CHAPTER THREE
SYSTEM DRIVER
3.1 MOSFET Control Circuit: -
MOSFET works in three regions cut off region triode region and saturation
region. When MOSFET is in cut off triode region, it can work as switch.
To control this switching process a so called PWM technique is a suitable and
simple way to perform, in which generate a track of square wave pulse to conduct the
semiconductor gate.
3.2 Multivibrator
A multivibrator is an electronic circuit used to implement a variety of simple
two-state systems such as oscillators, timers and flip-flops. It is characterized by two
amplifying devices (transistors, electron tubes or other devices) cross-coupled by
resistors and capacitors.
There are three types of multivibrator circuit:
astable, in which the circuit is not stable in either state—it continuously
oscillates from one state to the other.
monostable, in which one of the states is stable, but the other is not—the
circuit will flip into the unstable state for a determined period, but will eventually
return to the stable state. Such a circuit is useful for creating a timing period of fixed
duration in response to some external event. This circuit is also known as a one shot.
A common application is in eliminating switch bounce.
bistable, in which the circuit will remain in either state indefinitely. The
circuit can be flipped from one state to the other by an external event or trigger. Such
a circuit is important as the fundamental building block of
a register or memory device. This circuit is also known as a flip-flop.
In its simplest form the multivibrator circuit consists of two cross-
coupled transistors. Using resistor-capacitor networks within the circuit to define the
time periods of the unstable states, the various types may be implemented.
Multivibrators find applications in a variety of systems where square waves or timed
intervals are required. Simple circuits tend to be inaccurate since many factors affect
their timing, so they are rarely used where very high precision is required.
Before the advent of low-cost integrated circuits, chains of multivibrators found
use as frequency dividers. A free-running multivibrator with a frequency of one-half
to one-tenth of the reference frequency would accurately lock to the reference
frequency. This technique was used in early electronic organs, to keep notes of
different octaves accurately in tune. Other applications included
early television systems, where the various line and frame frequencies were kept
synchronized by pulses included in the video signal.
3.2.1 A stable multivibrator circuit: -
Figure 3.1: Circuit diagram of Basic BJT a stable multivibrator
This circuit shows a typical simple astable circuit, with an output from the
collector of Q1, and an inverted output from the collector of Q2.
Suggested values will yield a frequency of about f = 0.24 Hz.
3.2.1.1 Basic mode of operation: -
The circuit keeps one transistor switched on and the other switched off.
Suppose that initially, Q1 is switched on and Q2 is switched off.
State 1:
Q1 holds the bottom of R1 (and the left side of C1) near ground (0 V).
The right side of C1 (and the base of Q2) is being charged by R2 from below
ground to 0.6 V.
R3 is pulling the base of Q1 up, but its base-emitter diode prevents the voltage
from rising above 0.6 .
R4 is charging the right side of C2 up to the power supply voltage (+V).
Because R4 is less than R2, C2 charges faster than C1.
When the base of Q2 reaches 0.6 V, Q2 turns on, and the following positive
feedback loop occurs:
Q2 abruptly pulls the right side of C2 down to near 0 V.
Because the voltage across a capacitor cannot suddenly change, this causes the
left side of C2 to suddenly fall to almost −V, well below 0 V.
Q1 switches off due to the sudden disappearance of its base voltage.
R1 and R2 work to pull both ends of C1 toward +V, completing Q2's turn on.
The process is stopped by the B-E diode of Q2, which will not let the right side of
C1 rise very far.
This now takes us to State 2, the mirror image of the initial state, where Q1 is
switched off and Q2 is switched on. Then R1 rapidly pulls C1's left side toward +V,
while R3 more slowly pulls C2's left side toward +0.6 V. When C2's left side reaches
0.6 V, the cycle repeats.
3.2.2 Multivibrator frequency:-
The period of each half of the multivibrator is given by t = ln(2)RC. The total
period of oscillation is given by:
T= t1+ t2=nl 2(CR) 1+nl 2( R
) 3C 2
1 1 1
f= + ≈
T ln ( 2 )∗( R 2 C 1+ R 3 C 2) 0.693∗(R 2 C 1+ R 3 C 2)
where...
f is frequency in hertz.
R2 and R3 are resistor values in ohms.
C1 and C2 are capacitor values in farads.
T is period time (In this case, the sum of two period durations).
For the special case where
t1 = t2 (50% duty cycle) R2 = R3 C1 = C2
1 1 0.721
f= + ≈
T ln ( 2 )∗2 RC RC
3.2.3 Initial power-up
When the circuit is first powered up, neither transistor will be switched on.
