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.
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.
A Transistor works as a switch in ON condition, when it is operated in saturation region. It works as a switch in OFF condition, when it is operated in cut off region. It works as an amplifier in linear region, which lies between transistor and cut off. To have an idea regarding these regions of operation, refer to the transistors chapter from BASIC ELECTRONICS tutorial.
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.
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.
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.
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: _circuits/index.htm
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
This concept depends upon the phenomenon called as Hysteresis. The transfer characteristics of electronic circuits exhibit a loop called as Hysteresis. It explains that the output values depends upon both the present and the past values of the input. This prevents unwanted frequency switching in Schmitt trigger circuits
After having discussed the fundamentals of pulse circuits, let us now go through different circuits that generate and deal with Saw tooth waves. A Saw tooth wave increases linearly with time and has a sudden decrease. This is also called as a Time base signal. Actually, this is the ideal output of a time base generator.
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.
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.
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.
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.
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. 59ce067264