Discuss Pulse Circuits 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. Learning working make money
Pulse Circuits – Multivibrator Overview A multivibrator circuit is nothing but a switching circuit. It generates non-sinusoidal waves such as Square waves, Rectangular waves and Saw tooth waves etc. Multivibrators are used as frequency generators, frequency dividers and generators of time delays and also as memory elements in computers etc. A Transistor basically functions as an amplifier in its linear region. If a transistor amplifier output stage is joined with the previous amplifier stage, such a connection is said to be coupled. If a resistor is used in coupling two stages of such an amplifier circuit, it is called as Resistance coupled amplifier. For more details, refer to the AMPLIFIERS tutorial. What is a Multivibrator? According to the definition, A Multivibrator is a two-stage resistance coupled amplifier with positive feedback from the output of one amplifier to the input of the other. Two transistors are connected in feedback so that one controls the state of the other. Hence the ON and OFF states of the whole circuit, and the time periods for which the transistors are driven into saturation or cut off are controlled by the conditions of the circuit. The following figure shows the block diagram of a Multivibrator. Types of Multivibrators There are two possible states of a Multivibrator. In first stage, the transistor Q1 turns ON while the transistor Q2 turns OFF. In second stage, the transistor Q1 turns OFF while the transistor Q2 turns ON. These two states are interchanged for certain time periods depending upon the circuit conditions. Depending upon the manner in which these two states are interchanged, the Multivibrators are classified into three types. They are Astable Multivibrator An Astable Multivibrator is such a circuit that it automatically switches between the two states continuously without the application of any external pulse for its operation. As this produces a continuous square wave output, it is called as a Free-running Multivibrator. The dc power source is a common requirement. The time period of these states depends upon the time constants of the components used. As the Multivibrator keeps on switching, these states are known as quasi-stable or halfstable states. Hence there are two quasi-stable states for an Astable Multivibrator. Monostable Multivibrator A Monostable Multivibrator has a stable state and a quasi-stable state. This has a trigger input to one transistor. So, one transistor changes its state automatically, while the other one needs a trigger input to change its state. As this Multivibrator produces a single output for each trigger pulse, this is known as One-shot Multivibrator. This Multivibrator cannot stay in quasi-stable state for a longer period while it stays in stable state until the trigger pulse is received. Bistable Multivibrator A Bistable Multivibrator has both the two states stable. It requires two trigger pulses to be applied to change the states. Until the trigger input is given, this Multivibrator cannot change its state. It’s also known as flip-flop multivibrator. As the trigger pulse sets or resets the output, and as some data, i.e., either high or low is stored until it is disturbed, this Multivibrator can be called as a Flip-flop. To know more about flip-flops, refer our DIGITAL CIRCUITS tutorial at: To get a clear idea on the above discussion, let us have a look at the following figure. All these three Multivibrators are clearly discussed in the next chapters. Learning working make money
Pulse Circuits – Transistor as a Switch A transistor is used as an electronic switch by driving it either in saturation or in cut off. The region between these two is the linear region. A transistor works as a linear amplifier in this region. The Saturation and Cut off states are important consideration in this regard. ON & OFF States of a Transistor There are two main regions in the operation of a transistor which we can consider as ON and OFF states. They are saturation and cut off states. Let us have a look at the behavior of a transistor in those two states. Operation in Cut-off condition The following figure shows a transistor in cut-off region. When the base of the transistor is given negative, the transistor goes to cut off state. There is no collector current. Hence IC = 0. The voltage VCC applied at the collector, appears across the collector resistor RC. Therefore, VCE = VCC Operation in Saturation region The following figure shows a transistor in saturation region. When the base voltage is positive and transistor goes into saturation, IC flows through RC. Then VCC drops across RC. The output will be zero. $$I_C = I_{C(sat)} : = : frac{V_{CC}}{R_C} : and : V_{CE} = 0$$ Actually, this is the ideal condition. Practically, some leakage current flows. Hence we can understand that a transistor works as a switch when driven into saturation and cut off regions by applying positive and negative voltages to the base. The following figure gives a better explanation. Observe the dc load line that connects the IC and VCC. If the transistor is driven into saturation, IC flows completely and VCE = 0 which is indicated by the point A. If the transistor is driven into cut off, IC will be zero and VCE = VCC which is