after switching on the power supply, the voltage on the
capacitor is 0V and the output of the logic gate is in “1”. The capacitor starts the process
of charging trough resistor R trying to reach the voltage of logic “1”, but at the moment it
reaches the threshold level (Е1) the Schmidt flip-flop switches its output to “0”.
Then the capacitor starts to
discharge trying to reach 0V,
but at the moment it reaches
the threshold level (Е2) the
Schmidt flip-flop switches its
output to “1” and again starts
charging.
DIGITAL ELECTRONICS
The pulse duration (tu), pause duration (tП), period (Т) and frequency (f) of generated
pulse train are calculated using the following relations:
E2  Uo1
tu  (R  Rout).C ln
 (R  Rout).C ln 1,4  0,34.(R  Rout).C
E1  Uo1
E1
tп  R.C ln
 R.C ln 2  0,7R.C
E2
E2  Uo1
E1
T  (R  Rout).C ln
 R.C ln
 0,34.(R  Rout).C  0,7.R.C
E1  Uo1
E2
1
1
1
f 

T (R  Rout).C ln E2  Uo1  R.C ln E1 0,34.(R  Rout).C  0,7.R.C
E1  Uo1
E2
where: Rout = 130Ω, E1=1.6V, E2 = 0.8V and Uo1= 3.6V for TTL circuits.
Pulse train generators with stabilization of frequency
Whenever the pulse train with stabilization of frequency is necessary the pulse
generators is implemented with crystal resonator and logic gates. The crystal resonator
is electro-mechanical system, composed by crystal plate, cut off with proper orientation
according to the axes in the crystal structure, and attached electrodes. The resonator
vibrates with fixed frequency determined by mechanical characteristics of the crystal
plate. The frequency of vibration has high stability (in order of 10-6), which determines
very high stability of frequency of generated pulse train from this type of multi-vibrators.
DIGITAL ELECTRONICS
Multi-vibrators in integral realization
Many companies produce multi-vibrators in integral realization, which cover a wide
range of frequencies often used in digital electronics and microprocessor technique.
An example of Voltage
Controlled Oscilator (VCO) IC 74624.
DIGITAL ELECTRONICS
All VCOs have one external capacitor and at least one input for voltage control of frequency (Uf). Some of VCOs have second
input for voltage control (Ur) and one more additional enable input (Е).
From shown on the figure internal structure of IC 74624 it is visible that except pulse generator it consists from additional logic
responsible for synchronization of start and stop of generations. The result is that all pulses in the train are with equal duration.
The frequency of pulse train is determined by the values of connected capacitor (С) and control voltages Uf and Ur. The
presented characteristics show that for a given value of C, the voltage Uf controls the frequency linearly, while voltage Ur control
the slope of characteristic.
Multi-vibrators realized by two mono-vibrators in integral realization
The mono-vibrators IC 74123 and 74221, discussed in previous lecture, are arranged in couples in one integrated circuit. This is
very convenient for realization of multi-vibrators. The two mono-vibrators could be connected in such a way that every one of
them activates the other when terminates the generated pulse.
An example realization is presented at the following figure. The pulse duration for this circuit is determined by the RC-chain of
mono- vibrator ЧМ2 (R2-C2), while the pause duration is determined by the RC-chain of
mono- vibrator ЧМ1 (R1-C1).
DIGITAL ELECTRONICS
The pulse duration, pause duration, period, frequency and duty cycle of generated pulse train are calculated using the following
relations:
tu  k.R2.C2;
f
tп  k.R1.C1;
1
;
k.(R1.C1  R2.C2)
Кз 
T  k.(R1.C1  R2.C2);
R2.C2
R1.C1  R2.C2
Where к is coefficient of
proportion in the range of
0,2÷0,7, depending on the type of
TTL circuits.
DIGITAL ELECTRONICS
Multi-vibrator’ realization with specialized integrated circuits
One of the specialized integrated circuits used for realization of mono- and multivibrators is integral timer 555. It is integrated circuit with mixed analogue and digital
structure, composed by resistors divider, two analogue comparators, R-S type flip-flop,
output inverter buffer stage and key transistor.
