Category: pulse Circuits

Pulse Circuits – Miller Sweep Generator The transistor Miller time base generator circuit is the popular Miller integrator circuit that produces a sweep waveform. This is mostly used in horizontal deflection circuits. Let us try to understand the construction and working of a Miller time base generator circuit. Construction of Miller Sweep Generator The Miller time base generator circuit consists of a switch and a timing circuit in the initial stage, whose input is taken from the Schmitt gate generator circuit. The amplifier section is the following one which has three stages, first being an emitter follower, second an amplifier and the third one is also an emitter follower. An emitter follower circuit usually acts as a Buffer amplifier. It has a low output impedance and a high input impedance. The low output impedance lets the circuit drive a heavy load. The high input impedance keeps the circuit from not loading its previous circuit. The last emitter follower section will not load the previous amplifier section. Because of this, the amplifier gain will be high. The capacitor C placed between the base of Q1 and the emitter of Q3 is the timing capacitor. The values of R and C and the variation in the voltage level of VBB changes the sweep speed. The figure below shows the circuit of a Miller time base generator. Operation of Miller Sweep Generator When the output of Schmitt trigger generator is a negative pulse, the transistor Q4 turns ON and the emitter current flows through R1. The emitter is at negative potential and the same is applied at the cathode of the diode D, which makes it forward biased. As the capacitor C is bypassed here, it is not charged. The application of a trigger pulse, makes the Schmitt gate output high, which in turn, turns the transistor Q4 OFF. Now, a voltage of 10v is applied at the emitter of Q4 that makes the current flow through R1 which also makes the diode D reverse biased. As the transistor Q4 is in cutoff, the capacitor C gets charged from VBB through R and provides a rundown sweep output at the emitter of Q3. The capacitor C discharges through D and transistor Q4 at the end of the sweep. Considering the effect of capacitance C1, the slope speed or sweep speed error is given by $$e_s = frac{V_s}{V} left( 1- A + frac{R}{R_i} + frac{C}{C_i} right )$$ Applications Miller sweep circuits are the most commonly used integrator circuit in many devices. It is a widely used saw tooth generator. Learning working make money

Bidirectional Sampling Gates Bidirectional gates, unlike unidirectional ones, transmit signals of both positive and negative polarities. These gates can be constructed using either transistors or diodes. From different types of circuits, let us go through a circuit made up of transistors and another made up of diodes. Bidirectional Sampling Gates using Transistors A basic bidirectional sampling gate consists of a transistor and three resistors. The input signal voltage VS and the control input voltage VC are applied through the summing resistors to the base of the transistor. The circuit diagram given below shows the bidirectional sampling gate using transistor. The control input VC applied here is a pulse waveform with two levels V1 and V2 and pulse width tp. This pulse width decides the desired transmission interval. The gating signal allows the input to get transmitted. When the gating signal is at its lower level V2, the transistor goes into active region. So, until the gating input is maintained at its upper level, signals of either polarity, which appear at the base of the transistor will be sampled and appear amplified at the output. Four Diode Bidirectional Sampling Gate Bidirectional sampling gate circuit is made using diodes also. A two diode bidirectional sampling gate is the basic one in this model. But it has few disadvantages such as It has low gain It is sensitive to the imbalances of control voltage Vn (min) may be excessive Diode capacitance leakage is present A four diode bidirectional sampling gate was developed, improving these features. A two bidirectional sampling gate circuit was improved adding two more diodes and two balanced voltages +v or –v to make the circuit of a four diode bidirectional sampling gate as shown in the figure. The control voltages VC and –VC reverse bias the diodes D3 and D4 respectively. The voltages +v and –v forward bias the diodes D1 and D2 respectively. The signal source is coupled to the load through the resistors R2 and the conducting diodes D1 and D2. As the diodes D3 and D4 are reverse biased, they are open and disconnect the control signals from gate. So, an imbalance in control signals will not affect the output. When the control voltages applied are Vn and –Vn, then the diodes D3 and D4 conduct. The points P2 and P1 are clamped to these voltages, which make the diodes D1 and D2 revere biased. Now, the output is zero. During transmission, the diodes D3 and D4 are OFF. The gain A of the circuit is given by $$A = frac{R_C}{R_C + R_2} times frac{R_L}{R_L + (R_s/2)}$$ Hence the choice of application of control voltages enables or disables the transmission. The signals of either polarities are transmitted depending upon the gating inputs. Applications of Sampling Gates There are many applications of sampling gate circuits. The most common ones are as follows − Sampling scopes Multiplexers Sample and hold circuits Digital to Analog Converters Chopped Stabilizer Amplifiers Among the applications of sampling gate circuits, the Sampling scope circuit is prevalent. Let us try to have an idea on the block diagram of sampling scope. Sampling Scope In the sampling scope, the display consists of a sequence of samples of input waveform. Each of those samples are taken at a time progressively delayed with respect to some reference point in the waveform. This is the working principle of sampling scope which is shown below in the block diagram. The ramp generator and the stair case generator generates the waveforms according to the trigger inputs applied. The comparator compares both of these signals and generates the output which is then given to the sampling gate circuit as a control signal. As and when the control input is high the input at the sampling gate is delivered to the output and whenever the control input is low, the input is not transmitted. While taking the samples, they are chosen at the time instants, which are progressively delayed by equal increments. The samples consist of a pulse whose duration is equal to the duration of the sampling gate control and whose amplitude is determined by the magnitude of the input signal at the sampling time. The pulse width then produced will be low. Just like in the Pulse modulation, the signal has to be sampled and hold. But as the pulse width is low, it is amplified by an amplifier circuit so as to stretch and then given to a diode-capacitor combination circuit so as to hold the signal, to fill the interval of the next sample. The output this circuit is given to the vertical deflection plates and the output of sweep circuit is given to the horizontal deflection plates of the sampling scope to display the output waveform. Learning working make money

Pulse Circuits – Useful Resources The following resources contain additional information on Pulse Circuits. Please use them to get more in-depth knowledge on this. Useful Video Courses 137 Lectures 10 hours 8 Lectures 49 mins 45 Lectures 9.5 hours 88 Lectures 8 hours 11 Lectures 3.5 hours Most Popular 8 Lectures 2 hours Learning working make money

Pulse Circuits – Unijunction Transistor Unijunction Transistor is such a transistor that has a single PN junction, but still not a diode. Unijunction Transistor, or simply UJT has an emitter and two bases, unlike a normal transistor. This component is especially famous for its negative resistance property and also for its application as a relaxation oscillator. Construction of UJT A bar of highly resistive n-type silicon, is considered to form the base structure. Two Ohmic contacts are drawn at both the ends being both the bases. An aluminum rod like structure is attached to it which becomes the emitter. This emitter lies near to the base 2 and a bit far to the base1. Both of these join to form a PN junction. As single PN junction is present, this component is called as a Unijunction transistor. An internal resistance called as intrinsic resistance is present inside the bar whose resistance value depends upon the doping concentration of the bar. The construction and symbol of UJT are as shown below. In the symbol, the emitter is indicated by an inclined arrow and the remaining two ends indicate the bases. As the UJT is understood as a combination of diode and some resistance, the internal structure of UJT can be indicated by an equivalent diagram to explain the working of UJT. Working of UJT The working of UJT can be understood by its equivalent circuit. The voltage applied at the emitter is indicated as VE and the internal resistances are indicated as RB1 and RB2 at bases 1 and 2 respectively. Both resistances present internally are together called as intrinsic resistance, indicated as RBB. The voltage across RB1 can be denoted as V1. The dc voltage applied for the circuit to function is VBB. The UJT equivalent circuit is as given below. Initially when no voltage is applied, $$V_E = 0$$ Then the voltage VBB is applied through RB2. The diode D will be in reverse bias. The voltage across the diode will be VB which is the barrier voltage of the emitter diode. Due to the application of VBB, some voltage appears at point A. So, the total voltage will be VA + VB. Now if the emitter voltage VE is increased, the current IE flows through the diode D. This current makes the diode forward biased. The carriers get induced and the resistance RB1 goes on decreasing. Therefore, the potential across RB1 which means VB1 also decreases. $$V_{B1} = left( frac{R_{B1}}{R_{B1} + R_{B2}} right )V_{BB}$$ As VBB is constant and RB1 decreases to its minimum value due