indicated by the point B. the line joining the saturation point A and cut off B is called as Load line. As the voltage applied here is dc, it is called as DC Load line. Practical Considerations Though the above-mentioned conditions are all convincing, there are a few practical limitations for such results to occur. During the Cut off state An ideal transistor has VCE = VCC and IC = 0. But in practice, a smaller leakage current flows through the collector. Hence IC will be a few μA. This is called as Collector Leakage Current which is of course, negligible. During the Saturation State An ideal transistor has VCE = 0 and IC = IC(sat). But in practice, VCE decreases to some value called knee voltage. When VCE decreases more than knee voltage, β decreases sharply. As IC = βIB this decreases the collector current. Hence that maximum current IC which maintains VCE at knee voltage, is known as Saturation Collector Current. Saturation Collector Current = $I_{C(sat)} : = : frac{V_{CC} – V_{knee}}{R_C}$ A Transistor which is fabricated only to make it work for switching purposes is called as Switching Transistor. This works either in Saturation or in Cut off region. While in saturation state, the collector saturation current flows through the load and while in cut off state, the collector leakage current flows through the load. Switching Action of a Transistor A Transistor has three regions of operation. To understand the efficiency of operation, the practical losses are to be considered. So let us try to get an idea on how efficiently a transistor works as a switch. During Cut off (OFF) state The Base current IB = 0 The Collector current IC = ICEO (collector lekeage current) Power Loss = Output Voltage × Output Current $$= V_{CC} times I_{CEO}$$ As ICEO is very small and VCC is also low, the loss will be of very low value. Hence, a transistor works as an efficient switch in OFF state. During Saturation (ON) state As discussed earlier, $$I_{C(sat)} = frac{V_{CC} – V_{knee}}{R_C}$$ The output voltage is Vknee. Power loss = Output Voltage × Output Current $$= :V_{knee} times I_{c(sat)}$$ As Vknee will be of small value, the loss is low. Hence, a transistor works as an efficient switch in ON state. During Active region The transistor lies between ON & OFF states. The transistor operates as a linear amplifier where small changes in input current cause large changes in the output current (ΔIC). Switching Times The Switching transistor has a pulse as an input and a pulse with few variations will be the output. There are a few terms that you should know regarding the timings of the switching output pulse. Let us go through them. Let the input pulse duration = T When the input pulse is applied the collector current takes some time to reach the steady state value, due to the stray capacitances. The following figure explains this concept. From the figure above, Time delay(td) − The time taken by the collector current to reach from its initial value to 10% of its final value is called as the Time Delay. Rise time(tr) − The time taken for the collector current to reach from 10% of its initial value to 90% of its final value is called as the Rise Time. Turn-on time (TON) − The sum of time delay (td) and rise time (tr) is called as Turn-on time. TON = td + tr Storage time (ts) − The time interval between the trailing edge of the input pulse to the 90% of the maximum value of the output, is called as the Storage time. Fall time (tf) − The time taken for the collector current to reach from 90% of its maximum value to 10% of its initial value is called as the Fall Time. Turn-off time (TOFF) − The sum of storage time (ts) and fall time (tf) is defined as the Turn-off time. TOFF = ts + tf Pulse Width(W) − The time duration of the output pulse measured between two 50% levels of rising and falling waveform is defined as Pulse Width. Learning
Pulse Circuits – Signal A Signal not only carries information but it also represents the condition of the circuit. The functioning of any circuit can be studies by the signal it produces. Hence, we will start this tutorial with a brief introduction to signals. Electronic Signal An electronic signal is similar to a normal signal we come across, which indicates something or which informs about something. The graphical representation of an electronic signal gives information regarding the periodical changes in the parameters such as amplitude or phase of the signal. It also provides information regarding the voltage, frequency, time period, etc. This representation brings some shape to the information conveyed or to the signal received. Such a shape of the signal when formed according to a certain variation, can be given different names, such as sinusoidal signal, triangular signal, saw tooth signal and square wave signal etc. These signals are mainly of two types named as Unidirectional and Bidirectional signals. Unidirectional Signal − The signal when flows only in one direction, which is either positive or negative, such a signal is termed as Unidirectional signal. Example − Pulse signal. Bidirectional Signal − The signal when alters in both positive and negative directions crossing the zero point, such a signal is termed as a Bidirectional signal. Example − Sinusoidal signal. In this chapter, we are going to discuss pulse signals and their characteristic