The main characteristics and specific features of integral timer 555 are as follows:
1. Wide range for operation of power supply voltages - 3 - 15 V.
2. High stability of tu and f of generated pulse train.
3. Low output resistance for both logic levels – approximately 10Ω, which assures rapid
transitions at the output (good shape of output pulses).
8
4 (Reset)
5к
6
7
K1
5
R R
5к
S
K2
2
5к
1
Q
3 (Output)
4. High output power – output
current of 200mA, which allows
direct control of relays, LED
indicators, etc.
5. Low power consumption
(without load) – 0.7mA/ V of power
supply.
DIGITAL ELECTRONICS
 Multi-vibrator with IТ555.
The circuit of multi-vibrator with IT555 with timing diagrams of operation are shown on
the figure below. Timing diagrams illustrate the process of generation of pulse train.
When the power supply is switched on, because the voltage on capacitor is equal to 0V,
the R-S flip-flop is in reset (‘0’) state on its complementary output Q. Then the internal
key transistor is blocked and the output voltage is high. At this moment the capacitor
starts the process of charging from the positive pole of power supply source (+Ucс),
through resistors RA and RB, capacitor С itself to negative pole of power supply source.
(8) +Ucc
Uc
2/3 Ucс
RA
(4)
1/3 Ucс
(7)
t
555
RB
(6)
(2)
Output
Uout
(3)
C
t
(1)
t1
t2
t3
t4
DIGITAL ELECTRONICS
The capacitor is trying to charge to the value of power supply voltage (+Ucс), but at the
moment of time (t1), when the voltage reaches 2/3Ucс, the comparator К1 switches from
‘0’ to ‘1’, which set the R-S flip-flop to state ‘1’ on the complementary output. Then the
key transistor switches to saturation state and the output level becomes low. From this
moment starts the process of capacitor discharging through resistor RB and saturated
key transistor to negative pole of power supply.
The process of capacitor discharging is going on till the moment of time (t2), when the
voltage on capacitor reaches 1/3Ucс. At this moment the comparator К2 switches from
‘0’ to ‘1’ and reset again the R-S flip-flop to state ‘0’ on its complementary output. From
this moment again starts the process of charging the capacitor.
The processes of charging and discharging the capacitor are changed and this leads to
generation of pulse train at the output.
The pulse (tu) and pause duration (tП) could be calculated using the basic equation for
determination of time intervals during the transition process:
 1 3UCC  UCC  
  R A  RB   C  ln 2  0,7.R A  RB   C
tu  t3  t2  R A  RB   C  ln
 2 3UCC  UCC  
 2 3UCC  0 
  RB  C  ln 2  0,7.RB  C
tП  t2  t1  RB  C  ln
 1 3UCC  0 
DIGITAL ELECTRONICS
The period, frequency and duty cycle of the pulse train are calculated
following relations:
from
the
T  tu  tП  R A  2RB   C  ln 2  0.7  R A  2RB   C
f  1/ T 
1
1

R A  2RB   C  ln 2 0,7.R A  2RB   C
Kз 
R A  RB   C  ln 2 R A  RB 

R A  2RB   C  ln 2 R A  2RB 
It is visible that for this circuit Kз > 0,5.
When is necessary the duty cycle of pulse train to be Кз<0,5 or Кз=0,5, some
modifications of the multi-vibrator circuit should be done.
Usually it is realized by connecting one or
two diodes in the circuit in order to by-bass
some of the resistors during the processes
of capacitor charge or discharge.
(8) +Ucc
RA
(4)
(7)
П
555
D
RB
(6)
(2)
C
(1)
Output
(3)
Example question: For pulse train
generator with IT 555 shown on the figure,
derive the equations for tu, tП, T, f and Кз
for middle, extremely up and extremely low
position of potentiometer slide.
DIGITAL ELECTRONICS
Multi-vibrators with operation amplifier or analogue comparator
Except multi-vibrators generating pulse trains with standard logic levels in some
applications bipolar pulse train is necessary. Such type multi-vibrators could be realized
using operation amplifier /or analogue comparator/ as an active device in the circuit. The
principle is the same – the alternating with each other processes of capacitor charging or
discharging.