to the doping concentration of the channel, VB1 also decreases. Actually, the resistances present internally are together called as intrinsic resistance, indicated as RBB. The resistance mentioned above can be indicated as $$R_{BB} = R_{B1} + R_{B2}$$ $$left( frac{R_{B1}}{R_{BB}} right ) = eta$$ The symbol η is used to represent the total resistance applied. Hence voltage across VB1 is represented as $$V_{B1} = eta V_{BB}$$ The emitter voltage is given as $$V_E = V_D + V_{B1}$$ $$V_E = 0.7 + V_{B1}$$ Where VD is the voltage across the diode. As the diode gets forward biased, the voltage across it will be 0.7v. So, this is constant and VB1 goes on decreasing. Hence VE goes on decreasing. It decreases to a least value which may be denoted VV called as Valley voltage. The voltage at which the UJT gets switched ON is the Peak Voltage denoted as VP. V-I Characteristics of UJT The concept discussed till now is clearly understood from the following graph shown below. Initially when VE is zero, some reverse current IE flows until, the value of VE reaches a point at which $$V_E = eta V_{BB}$$ This is the point where the curve touches the Y-axis. When VE reaches a voltage where $$V_E = eta V_{BB} + V_D$$ At this point, the diode gets forward biased. The voltage at this point is called as VP (Peak Voltage) and the current at this point is called as IP (Peak Current). The portion in the graph till now, is termed as Cut off region as the UJT was in OFF state. Now, when VE is further increased, the resistance RB1 and then the voltage V1 also decreases, but the current through it increases. This is the Negative resistance property and hence this region is called as Negative resistance region. Now, the voltage VE reaches a certain point where further increase leads to the increase in voltage across RB1. The voltage at this point is called as VV (Valley Voltage) and the current at this point is called as IV (Valley Current). The region after this is termed as Saturation region. Applications of UJT UJTs are most prominently used as relaxation oscillators. They are also used in Phase Control Circuits. In addition, UJTs are widely used to provide clock for digital circuits, timing control for various devices, controlled firing in thyristors, and sync pulsed for horizontal deflection circuits in CRO. Learning working make money

Pulse Circuits – Blocking Oscillators An oscillator is a circuit that provides an alternating voltage or current by its own, without any input applied. An Oscillator needs an amplifier and also a feedback from the output. The feedback provided should be regenerative feedback which along with the portion of the output signal, contains a component in the output signal, which is in phase with the input signal. An oscillator that uses a regenerative feedback to generate a nonsinusoidal output is called as Relaxation Oscillator. We have already seen UJT relaxation oscillator. Another type of relaxation oscillator is the Blocking oscillator. Blocking Oscillator A blocking oscillator is a waveform generator that is used to produce narrow pulses or trigger pulses. While having the feedback from the output signal, it blocks the feedback, after a cycle, for certain predetermined time. This feature of blocking the output while being an oscillator, gets the name blocking oscillator to it. In the construction of a blocking oscillator, the transistor is used as an amplifier and the transformer is used for feedback. The transformer used here is a Pulse transformer. The symbol of a pulse transformer is as shown below. Pulse Transformer A Pulse transformer is one which couples a source of rectangular pulses of electrical energy to the load. Keeping the shape and other properties of pulses unchanged. They are wide band transformers with minimum attenuation and zero or minimum phase change. The output of the transformer depends upon the charge and discharge of the capacitor connected. The regenerative feedback is made easy by using pulse transformer. The output can be fed back to the input in the same phase by properly choosing the winding polarities of the pulse transformer. Blocking oscillator is such a free-running oscillator made using a capacitor and a pulse transformer along with a single transistor which is cut off for most of the duty cycle producing periodic pulses. Using the blocking oscillator, Astable and Monostable operations are possible. But Bistable operation is not possible. Let us go through them. Monostable Blocking Oscillator If the blocking oscillator needs a single pulse, to change its state, it is called as a Monostable blocking oscillator circuit. These Monostable blocking oscillators can be of two types. They are Monostable blocking oscillator with base timing Monostable blocking oscillator with emitter timing In both of these, a timing resistor R controls the gate width, which when placed in the base of transistor becomes base timing circuit and when placed in the emitter of transistor becomes emitter timing circuit. To have a clear