features. Pulse Signal A Pulse shape is formed by a rapid or sudden transient change from a baseline value to a higher or lower level value, which returns to the same baseline value after a certain time period. Such a signal can be termed as Pulse Signal. The following illustration shows a series of pulses. A Pulse signal is a unidirectional, non-sinusoidal signal which is similar to a square signal but it is not symmetrical like a square wave. A series of continuous pulse signals is simply called as a pulse train. A train of pulses indicate a sudden high level and a sudden low level transition from a baseline level which can be understood as ON/OFF respectively. Hence a pulse signal indicates ON & OFF of the signal. If an electric switch is given a pulse input, it gets ON/OFF according to the pulse signal given. These switches which produce the pulse signals can be discussed later. Terms Related to Pulse signals There are few terms related to pulse signals which one should know. These can be understood with the help of the following figure. From the above figure, Pulse width − Length of the pulse Period of a waveform − Measurement from any point on one cycle to the same point on next cycle Duty cycle − Ratio of the pulse width to the period Rise time − Time it takes to rise from 10% to 90% of its maximum amplitude. Fall time − Time signal takes to fall from 90% to 10% of its maximum amplitude. Overshoot − Said to be occurred when leading edge of a waveform exceeds its normal maximum value. Undershoot − Said to be occurred when trailing edge of a waveform exceeds its normal maximum value. Ringing − Both undershoot and overshoot are followed by damped oscillations known as ringing. The damped oscillations are the signal variations that indicate the decreasing amplitude and frequency of the signal which are of no use and unwanted. These oscillations are simple disturbances known as ringing. In the next chapter, we will explain the concept of switching in electronics done using BJTs. We had already discussed switching using diodes in our ELECTRONIC CIRCUITS tutorial. Please refer. Learning working make money
Pulse Circuits – Quick Guide Pulse Circuits – Signal A Signal not only carries information but it also represents the condition of the circuit. The functioning of any circuit can be studies by the signal it produces. Hence, we will start this tutorial with a brief introduction to signals. Electronic Signal An electronic signal is similar to a normal signal we come across, which indicates something or which informs about something. The graphical representation of an electronic signal gives information regarding the periodical changes in the parameters such as amplitude or phase of the signal. It also provides information regarding the voltage, frequency, time period, etc. This representation brings some shape to the information conveyed or to the signal received. Such a shape of the signal when formed according to a certain variation, can be given different names, such as sinusoidal signal, triangular signal, saw tooth signal and square wave signal etc. These signals are mainly of two types named as Unidirectional and Bidirectional signals. Unidirectional Signal − The signal when flows only in one direction, which is either positive or negative, such a signal is termed as Unidirectional signal. Example − Pulse signal. Bidirectional Signal − The signal when alters in both positive and negative directions crossing the zero point, such a signal is termed as a Bidirectional signal. Example − Sinusoidal signal. In this chapter, we are going to discuss pulse signals and their characteristic features. Pulse Signal A Pulse shape is formed by a rapid or sudden transient change from a baseline value to a higher or lower level value, which returns to the same baseline value after a certain time period. Such a signal can be termed as Pulse Signal. The following illustration shows a series of pulses. A Pulse signal is a unidirectional, non-sinusoidal signal which is similar to a square signal but it is not symmetrical like a square wave. A series of continuous pulse signals is simply called as a pulse train. A train of pulses indicate a sudden high level and a sudden low level transition from a baseline level which can be understood as ON/OFF respectively. Hence a pulse signal indicates ON & OFF of the signal. If an electric switch is given a pulse input, it gets ON/OFF according to the pulse signal given. These switches which produce the pulse signals can be discussed later. Terms Related to Pulse signals There are few terms related to pulse signals which one should know. These can be understood with the help of the following figure. From the above figure, Pulse width − Length of the pulse Period of a waveform − Measurement from any point on one cycle to the same point on next cycle Duty cycle − Ratio of the pulse width to the period Rise time − Time it takes to rise from 10% to 90% of its maximum amplitude. Fall time − Time signal takes to fall from 90% to 10% of its maximum amplitude. Overshoot − Said to be occurred when leading edge of a waveform exceeds its normal maximum value. Undershoot − Said to be occurred when trailing edge of a waveform exceeds its normal maximum value. Ringing − Both undershoot and overshoot are followed by damped oscillations known as ringing. The