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Applied Electronics – SYET – Mr. Manoj Kavedia 1 Non Sinusoidal Generators-Multivibrators The oscillators which generate waveforms other than sine waveform are called nonsinusoidal oscillators. The waveforms generated by nonsinusoidal oscillators include 1. Square, 2. Triangular, 3. Rectangular and 4. 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 i. Multivibrator ii. Blocking oscillator and iii. 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 switch operates between two states namely 1. saturation and 2. cut-off state. The saturation state occurs when both the junctions (i.e., emitter-base junction and collector base junction) of a transistor 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 collectorbase 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 the transistor is saturated. In this interval (i.e, T1 < t <T2 ) the output voltage, Vo = Vcesat. The value of Vcesat for silicon transistor is 0.2 V and the collector current is maximum. Its value is approximately equal to Vcc/Rc. These values indicate that the transistor acts as a closed switch and 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) Applied Electronics – SYET – Mr. Manoj Kavedia 2 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 fall time 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, Applied Electronics – SYET – Mr. Manoj Kavedia 3 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 1. one stable state and 2. 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 of an 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 1 Astable multivibrator (free running) 2 Monostable multiyibrator (one shot or gating circuit) 3 Bistable multivibrator (ifip-flop or binary) Schmitt trigger 4 Description Application Two quasi-stable states with the length or time in each one controllable. One stable state and one quasi-stable state of controlled duration Two stable states Oscillator, timing circuits, square wave generators. One stable state and one Delay or gate circuits Scalers, memory, counter, Arithmetic operations. Voltage discriminators, Applied Electronics – SYET – Mr. Manoj Kavedia 4 state that is maintained only as long as a minimum input is present. Analog to Digital conversion. Astable Multi Vibrator It is also called a free-running or 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 supply through 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 be explained as follows: 1. 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. 2. 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 Applied Electronics – SYET – Mr. Manoj Kavedia 5 Therefore the voltage Vb1 also increases exponentially towards Vcc. 3. 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) and T2= 0.693 R2 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 time periods 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) Rc1 and 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 Applied Electronics – SYET – Mr. Manoj Kavedia 6 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 uni vibrator 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 of transistor Q1 is connected to the negative supply through a resistor R2 The advantage of this biasing 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 either transistor (i.e. Q1 or Q2) as shown in the fig. However, the two outputs are the complement of each 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 is applied: 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. Applied Electronics – SYET – Mr. Manoj Kavedia 7 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 of the drop across resistor Rc This drop is coupled 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 monostable multivibrator is given by the expression, = 0.693 R3 C3 Application of MMV The monostable multivibrators are used for generation of well defined pulses, the logic design of pulse delay, variable pulse width etc. Bistable MultiVibrator It is also called flip-flop, Eccles-Jordan circuit, trigger-circuit or binary. The bistable multivibrator is used for counting and storing of binary information in computer circuits. It also used in the generation and processing of pulse type waveforms. The bistable multivibrator has two stable states and can stay in any one of these two states, indefinitely, as long as the power is supplied. It changes to other state only when it receives a trigger (a short direction pulse) from outside. Fig. shows the circuit of a bistable multivibrator using two NPN transistors. Here, the output of a transistor Q2 is coupled to the base of transistor Q1 through a resistor R1 Similarly, the output of a transistor Q1 is coupled to the base of transistor Q2 through a resistor R2. The capacitors C1 and C2 are known as speed up capacitors. Theft function is to increase the speed of the circuit in making abrupt transition from one stable state to another stable state. The base resistors R3 and R4 of both the transistors are connected to a common source VBB. The output of a bistable multivibrator is available at the collector terminal of both the transistors Q1 and Q2. However, the two outputs are the complements of each other. Operation Monostable Multivibrator When Vcc supply is switched on one of the transistor will start conducting more than the other. Then because of the feedback action, this transistor will be driven into s4turation and the other to cut-off. Let us assume that the transistor Q1 is in saturation (i.e ON) and Q2 is cut-off (i.e., OFF). It is a stable state of the circuit and will remain in this state, till a trigger pulse is applied from outside. A negative pulse applied to the set input will turn OFF the transistor Q1 and Q2 to ON. A similar action can also be achieved by applying a positive pulse at reset input. Suppose a positive pulse is applied at the reset input. It will cause the transistor Q2 to conduct. As the collector voltage of Q2 falls, it cuts-off the transistor Q1 Thus the circuit switches to the other stable state i.e., a state where Q1 is OFF and Q2 is ON. Now if a positive pulse is applied at the set input, it will switch the circuit back to its original stable state i.e. Q1 ON and Q2 OFF. Applied Electronics – SYET – Mr. Manoj Kavedia 8 Fig shows the waveforms at the collector of transistor Q1 and Q2 of the bistable multivibrator in response to the trigger pulses applied at the set and reset input. It is seen from the waveforms, that the output waveforms are the complement of each other. Notes: There are two methods of triggering bistable multivibrators namely unsymmetrical triggering and symmetrical triggering. The unsymmetrical triggering requires two trigger inputs called set and reset. These are two independent trigger sources. This method of triggering is used in computer logic circuits. The symmetrical trigging requires a common trigger source and is used In counter circuits. It may be noted that we have explained the operation of a bistable multivibrator using a method of unsymmetrical triggering. The trigger pulses may be applied either at the base or collector of the transistors, as it does not make any difference. Application of Bistable MV Some of the important applications of bistable multivibrators are 1) It is used as a basic memory element which stores a single bit. It is useful in digital circuits. 