understanding, let us discuss the working of base timing Monostable Multivibrator. Transistor Triggered Monostable blocking oscillator with Base timing A transistor, a pulse transformer for feedback and a resistor in the base of the transistor constitute the circuit of a transistor triggered Monostable blocking oscillator with base timing. The pulse transformer used here has a turns ratio of n: 1 where the base circuit has n turns for every turn on the collector circuit. A resistance R is connected in series to the base of the transistor which controls the pulse duration. Initially the transistor is in OFF condition. As shown in the following figure, VBB is considered zero or too low, which is negligible. The voltage at the collector is VCC, since the device is OFF. But when a negative trigger is applied at the collector, the voltage gets reduced. Because of the winding polarities of the transformer, the collector voltage goes down, while the base voltage rises. When the base to emitter voltage becomes greater than the cut-in voltage, i.e. $$V_{BE} > V_gamma$$ Then, a small base current is observed. This raises the collector current which decreases the collector voltage. This action cumulates further, which increases the collector current and decreases the collector voltage further. With the regenerative feedback action, if the loop gain increases, the transistor gets into saturation quickly. But this is not a stable state. Then, a small base current is observed. This raises the collector current which decreases the collector voltage. This action cumulates further, which increases the collector current and decreases the collector voltage further. With the regenerative feedback action, if the loop gain increases, the transistor gets into saturation quickly. But this is not a stable state. When the transistor gets into saturation, the collector current increases and the base current is constant. Now, the collector current slowly starts charging the capacitor and the voltage at the transformer reduces. Due to the transformer winding polarities, the base voltage gets increased. This in turn decreases the base current. This cumulative action, throws the transistor into cut off condition, which is the stable state of the circuit. The output waveforms are as follows − The main disadvantage of this circuit is that the output Pulse width cannot be maintained stable. We know that the collector current is $$i_c = h_{FE}i_B$$ As the hFE is temperature dependent and the pulse width varies linearly with this, the output pulse width cannot be stable. Also hFE varies with the transistor used. Anyways, this disadvantage can be eliminated if the resistor is placed in emitter, which means the solution is the emitter timing circuit. When the above condition occurs, the transistor turns OFF in the emitter timing circuit and so a stable output is obtained. Astable Blocking Oscillator If the blocking oscillator can change its state automatically, it is called as an Astable blocking oscillator circuit. These Astable blocking oscillators can be of two types. They are Diode controlled Astable blocking oscillator RC controlled Astable blocking oscillator In diode controlled Astable blocking oscillator, a diode placed in the collector changes the state of the blocking oscillator. While in the RC controlled Astable blocking oscillator, a timing resistor R and capacitor C form a network in the emitter section to control the pulse timings. To have a clear understanding, let us discuss the working of Diode controlled Astable blocking oscillator. Diode controlled Astable blocking oscillator The diode controlled Astable blocking oscillator contains

Pulse Circuits – Bistable Multivibrator A Bistable Multivibrator has two stable states. The circuit stays in any one of the two stable states. It continues in that state, unless an external trigger pulse is given. This Multivibrator is also known as Flip-flop. This circuit is simply called as Binary. There are few types in Bistable Multivibrators. They are as shown in the following figure. Construction of Bistable Multivibrator Two similar transistors Q1 and Q2 with load resistors RL1 and RL2 are connected in feedback to one another. The base resistors R3 and R4 are joined to a common source –VBB. The feedback resistors R1 and R2 are shunted by capacitors C1 and C2 known as Commutating Capacitors. The transistor Q1 is given a trigger input at the base through the capacitor C3 and the transistor Q2 is given a trigger input at its base through the capacitor C4. The capacitors C1 and C2 are also known as Speed-up Capacitors, as they reduce the transition time, which means the time taken for the transfer of conduction from one transistor to the other. The following figure shows the circuit diagram of a self-biased Bistable Multivibrator. Operation of Bistable Multivibrator When the circuit is switched ON, due to some circuit imbalances as in Astable, one of the transistors, say Q1 gets switched ON, while the transistor Q2 gets switched