damped oscillations are the signal variations that indicate the decreasing amplitude and frequency of the signal which are of no use and unwanted. These oscillations are simple disturbances known as ringing. In the next chapter, we will explain the concept of switching in electronics done using BJTs. We had already discussed switching using diodes in our ELECTRONIC CIRCUITS tutorial. Please refer. Pulse Circuits – Switch A Switch is a device that makes or breaks a circuit or a contact. As well, it can convert an analog data into digital data. The main requirements of a switch to be efficient are to be quick and to switch without sparking. The essential parts are a switch and its associated circuitry. There are three types of Switches. They are − Mechanical switches Electromechanical switches or Relays Electronic switches Mechanical Switches The Mechanical Switches are the older type switches, which we previously used. But they had been replaced by Electro-mechanical switches and later on by electronic switches also in a few applications, so as to get over the disadvantages of the former. The drawbacks of Mechanical Switches are as follows − They have high inertia which limits the speed of operation. They produce sparks while breaking the contact. Switch contacts are made heavy to carry larger currents. The mechanical switches look as in the figure below. These mechanical switches were replaced by electro-mechanical switches or relays that have good speed of operation and reduce sparking. Relays Electromechanical switches are also called as Relays. These switches are partially mechanical and partially electronic or electrical. These are greater in size than electronic switches and lesser in size than mechanical switches. Construction of a Relay A Relay is made such that the making of contact supplies power to the load. In the external circuit, we have load power supply for the load and coil power supply for controlling the relay operation. Internally, a lever is connected to the iron yoke with a hard spring to hold the lever up. A Solenoid is connected to the yoke with an operating coil wounded around it. This coil is connected with the coil power supply as mentioned. The figure below explains the construction and working of a Relay. Working of a Relay When the Switch is closed, an electrical path is established which energizes the solenoid. The lever is connected by a heavy spring which pulls up the lever and holds. The solenoid when gets energized, pulls the lever towards it, against the pulling force of the spring. When the lever gets pulled, the moving contact meets the fixed contact in order to connect the circuit. Thus the circuit connection is ON or established and the lamp glows indicating this. When the switch is
Unidirectional with More Inputs The unidirectional sampling gate circuits that we have discussed so far have a single input. In this chapter, let us discuss a few more unidirectional sampling gate circuits that can handle more than one input signals. A unidirectional sampling gate circuit consists of the capacitors and resistors of same value. Here two input unidirectional diode sampling gate with two inputs is considered. In this circuit we have two capacitors and two resistors of same value. They are connected with two diodes each. The control signal is applied at the resistors. The output is taken across the load resistor. The figure below shows the circuit diagram for unidirectional diode sampling gate with more than one input signal. When the control input is given, At VC = V1 which is during the transmission period, both the diodes D1 and D2 are forward biased. Now, the output will be the sum of all the three inputs. $$V_O = V_{S1} + V_{S2} + V_C$$ For V1 = 0v which is the ideal value, $$V_O = V_{S1} + V_{S2}$$ Here we have a major limitation that at any instant of time, during the transmission period, only one input should be applied. This is a disadvantage of this circuit. During the non-transmission period, $$V_C = V_2$$ Both the diodes will be in reverse bias which means open circuited. This makes the output $$V_O = 0V$$ The main disadvantage of this circuit is that the loading of the circuit increases as the number of inputs increase. This limitation can be avoided by another circuit in which the control input is given after the input signal diodes. Pedestal Reduction While going through different types of sampling gates and the outputs they produce, we have come across an extra voltage level in the output waveforms called as Pedestal. This is unwanted and creates some noise. Reduction of Pedestal in a Gate circuit The difference in the output signals during transmission period and non-transmission period though the input signals is not applied, is called as Pedestal. It can be a positive or a negative pedestal. Hence it is the output observed because of the gating voltage though the input signal is absent. This is unwanted and has to be reduced. The circuit below is designed for the reduction of pedestal in a gate circuit. When the control signal is applied, during the transmission period i.e. at V1, Q1 turns ON and Q2 turns OFF and the VCC is applied through RC to Q1. Whereas during the nontransmission period i.e. at V2, Q2 turns ON and Q1 turns OFF and the VCC is applied through RC to Q2. The base voltages –VBB1 and –VBB2 and the