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Applied Electronics – SYET – Mr. Manoj Kavedia
Non Sinusoidal Generators-Multivibrators
The oscillators which generate waveforms other than sine waveform are called nonsinusoidal oscillators. The waveforms generated by nonsinusoidal oscillators include
- Square,
- Triangular,
- Rectangular and
- Saw-tooth waveforms.
These oscillators use one or two transistors (BJTs or FETs). The transistor is used as switch which operates in two states, namely saturation and cut-off states. When the switch operates, there is a sharp transition from one state to another. Usually, a transistor remains in the cut-off state for a certain period of time which is followed by another interval of time during which it remains in saturation state.
Since the transistor alternately supplies power to the load and relaxes when it is in cut-off state, therefore, the nonsinusoidal oscillators are also known as relaxation oscillators.
The nonsinusoidal oscillator may be classified into the following three types
- Multivibrator
- Blocking oscillator and
- Saw tooth generator or sweep generator (time base Generator chapter )
Transistor As a Switch
The nonsinusoidal oscillators employ one or two transistors, which
are used as a switch. The switchoperates between two states namely
- saturation and
- cut-off state.
The saturation state occurs when both the junctions (i.e., emitter-base junction and collector base junction) of atransistor are forward biased, the cut-off state occurs, when both the junctions are reverse biased.
Fig. Shows common emitter circuit. A pulse input (Vin) as shown in the figure is applied at the based of the transistor. It is used to control the voltage (or state of the switch) between collector and emitter of the transistor.
Case 1 : Vin = -V1
When the input at time t < T1 the input voltage (Vin) is equal to –V1 and the emitter-base diode is reverse biased. Since the collector-base diode is also reverse biased, therefore the transistor is cut-off, and practically no current exists anywhere in the circuit. As a result of this, the collector-to-emitter or the output voltage (Vo) is approximately equal to the supply voltage (Vcc). Thus when the collector current (Ic) is zero and Vo = Vcc, the transistor acts as an open switch.
Case 2 : Vin = -V1
When the input voltage becomes equal to V for the time interval T1<t<T2, both the emitter-base and collector-base diodes are forward biased and thetransistor is saturated. In this interval (i.e, T1t <T2 ) the output voltage, Vo = Vcesat. The valueof Vcesat for silicon transistor is 0.2 V and the collector current is maximum. Its value isapproximately equal to Vcc/Rc. These values indicate that the transistor acts as a closed switchand the current in the closed switch is determined by the supply voltage Vcc and the load resistor Rc.
At time T = T2, the input waveform returns back to -V
This causes the transistor to switch back to cut-off state.
Hence the results is as follows:
Ic = 0 and Vce = Vcc(Open switch)
Ic = Vcc/Rc and Vce = Vcesat (Closed switch)
Transistor Switching Time
By controlling the base current, the collector current can be controlled and thus make the BJT acts like a switch. Ideally, as soon as the base current exceeds the threshold value IB(sat) the BJT should saturate. But, practically it does not happen because of the capacitive effect of transistor
The collector voltage, instead of changing instantaneously, always takes time to change from one voltage level to another. Therefore, it is observed that the application of a pulse input at the base of a BJT is not followed directly by a change in output voltage. In other words, there is always some delay before the output changes as compared to the input.
Fig shows the waveform of the input pulse applied to the BJT and Fig shows the resulting waveform of collector current Ic along with the various time delays involved. The various-important parameters are defined as below
Delay time (td)
The delay time is the finite time that, lapses between application of the base input voltage and the start of collector current flow in the BJT. The time t is measured at values of input voltage and output current that are 10% of the maximum values. This is indicated by T The time that elapses during these delay together with the time required for the collector current to rise to 10% of its maximum, i.e. saturation value Icsat equal to Vcc/RL is called the delay time (td). The following two factors contribute to delay time:
1) The time required for carriers to pass from emitter to collector, and
ii) The base-emitter capacitance.
Rise time (tr)
The time required for the collector current to increase from 10% to 90%of its maximum value is called rise time t,.. The value of rise time for a transistor 2N2222 A is about 25 nanosecond.