OFF. This is a stable state of the Bistable Multivibrator. By applying a negative trigger at the base of transistor Q1 or by applying a positive trigger pulse at the base of transistor Q2, this stable state is unaltered. So, let us understand this by considering a negative pulse at the base of transistor Q1. As a result, the collector voltage increases, which forward biases the transistor Q2. The collector current of Q2 as applied at the base of Q1, reverse biases Q1 and this cumulative action, makes the transistor Q1 OFF and transistor Q2 ON. This is another stable state of the Multivibrator. Now, if this stable state has to be changed again, then either a negative trigger pulse at transistor Q2 or a positive trigger pulse at transistor Q1 is applied. Output Waveforms The output waveforms at the collectors of Q1 and Q2 along with the trigger inputs given at the bases of QW and Q2 are shown in the following figures. Advantages The advantages of using a Bistable Multivibrator are as follows − Stores the previous output unless disturbed. Circuit design is simple Disadvantages The drawbacks of a Bistable Multivibrator are as follows − Two kinds of trigger pulses are required. A bit costlier than other Multivibrators. Applications Bistable Multivibrators are used in applications such as pulse generation and digital operations like counting and storing of binary information. Fixed-bias Binary A fixed-bias binary circuit is similar to an Astable Multivibrator but with a simple SPDT switch. Two transistors are connected in feedback with two resistors, having one collector connected to the base of the other. The figure below shows the circuit diagram of a fixed-bias binary. To understand the operation, let us consider the switch to be in position 1. Now the transistor Q1 will be OFF as the base is grounded. The collector voltage at the output terminal VO1 will be equal to VCC which turns the transistor Q2 ON. The output at the terminal VO2 goes LOW. This is a stable state which can be altered only by an external trigger. The change of switch to position 2, works as a trigger. When the switch is altered, the base of transistor Q2 is grounded turning it to OFF state. The collector voltage at VO2 will be equal to VCC which is applied to transistor Q1 to turn it ON. This is the other stable state. The triggering is achieved in this circuit with the help of a SPDT Switch. There are two main types of triggering given to the binary circuits. They are Symmetrical Triggering Asymmetrical Triggering Schmitt Trigger Another type of binary circuit which is ought to be discussed is the Emitter Coupled Binary Circuit. This circuit is also called as Schmitt Trigger circuit. This circuit is considered as a special type of its kind for its applications. The main difference in the construction of this circuit is that the coupling from the output C2 of the second transistor to the base B1 of the first transistor is missing and that feedback is obtained now through the resistor Re. This circuit is called as the Regenerative circuit for this has a positive feedback and no Phase inversion. The circuit of Schmitt trigger using BJT is as shown below. Initially we have Q1 OFF and Q2 ON. The voltage applied at the base of Q2 is VCC through RC1 and R1. So the output voltage will be $$V_0 = V_{CC} – (I_{C2}R_{c2})$$ As Q2 is ON, there will be a voltage drop across RE, which will be (IC2 + IB2) RE. Now this voltage gets applied at the emitter of Q1. The input voltage is increased and until Q1 reaches cut-in voltage to turn ON, the output remains LOW. With Q1 ON, the output will increase as Q2 is also ON. As the input voltage continues to rise, the voltage at the points C1 and B2 continue to fall and E2 continues to rise. At certain value of the input voltage, Q2 turns OFF. The output voltage at this point will be VCC and remains constant though the input voltage is further increased. As the input voltage rises, the output remains LOW until the input voltage reaches V1 where $$V_1 = [V_{CC} – (I_{C2}R_{C2})]$$ The value where the input voltage equals V1, lets the transistor Q1 to enter into saturation, is called UTP (Upper Trigger Point). If the voltage is already greater than V1, then it remains there until the input voltage reaches V2, which is a low level transition. Hence the value for which input voltage will be V2 at which Q2 gets into ON condition, is

Pulse Circuits Tutorial Job Search In this tutorial, we will discuss all the important circuits that are related to pulse signals. In addition, we will also cover the circuits that generate and work with pulse signals. Audience A reader who is interested in the basics of pulse and sweep related circuits and who aspires to have an idea regarding the generation and applications of pulse and sweep signals, can go ahead with this tutorial. Prerequisites We assume that the readers have prior knowledge on the fundamental concepts of Basic Electronic Circuits and the behavior of different electronic components. For reference, the readers can browse through our ELECTRONIC CIRCUITS tutorial at . Learning working make money