2) It is used for divide by two operation. 3) It is used as a scaler. 4) It is used as counters and registers. 5) It is also used for arithmetic operations. Application Of Bistable Multivibrator : Schmitt Trigger Figure shows the circuit of a Schmitt trigger (after the name of its inventor). Even though it is multivibrator circuits, Schmitt trigger is used for wave shaping circuits. It is used for generation of square wave from a sine-wave input. Basically, the circuit has two opposite operating states as in all other multivibrator circuits. Here the trigger signal is not, a pulse waveform but a slowly varying ac. voltage. The Schmitt trigger is level sensitive and switches the output state at two distinct trigger levels. One of the triggering levels is called a lowertrigger level (abbreviated as L.T.L) and the other as upper-trigger level (abbreviated as U.T.L.) Circuit Description The circuit of a Schmitt trigger consist of two identical transistors Q1 and Q2 coupled through an emitter RE. The resistors R1 and R2 form a voltage divider across the Vcc supply and ground. These resistors provide a small forward bias on the base of transistor Q2 Operation of the circuit Initially when there is no signal at the input. and the power supply is switched on, the transistor Q2 starts conducting. The flow of its current through resistor RE produces a voltage drop across it. This voltage drop acts as a reverse bias across the emitter-base junction of transistor Q1 due to which it cuts-off. As a result, the voltage at its collector rises to Vcc. This rising voltage is coupled to the base of transistor Q2 through the resistor R1. It increases the forward bias at the base of transistor Q2 and therefore drives it into saturation and holds it there. At this instant, the collector voltage, levels are Vc1 = Vcc and Vc2 = vcE (sat) as Applied Electronics – SYET – Mr. Manoj Kavedia 9 shown in Figure. When an a.c. signal is applied at the input of the Schmitt trigger (i.e., at the base of the transistor Q1, as the input voltage increases above zero, nothing will happen until it crosses the upper trigger level (U.L.T.). As the input voltage increases, above the upper-trigger level, the transistor Q1 conducts. The point, at which it starts conducting, is known as upper-trigger point (U.T.P.). As the transistor Q1 conducts, its collector voltage falls below Vcc. This fall is coupled through resistor R1 to the base of transistor Q2 which reduces its forward bias. This in turn reduces the current of transistors Q2 and hence the voltage drop across the resistor RE. As a result of this, the reverse bias of transistor Q1 is reduced, and it conducts more. As the transistor Q1 conducts more heavily, its collector current further reduces due to which the transistor Q2 conducts near cut-off. This process continues till the transistor Q1 is driven into saturation and Q2 into cut-off At this instant, the collector voltage levels are Vc1 =Vcecsat and Vc2= Vcc as shown in the figure The transistor Q1 continue to conduct till the input voltage falls below the lowertrigger level (L.T.L.). When the input voltage becomes equal to the lower-trigger level, the emitter-base junction of transistor Q1 becomes reverse biased. As a result of this, its collector voltages starts rising towards Vcc. This rising voltage increases the forward bias across transistor Q2, due to which it conducts. The points, at which transistor Q2 starts conducting is called lower-trigger point (L.T.P.). Soon the transistor Q2 is driven into saturation and Q1 to cut-off. This completes one cycle. The collector voltage levels at this instant are Vc1=Vcc and Vc2 = Vce (sat). No change in state will occur during the negative half cycle of the input voltage. It is clear that the output of a Schmitt trigger is a positive going pulse, whose width depends upon the time during which transistor Q1 is conducting. The conduction time is set by the upper- and lower-trigger levels. Application of Schmitt Trigger The application of a Schmitt trigger are as given below: 1) It is used as a comparator, i.e. voltage discriminator. 2) It is used as a pulse counter. 3) It is used as asynchronization driven multivibrator. 4) It is used as a rectangular waveform generator for squaring sinusoidal, sawtooth or other wave-shapes. 5) It is used as analog to digital converter. Comparision Between Multivibrators Sr.N Astable MV Monostable MV Bistable MV o 1 It has two quasi-stable It has one stable state and It has two stable states. states with the length of one quasi-stable state of time in each one controlled duration. controllable. 2 It is self-triggered. It is externally triggered. It is externally triggered. Applied Electronics – SYET – Mr. Manoj Kavedia 3 4 It requires no external trigger pulse to generate one square wave, It uses capacitive coupling. 10 It requires one trigger pulse to square wave. external It requires two external generate trigger pulses to generate one square wave. It uses resistive capacitive. It uses resistive coupling. Assignment Question 1) What is a nonsinusoidal oscillator? Explain it briefly. 2) Explain the switching action of a transistor 3) What is a multivibrator? Explain the difference between the three types of multivibrators. 4) With a neat sketch, explain the working of an astable multivibrator. 5) Draw the switching waveforms for the astable multivibrator. 6) What is monostable multivibrator? Explain its working with the help of waveforms. 7) With a neat sketch, explain the operation of a bistable multivibrator. 8) How a Schmitt trigger is different from a multivibrator? 9) Draw any one type of a Schmitt trigger circuit and explain with waveforms. 10) Compare AMV , MMV, BMV 11) What is Hysteresis ? Define UTP , LTP . 12) Define the terms delay time , Rise Time , Turn –On time , Fall tine , turn oFF Time , Storage time. 13) Explain the operation of the transistor as the switch 14) State the device which are used multivibrator.