Turn-ON time (Ton)
The sum of the delay time td and the rise time tr, is called the turn-ON time. Mathematically,
Ton=td
Storage time (ts)
when the input signal returns back to its initial state, Let –V1 at time t = T2 the collector current again fails to response immediately.
The time required for the collector current to decrease to 90% of its maximum value after the input has decreased to 90% of its maximum value is called storage time or saturated delay time ts.
It arises because a large number of carriers are stored at the collector junction when the BJT is saturated. These excess carriers must first be cleared out of the junction before the BJT can cut-off.
Falltime(tf)
The time required for the collector to fall from 90% to 10% of its maximum value is called falltime tf. Like rise time, fall time is also related to the transistor cut-off frequency and external capacitances.
Turn-OFF time (Toff)
The turn-off time is the sum of storage time ts and fall time tf Mathematically,
Toff = ts + tf
Types of MultiVibrator
Depending upon the type of coupling network, the multivibrators are classified as:
1. Astable (or free running) multivibrator,
2. Monostable (or one-shot) multivibrator, and
3. Bistable (or ifip-flop) multivibrator.
Astable Multivibrator (AMV)
The astable multivibrator also known as free running multivibrator which alternates automatically between the two states (ON or OFF) and remains in each state for a time dependent upon circuit constants. Thus, it is just an oscillator since it requires no external pulse for its operation. But it requires a source of d.c. power. It generates a square wave of known period.
It uses two capacitor and Astable multivibrator uses only capacitive couplings
It does not have any permanent stable stages, but it has two quasi-stable, i.e. temporary states. The circuit changes the state continuously from one quasi-stable to another without any external stimulus or trigger after a predetermined length of time.
This predetermined length of time is decided by the circuit time constants and parameters. Thus, an astable multivibrator generates continuous square waveform without any external signal.
Monostable Multivibrator
The monostable multivibrator also known as one shot multivibrator or delay Multivibrator of Univibrator, it has
- one stable state and
- another quasi-stable state,
i.e. half-stable state. Normally the circuit stay in a stable state.
It has one energy storage element ie capactor , Monostable multivibrator uses resistive-capacitive coupling.
When an external stimulus or trigger, i.e. pulse is applied, the stable state it changes into quasi-stable state for a predetermined length of time. After this period, the circuit returns back to its initial stable state automatically, i.e. by itself.
The process is repeated upon the application of each triggering pulse. The time duration of quasi-stable state is strictly decided by the circuit time constants and parameters and it is independent of pulse duration.
Bistable MultiVibrator
The bistable-multivibrator is also known flipFlop, binary and scaler of two circuits.
It doesnot use any type if charge storing element , Bistable multivibrator uses only resistive coupling.
It has two stable states and can stay in a particular state indefinitely. It requires the application ofan external triggering pulse to change the operation of circuit from either one state to the other.
At the occurrence of each triggering pulse, the circuit state changes abruptly from one stable state to another. Thus, one pulse is used to generate half-cycle of square wave and another pulse to generate the next half-cycle of square-wave. It is known as flip-flop because of the two possible states it can assume.
Application of Multivibrator
SrNo / MultiVibrator / Description / Application1 / Astable multivibrator (free running) / Two quasi-stable states with the length or time in each one controllable. / Oscillator, timing circuits, square wave generators.
2 / Monostable multiyibrator
(one shot or gating circuit) / One stable state and one quasi-stable state of controlled duration / Delay or gate circuits
3 / Bistable multivibrator (ifip-flop or binary) / Two stable states / Scalers, memory, counter, Arithmetic operations.
4 / Schmitt trigger / One stable state and one state that is maintained only as long as a minimum input is present. / Voltage discriminators, Analog to Digital conversion.
Astable Multi Vibrator
It is also called a free-runningor relaxation oscillator and is commonly used to generate square waveform. Figure shows the circuit of a collector-coupled astable multivibrator.
It uses two identical NPN transistors Q1 and Q2.
It is possible to have Rc1= Rc2 = Rc, R1 = R2 = R and C1 = C2 = C.
In such case, the circuit is known as symmetrical astable multivibrator. The transistor Q1 is forward biased by the Vcc supplythrough resistor R1.