Pulse Circuits – Synchronization In any system, having different waveform generators, all of them are required to be operated in synchronism. Synchronization is the process of making two or more waveform generators arrive at some reference point in the cycle exactly at the same time. Types of Synchronization Synchronization can be of the following two types − One-to-one basis All the generators are operated at a same frequency. All of them arrive at some reference point in the cycle exactly at the same time. Sync with frequency division Generators operate at different frequency which are integral multiples of each other. All of them arrive at some reference point in the cycle exactly at the same time. Relaxation devices Relaxation circuits are the circuits in which the timing interval is established through the gradual charging of a capacitor, the timing interval being terminated by the sudden discharge (relaxation) of a capacitor. Examples − Multivibrators, sweep circuits, blocking oscillators, etc. We have observed in the UJT relaxation oscillator circuit that the capacitor stops charging when the negative resistance device such as UJT turns ON. The capacitor then discharges through it to reach its minimum value. Both these points denote the maximum and minimum voltage points of a sweep waveform. Synchronization in Relaxation Devices If the high voltage or peak voltage or breakdown voltage of the sweep waveform has to be brought down to a lower level, then an external signal can be applied. This signal to be applied is the synchronized signal whose effect lowers the voltage of peak or breakdown voltage, for the duration of the pulse. A synchronizing pulse is generally applied at the emitter or at the base of a negative resistance device. A pulse train having regularly spaced pulses is applied to achieve synchronization. Though the synchronizing signal is applied first few pulses will have no effect on the sweep generator as the amplitude of the sweep signal at the occurrence of the pulse, in addition with the amplitude of the pulse is less than VP. Hence the sweep generator runs unsynchronized. The exact moment at which UJT turns ON is determined by the instant ofoccurrence of a pulse. This is the point where the sync signal achieves synchronization with the sweep signal. This can be observed from the following figure. Where, TP is the time period of the pulse signal TO is the time period of the sweep signal VP is the Peak or breakdown voltage VV is the Valley or Maintaining voltage To achieve synchronization, the pulse timing interval TP should be less than the time period of sweep generator TO, so that it terminates the sweep cycle prematurely. The synchronization cannot be achieved if the pulse timing interval TP is greater than the time period of sweep generator TO and also if the amplitude of the pulses is not large enough to bridge the gap between the quiescent breakdown and the sweep voltage, though TP is less than TO. Frequency Division in Sweep Circuits In the previous topic, we have observed that synchronization gets achieved when the following conditions are satisfied. They are When TP < TO When the amplitude of the pulse is sufficient to terminate each cycle prematurely. Satisfying these two conditions, though synchronization is achieved, we may often come across a certain interesting pattern in the sweep with regard to sync timing. The following figure illustrates this point. We can observe that the amplitude V’S of the sweep after synchronization is less than the unsynchronized amplitude VS. Also the time period TO of the sweep is adjusted according to the time period of the pulse but leaving a cycle in-between. Which means, one sweep cycle is made equal to two pulse cycles. The synchronization is achieved for every alternate cycle, which states $$T_o > 2T_P$$ The sweep timing TO be restricted to TS and its amplitude is reduced to V’S. As every second pulse is made in synchronism with the sweep cycle, this signal can be understood as a circuit that exhibits frequency division by a factor of 2. Hence the frequency division circuit is obtained by synchronization. Learning working make money

UJT as Relaxation Oscillator An oscillator is a device that produces a waveform by its own, without any input. Though some dc voltage is applied for the device to work, it will not produce any waveform as input. A relaxation oscillator is a device that produces a non-sinusoidal waveform on its own. This waveform depends generally upon the charging and discharging time constants of a capacitor in the circuit. Construction and Working The emitter of UJT is connected with a resistor and capacitor as shown. The RC time constant determines the timings of the output waveform of the relaxation oscillator. Both the bases are connected with a resistor each. The dc voltage supply VBB is given. The following figure shows how to use a UJT as a relaxation oscillator. Initially, the