An example realization of multi-vibrator with operation amplifier (op. amp) and the timing
diagrams showing the bipolar pulse train generation are presented on the figure below.
After switching on the bipolar power supply voltage at the moment of time (t0), the
capacitor voltage is equal to 0V and the voltage at the output of op. amp is in positive
saturation equal to Uout(+). For conventional type op. amps the difference between
Uout(+) and (+Ucc) is approximately 1V. Then the voltage on the non-inverting input of
op. amp (in point A) will be positive and determined by the resistor divider R2-R1 as:
UA     UИЗХ    R1/ R1  R2 
From this moment of time starts the process of capacitor charging through the chain
+Ucc - the op. amp. output - the resistor R3 - capacitor С – to the 0V pole. The process
of charging continuous to the moment of time (t1), when the capacitor voltage reaches
the value equal to UA(+). Then the output of op. amp. switches to negative saturation
Uout(-), which is approximately equal to the negative power supply voltage (-Ucc) - the
difference is app. 1V. Then the voltage on the non-inverting input of op. amp (in point A)
will be negative and determined by the resistor divider R2-R1 as:
DIGITAL ELECTRONICS
UA     UИЗХ    R1/ R1  R2 
From the moment of time (t1) starts the process of re-charging of capacitor through the
chain – from 0V pole - capacitor С - the resistor R3 - the op. amp. output – to (-Ucc).
The process of re-charging continuous to the moment of time (t2), when the capacitor
voltage reaches the voltage UA(-). Then the output of op. amp. switches again to the
positive saturation voltage Uout(+). The alternating processes of capacitor charging and
re-charging are going further in the successive time intervals. The output of op. amp.
switches from negative to positive saturation and vise versa, thus producing the bipolar
pulse train. In both alternating processes the time constant is the same (R3.C).
UUc
A(+)
t
R3
UA(-)
+Ucc
Uout
С
(Output)
Uout(+)
OA
t
-Ucc
R1
А
R2
t0
Uout(-)
t1
t2
t3
DIGITAL ELECTRONICS
The pulse duration (tu) and pause duration (tп) are calculated using following relations:
tu  t3  t2  R3  C  lnUA     UИЗХ    / UA     UИЗХ     R3  C  ln1  2R1/ R2 
tП  t2  t1  R3  C  lnUA     UИЗХ    / UA     UИЗХ     R3  C  ln1  2R1/ R2 
From the timing diagrams it is visible that only the first pulse is shorter because the
process of capacitor charging starts from 0V. The duration of all pulses in the pulse train
(except the first) are equal to the pauses duration. Hence, the duty cycle for this multivibrator circuit is equal to 0,5.
For practical calculations in order to determine the values of resistors and capacitor it is
convenient to chose one of the following cases:
1. If
R1=R2, then
tu  tП  R3  C  ln 3  1.1 R3  C
2. If
2R1=R2, then
tu  tП  R3  C  ln 2  0.7  R3  C
DIGITAL ELECTRONICS
For some applications pulse train with duty cycle Кз 0,5 is needed. Then it is necessary
to modify the circuit in a way to change the time constants of two processes. Usually it is
realized by suitable connection of one or two diodes in negative feedback of op. amp.
The duration of slops of generated pulse train depends on the time characteristics of op.
amp used. When steep slopes and high frequency pulse train are necessary, a highspeed op. amp. must be chosen with high value of parameter slew rate (SR) – the speed
for increasing the output voltage [V/s].
R3”
D1
D2
R3’
+Ucc
С
(Output)
OA
Example question: For the bipolar pulse train
generator shown on the figure, derive the
equations for tu, tП, T, f and Кз.
Then chose and calculate the values for
resistors and capacitor in order to generate
the pulse train with frequency f = 3kHz and
duty cycle Кз=1/3.
You can choose the value for capacitor from
the following possibilities:
-Ucc
R1
А
C = (1; 1,5; 2,2; 3,3; 4,7; 6,8; 8,2) x 10-i,
R2
where i depends on the dimension chosen –
pF, nF, µF, etc.