Similarly, the transistor Q2 is forward biased by the Vcc supply through resistor R2. The output of transistor Q1 is coupled to the input of transistor Q2 through the capacitor C. Similarly, the output of transistor Q2 is coupled to the input of transistor Q1 through the capacitor C2.
Since capacitive coupling is used one of the transistor can remain permanently cut-off or saturated. Instead the circuit has two quasi-stable states (ON and OFF) and it makes periodic transition between these two states.
The output of an astable multivibrator is available at the collector terminal of either transistor (i.e, Q1 and Q2) as shown in the figure. However, the two outputs are 180° out of phase with each other. Therefore one of the output is said to be the complement of the other shown in Fig.
Working of AMV
The circuit operation of an astable multivibrator is easy to understand with the waveforms are at the base and collector of transistors Q1 and Q2.
Fig. shows the waveform for the base voltage of transistor Q (i.e vb1 and Fig. for the collector voltage of transistor Q1 (i.e.Vc1). Similarly Figure shows the waveform for the base voltage of transistor Q2 (i.e., vb2 )and the collector voltage of transistor Q2 (i.e., Vc2). The circuit operation may beexplained as follows:
- When the d.c. power supply (Vcc) is switched ON, (say at t = 0) one of the transistor will start conducting more than the other due to difference in β of transistor . suppose that transistor Q1 start conducting more than that of transistor Q2.
Then because of positive feedback, the transistor Q1 will be driven into saturation and transistor Q2 to cut-off. Thus at t > 0, the transistor Q1 is ON and Q2 is Off. Thus at t > 0, Vb1 = Vbesat (i.e., 0.7 V for silicon transistor), Vc1 = Vcesat (i.e.Vcesat = 0.2. for silicon transistor)0.3 V for silicon transistor), v is negative and Vc, = Vcc.
- During the time t > 0 (i.e. when Q1 is ON and Q2 is OFF), the capacitor C is charging towards the voltage Vcc through R1 The charging takes place exponentially with time constant =R1C1. Since the base of transistor Q2 is directly connected to capacitor C1 as shown in Fig Therefore the voltage Vb1 also increases exponentially towards Vcc.
- As soon as the voltage VB2, increases, above the cut-in voltage (i.e., 0.7 V for silicon transistor), the transistor Q2 starts conducting. It occurs at t = t1 As the transistor Q2 goes into saturation, its collector voltage (Vc2) falls to Vce (sat). The fall in voltage Vc2 causes an equal fall, (i.e. Vcc — Vce(sat) = Vcc) in voltage Vb1 because the two are capacitively coupled.
The fall in voltage Vb1 cuts-off the transistor Q1 and its collector voltage (Vc1) starts rising towards Vcc with a time constant ’ = RC2C2.
The rise in voltage Vc1 is coupled through capacitor C to the base of the transistor Q2, causing a small overshoot in voltage Vb2. Soon the voltage Vb2 settles at Vbe(sat) i.e., 0.7 V level. Thus at t >= t1, the transistor Q1 is OFF and Q2 is ON. The voltage levels at this instant are Vb1 is negative, Vc1=Vcc, Vbe2 = Vbe = Vbe(sat) and Vc2 = Vce(sat)
During the time t>t1 (i.e., when Q1 is OFF and Q2 is ON) the voltage Vb1 rises exponentially with time constant 2 = R2.C2 towards Vcc. At t = T2, the voltage Vb1, reaches the cut-in level (i.e., 0.7 V) and a reverse transition takes place (i.e. Q1 turns ON and Q2 turns OFF). The voltage levels for t>t2 Vb1 = BE (sat),Vc1 = Vce(sat), Vb2 is negative and Vc2 = Vcc. Thus the voltage level for t> t2 are the same as for t > 0.
Time period and Duty cycle
The time period T1 represents the duration for which the transistor Q1 is ON and Q2 OFF.