voltage across the capacitor is zero. $$V_c = 0$$ The UJT is in OFF condition. The resistor R provides a path for the capacitor C to charge through the voltage applied. The capacitor charges according to the voltage $$V = V_0(1 – e^{-t/RC})$$ The capacitor usually starts charging and continues to charge until the maximum voltage VBB. But in this circuit, when the voltage across capacitor reaches a value, which enables the UJT to turn ON (the peak voltage) then the capacitor stops to charge and starts discharging through UJT. Now, this discharging continues until the minimum voltage which turns the UJT OFF (the valley voltage). This process continues and the voltage across the capacitor, when indicated on a graph, the following waveform is observed. So, the charge and discharge of capacitor produces the sweep waveform as shown above. The charging time produces increasing sweep and the discharging time produces decreasing sweep. The repetition of this cycle, forms a continuous sweep output waveform. As the output is a non-sinusoidal waveform, this circuit is said to be working as a relaxation oscillator. Applications of Relaxation Oscillator Relaxation oscillators are widely used in function generators, electronic beepers, SMPS, inverters, blinkers, and voltage controlled oscillators. Learning working make money

Pulse Circuits – Astable Multivibrator An astable multivibrator has no stable states. Once the Multivibrator is ON, it just changes its states on its own after a certain time period which is determined by the RC time constants. A dc power supply or Vcc is given to the circuit for its operation. Construction of Astable Multivibrator Two transistors named Q1 and Q2 are connected in feedback to one another. The collector of transistor Q1 is connected to the base of transistor Q2 through the capacitor C1 and vice versa. The emitters of both the transistors are connected to the ground. The collector load resistors R1 and R4 and the biasing resistors R2 and R3 are of equal values. The capacitors C1 and C2 are of equal values. The following figure shows the circuit diagram for Astable Multivibrator. Operation of Astable Multivibrator When Vcc is applied, the collector current of the transistors increase. As the collector current depends upon the base current, $$I_c = beta I_B$$ As no transistor characteristics are alike, one of the two transistors say Q1 has its collector current increase and thus conducts. The collector of Q1 is applied to the base of Q2 through C1. This connection lets the increased negative voltage at the collector of Q1 to get applied at the base of Q2 and its collector current decreases. This continuous action makes the collector current of Q2 to decrease further. This current when applied to the base of Q1 makes it more negative and with the cumulative actions Q1 gets into saturation and Q2 to cut off. Thus the output voltage of Q1 will be VCE (sat) and Q2 will be equal to VCC. The capacitor C1 charges through R1 and when the voltage across C1 reaches 0.7v, this is enough to turn the transistor Q2 to saturation. As this voltage is applied to the base of Q2, it gets into saturation, decreasing its collector current. This reduction of voltage at point B is applied to the base of transistor Q1 through C2 which makes the Q1 reverse bias. A series of these actions turn the transistor Q1 to cut off and transistor Q2 to saturation. Now point A has the potential VCC. The capacitor C2 charges through R2. The voltage across this capacitor C2 when gets to 0.7v, turns on the transistor Q1 to saturation. Hence the output voltage and the output waveform are formed by the alternate switching of the transistors Q1 and Q2. The time period of these ON/OFF states depends upon the values of biasing resistors and capacitors used, i.e., on the RC values used. As both the transistors are operated alternately, the output is a square waveform, with the peak amplitude of VCC. Waveforms The output waveforms at the collectors of Q1 and Q2 are shown in the following figures. Frequency of Oscillations The ON time of transistor Q1 or the OFF time of transistor Q2 is given by t1 = 0.69R1C1 Similarly, the OFF time of transistor Q1 or ON time of transistor Q2 is given by t2 = 0.69R2C2 Hence, total time period of square wave t = t1 + t2 = 0.69(R1C1 + R2C2) As R1 = R2 = R and C1 = C2 = C, the frequency of square wave will be $$f = frac{1}{t} = frac{1}{1.38 R C} = frac{0.7}{RC}$$ Advantages The advantages of using an astable multivibrator are as follows − No external triggering required. Circuit design is simple Inexpensive Can function continuously Disadvantages The drawbacks of using an astable multivibrator are as follows − Energy absorption is more within the circuit. Output signal is of low energy. Duty cycle less than or equal to 50% can’t be achieved. Applications Astable Multivibrators are used in many applications such as amateur radio equipment, Morse code generators, timer circuits, analog circuits, and TV systems. Learning working make money