amplitude of gate signals are adjusted so that two transistor currents are identical and as a result the quiescent output voltage level will remain constant. If the gate pulse voltage is large compared with the VBE of the transistors, then each transistor is biased far below cut off, when it is not conducting. So, when the gate voltage appears, Q2 will be driven into cut off before Q1 starts to conduct, whereas at the end of the gate, Q1 will be driven to cut off before Q2 starts to conduct. The figure below explains this in a better fashion. Hence the gate signals appear as in the above figure. The gated signal voltage will appear superimposed on this waveform. These spikes will be of negligible value if the gate waveform rise time is small compared with the gate duration. There are few drawbacks of this circuit such as Definite rise and fall times, result in sharp spikes The continuous current through RC dissipates lot of heat Two bias voltages and two control signal sources (complement to each other) make the circuit complicated. Other than these drawbacks, this circuit is useful in the reduction of pedestal in a gate circuit. Learning working make money
Pulse Circuits – Monostable Multivibrator A monostable multivibrator, as the name implies, has only one stable state. When the transistor conducts, the other remains in non-conducting state. A stable state is such a state where the transistor remains without being altered, unless disturbed by some external trigger pulse. As Monostable works on the same principle, it has another name called as One-shot Multivibrator. Construction of Monostable Multivibrator Two transistors 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. The base Q1 is connected to the collector of Q2 through the resistor R2 and capacitor C. Another dc supply voltage –VBB is given to the base of transistor Q1 through the resistor R3. The trigger pulse is given to the base of Q1 through the capacitor C2 to change its state. RL1 and RL2 are the load resistors of Q1 and Q2. One of the transistors, when gets into a stable state, an external trigger pulse is given to change its state. After changing its state, the transistor remains in this quasi-stable state or Meta-stable state for a specific time period, which is determined by the values of RC time constants and gets back to the previous stable state. The following figure shows the circuit diagram of a Monostable Multivibrator. Operation of Monostable Multivibrator Firstly, when the circuit is switched ON, transistor Q1 will be in OFF state and Q2 will be in ON state. This is the stable state. As Q1 is OFF, the collector voltage will be VCC at point A and hence C1 gets charged. A positive trigger pulse applied at the base of the transistor Q1 turns the transistor ON. This decreases the collector voltage, which turns OFF the transistor Q2. The capacitor C1 starts discharging at this point of time. As the positive voltage from the collector of transistor Q2 gets applied to transistor Q1, it remains in ON state. This is the quasi-stable state or Meta-stable state. The transistor Q2 remains in OFF state, until the capacitor C1 discharges completely. After this, the transistor Q2 turns ON with the voltage applied through the capacitor discharge. This turn ON the transistor Q1, which is the previous stable state. Output Waveforms The output waveforms at the collectors of Q1 and Q2 along with the trigger input given at the base of Q1 are shown in the following figures. The width of this output pulse depends upon the RC time constant. Hence it depends on the values of R1C1. The duration of pulse is given by $$T = 0.69R_1 C_1$$ The trigger input given will be of very short duration, just to initiate the action. This triggers the circuit to change its state from Stable state to Quasi-stable or Meta-stable or Semi-stable state, in which the circuit remains for a short duration. There will be one output pulse for one trigger pulse. Advantages The advantages of Monostable Multivibrator are as follows − One trigger pulse is enough. Circuit design is simple Inexpensive Disadvantages The major drawback of using a monostable multivibrator is that the time between the applications of trigger pulse T has to be greater than the RC time constant of the circuit. Applications Monostable Multivibrators are used in applications such as television circuits and control system circuits. Learning working make money
Bootstrap Time Base Generator A bootstrap sweep generator is a time base generator circuit whose output is fed back to the input through the feedback. This will increase or decrease the input impedance of the circuit. This process of bootstrapping is used to achieve constant charging current. Construction of Bootstrap Time Base Generator The boot strap time base generator circuit consists of two transistors, Q1 which acts as a switch and Q2 which acts as an emitter follower. The transistor Q1 is connected using an input capacitor CB at its base and a resistor RB through VCC. The collector of the transistor Q1 is connected to the base of the transistor Q2. The collector of Q2 is connected to VCC while its emitter is provided with a resistor RE across which the output is taken. A diode D is taken whose anode is connected to VCC while cathode is connected to the capacitor C2 which is connected to the output. The