However, this means that at this stage they will both have high base voltages and
therefore a tendency to switch on, and inevitable slight asymmetries will mean that
one of the transistors is first to switch on. This will quickly put the circuit into one of
the above states, and oscillation will ensue. In practice, oscillation always occurs for
practical values of R and C.
However, if the circuit is temporarily held with both bases high, for longer
than it takes for both capacitors to charge fully, then the circuit will remain in this
stable state, with both bases at 0.6 V, both collectors at 0 V, and both capacitors
charged backwards to −0.6 V. This can occur at startup without external intervention,
if R and C are both very small. For example, a 10 MHz oscillator of this type will
often be unreliable. (Different oscillator designs, such as relaxation oscillators, are
required at high frequencies.)
3.2.4 Period of oscillation:-
Very roughly, the duration of state 1 (low output) will be related to the time
constant R2C1 as it depends on the charging of C1, and the duration of state 2 (high
output) will be related to the time constant R3C2 as it depends on the charging of C2.
Because they do not need to be the same, an asymmetric duty cycle is easily achieved.
However, the duration of each state also depends on the initial state of charge of the
capacitor in question, and this in turn will depend on the amount of discharge during
the previous state, which will also depend on the resistors used during discharge (R1
and R4) and also on the duration of the previous state, etc. The result is that when first
powered up, the period will be quite long as the capacitors are initially fully
discharged, but the period will quickly shorten and stabilise.
The period will also depend on any current drawn from the output and on the
supply voltage.
3.2.5 CD4047 Features:-
1. Operate in both Monostable and Astable operation
2. Require only a few external components that are one resistor and one
capacitor
3. Symmetrical buffered output characteristics
4. High Noise Immunity.
3.2.6 How to use CD4047?
Multivibrators are devices that change the state of electrical signals on either a
regular basis or according to the requirement. CD4047 is also a multivibrator IC. It
can operate in two modes. A capacitor is connected externally between pins 1 and 3 to
determine the pulse width of the output signal in the monostable mode and the output
frequency is determined in astable mode by connecting a resistor between pins 2 and
3. A reset input is provided to reset the output of Q to 0 and the other output will
become 1.
3.2.7 Astable Mode of Operation
The astable and ~ astable inputs of CD4047 enable this mode of operation.
The astable input connects with a high level. we can do it by applying low level on
input ~astable, the IC operates in an astable mode. The output frequency can be
calculated through timing components and is given by the following equation:
f= 1/( 4.4xRxC)
In astable mode, we have an additional oscillator output. The timing diagram
of the three outputs is shown below
Figure 3.2: Circuit Diagram of Astable Mode
The oscillator output at pin 13 is of the *basic frequency. The Q output
frequency is half to that of the basic frequency. Pin 11 output is the same as that of
pin 10. But the output signal is inverted to 180 degrees. The time required to generate
:pulses is given by the formula
t = 2.48×R×C
3.2.8 Square wave generator using CD4047
We should use 4047 IC in astable mode to generate a square wave. A square
wave is a PWM signal with equal width of logic high and low signal. We only need a
couple of resistors and capacitors with this multivibrator IC to generate a square
signal. This is a circuit diagram for the square PWM generator. We use RV1 as a
variable resistor to get the variable frequency. This circuit shows a complete circuit
diagram with an output signal.
Figure 3.3: Circuit Diagram of Astable Mode Using CD4047
Figure 3.4: Output Waveform of Astable Mode Using CD4047
Picture 3.1: Output Waveform of Astable Mode Using CD4047
3.2.9 Determination Of The Oscillating Frequency in Power
inverter Design:
By supplying a 12Volt DC to the CD4047 PWM, the frequency of the oscillating
signal was determined using a 25KΩ variable resistor and connected in parallel with
0.22µF to form the RC time constant network.
Frequency, F = 1/4.4*Ct*Rf where
Time Capacitor (CT) = 0.22µF
Variable Resistor (VR) = 25KΩ
Time Resistor (RT) =18KΩ
Therefore,
F= 1/(4.4*0.22*10^-6*18k)
F= 50Hz
3.2.10 CD4047 Applications:-
This IC is normally used for the conversion of DC signal to AC signal. Some of its
applications are:
Frequency discriminators
Timing circuits
Time-delay applications
Envelope detection
Frequency multiplication
Frequency division
3.2.11 CD4047 Pin out Diagram:-
This CD4047 monostable/Astable multivibrator pin out has 14 pins. The pinout is the
same for all packages. Three output pins provide PWM outputs such as Q, ~Q, and
Osc_out. We can depict the working of each from the pinout diagram. For details and
.functions of each pin, check the next section
Figure 3.5: Pin Configuration Description
It is a low gate IC having three outputs. It requires very few external
components for performing astable or monostable multivibrator operation.