Similarly, time period T2 represents the duration for which the transistor Q2 is ON and Q1 is OFF. Both the time periods T1 and T2 depend upon charge of capacitors C1 and C2
T1= 0.693 R1 C1... (i.e., ON time for Q1)
andT2= 0.693R2 C2... (Le., ON time for Q2)
Total period of the wave,
T = T1 + T2= 0.693(R1C1 + R2C2)
If R1 = R2 = R and C1 = C2 = C, then we have a symmetrical astable multivibrator, whose timeperiods T1 = T2 and the total time period,
T = 0.693R1 C1+0.693R2 . C2
= 1.386 R C
and the frequency of oscillation is given by the reciprocal of the period, i.e.,
F =1= 1
T 1.386RC
The frequency of oscillation may be varied by adjusting the values of R and C. However, it is more practicable to adjust R than C. It may be noted that if the values of resistors R1 and R2 have not been selected with d.c. current gain of the transistor (β or hfe) in mind, the oscillations may not take place. To ensure oscillations, t value of resistor
R1≤ hfe(min) Rc1and
R2≤hfe(min) Rc2
Where hfe(min) is the minimum value of the dc current gain of the transistor Q1 and Q2. If value of the Rc1= Rc2 =Rc and R1=R2=R then
R≤ hfe(min).Rc
F = 1
1.386RC
Application of Astable MV
Some of the important applications of an astable multivibrator are:
1. It is used as an oscillator.
2. It is used in timing circuits or time delay circuits, and
3. It is used as a square wave/rectangular wave generator
Monostable Multivibrator
It is also called one shot or univibrator and can be used to generate a gating pulse, whose width can be controlled.
The monostable multivibrator provides a single pulse of desired duration in response to an external trigger. The external trigger causes the circuit to go to the quasi-stable state.
After a certain interval of time, the circuit returns to its original stable state.Fig. shows the circuit of a monostable multivibrator using NPN transistors. Here, the output of transistor Q2 is coupled to the base of transistor Q1 through the resistance R1.
Whereas the output of transistor Q1 is coupled to the base of transistor Q2 through the capacitor C2. The capacitor C1 is known as commutating capacitor or speed up capacitor. Its function is to speed up the circuit in making abrupt transitions between the ON and OFF states.
The base of transistor Q2 is returned to the Vcc supply through a resistor R3, while the base oftransistor Q1 is connected to the negative supply through a resistor R2 The advantage of thisbiasing is that it keeps the transistor Q1 OFF and Q2 ON. This state is known as a stable state of the monostable multivibrator.
The output of a monostable multivibrator is available at the collector terminal of eithertransistor (i.e. Q1 or Q2) as shown in the fig. However, the two outputs are the complement ofeach other i.e., when one of the output is at Vcc level, the other is at Vce (sat) level.
Operation of Monostable MultiVibrator
1)In the Stable state i.e. Q1 is OFF and Q2 is ON. when the trigger isapplied:
When a positive trigger pulse of sufficient amplitude is applied to the base of transistor Q1 it overrides the reverse bias provided by the Vbb supply and gives it a forward bias.Because of this, the transistor Q1 starts conducting.
2)As the transistor Q1 conducts, its collector voltage falls due to the voltage drop across resistor This fall in voltage is coupled through capacitor C which decreases the forward bias of transistor Q2.
3)The reduced forward bias, the collector current of transistor Q2 starts decreasing and its collector voltage rises exponentially towards Vcc.R1/(R1 + RC2) with a time constant 2 = C1(R1||Rc2)
4)The rising collector voltage of transistor Q2 is coupled to the base of transistor Q1 through resistor R1, where it further increases its forward bias. Because of the increased forward bias, the transistor Q1 conducts more. This action is cumulative because of the positive feedback, and the collector voltage of transistor Q1 falls to VCE (sat)
5)The capacitor C starts charging exponentially towards Vcc with a time constant 1 = R3C2. As C1 charges, the voltage at the base of the transistor Q2 decreases. As C1 charges further, the transistor Q2 is pulled out from the cut-off and the reverse transition takes place i.e., Q2 turns ON and Q1 turn OFF.
6)When the transistor Q2 starts conducting, its collector voltage falls because ofthe drop across resistor Rc This drop iscoupled to the base of the transistor Q1, whose collector voltage rises towards Vcc with time constant 3 = Rc1.C2. Finally, the transistor Q1 turns fully ON and the transistor Q2 goes OFF. The circuit remains in this stable state till another pulse is applied.
Fig.shows the waveforms at the base and collector of the transistor Q1 and Q2 of a monostable multivibrator. The width or duration of the pulse obtained at the collector