cathode of diode D is also connected to a resistor R which is in turn connected to a capacitor C1. This C1 and R are connected through the base of Q2 and collector of Q1. The voltage that appears across the capacitor C1 provides the output voltage Vo. The following figure explains the construction of the boot strap time base generator. Operation of Bootstrap Time Base Generator Before the application of gating waveform at t = 0, as the transistor gets enough base drive from VCC through RB, Q1 is ON and Q2 is OFF. The capacitor C2 charges to VCC through the diode D. Then a negative trigger pulse from the gating waveform of a Monostable Multivibrator is applied at the base of Q1 which turns Q1 OFF. The capacitor C2 now discharges and the capacitor C1 charges through the resistor R. As the capacitor C2 has large value of capacitance, its voltage levels (charge and discharge) vary at a slower rate. Hence it discharges slowly and maintains a nearly constant value during the ramp generation at the output of Q2. During the ramp time, the diode D is reverse biased. The capacitor C2 provides a small current IC1 for the capacitor C1 to charge. As the capacitance value is high, though it provides current, it doesn’t make much difference in its charge. When Q1 gets ON at the end of ramp time, C1 discharges rapidly to its initial value. This voltage appears across VO. Consequently, the diode D gets forward biased again and the capacitor C2 gets a pulse of current to recover its small charge lost during the charging of C1. Now, the circuit is ready to produce another ramp output. The capacitor C2 which helps in providing some feedback current to the capacitor C1 acts as a boot strapping capacitor that provides constant current. Output Waveforms The output waveforms are obtained as shown in the following figure. The pulse given at the input and the voltage VC1 which denotes the charging and discharging of the capacitor C1 which contributes the output are shown in the figure above. Advantage The main advantage of this boot strap ramp generator is that the output voltage ramp is very linear and the ramp amplitude reaches the supply voltage level. Learning working make money
Types of Time Base Generators As we have an idea that there are two types of time base generators, let us try to know about the basic circuits of those time base generator circuits. Voltage Time base Generator A time base generator that provides an output voltage waveform that varies linearly with time is called as a Voltage Time base Generator. Let us try to understand the basic voltage time base generator. A Simple Voltage Time base Generator A basic simple RC time base generator or a Ramp generator or a sweep circuit consists of a capacitor C which charges through VCC via a series connected resistor R2. It contains a BJT whose base is connected through the resistor R1. The capacitor charges through the resistor and discharges through the transistor. The following figure shows a simple RC sweep circuit. By the application of a positive going voltage pulse, the transistor Q turns ON to saturation and the capacitor rapidly discharges through Q and R1 to VCE (sat). When the input pulse ends, Q switches OFF and the capacitor C starts charging and continues to charge until the next input pulse. This process repeats as shown in the waveform below. When the transistor turns ON it provides a low resistance path for the capacitor to discharge quickly. When the transistor is in OFF condition, the capacitor will charge exponentially to the supply voltage VCC, according to the equation $$V_0 = V_{CC}[1 – exp(-t/RC)]$$ Where VO = instantaneous voltage across the capacitor at time t VCC = supply voltage t = time taken R = value of series resistor C = value of the capacitor Let us now try to know about different types of time base generators. The circuit just we had discussed, is a voltage time base generator circuit as it offers the output in the form of voltage. Current Time base Generator A time base generator that provides an output current waveform that varies linearly with time is called as a Current Time base Generator. Let us try to understand the basic current time base generator. A Simple Current Time base Generator A basic simple RC time base generator or a Ramp generator or a sweep circuit consists of a common-base configuration transistor and two resistors, having one in emitter and another in collector. The VCC is given to the collector of the transistor. The circuit diagram of a basic ramp current generator is as shown here under. A transistor connected in common-base configuration has its collector current vary linearly with its emitter current. When the emitter current is held constant, the collector current also will be near constant value, except for very smaller values of collector base voltages. As the input voltage Vi is applied at the base of the transistor, it appears at the emitter which produces the emitter current iE and this increases linearly as Vi increase from zero to its peak value. The collector current increases as the emitter current increases, because iC is closely equal to iE. The instantaneous value of load current is $$i_L i_C thickapprox (v_i – V_{BE})/R_E$$ The input and output waveforms are as shown below. Learning working make money
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