MULTIVIBRATORS
Multivibrators are circuits suitable for supplying square, rectangular and impulsive waves. Made in various forms, with transistors, operational amplifiers, logic gates, integrated circuits, they are divided into astable, monostable and bistable:
- Astable Multivibrator: it is characterized by two states between which the multivibrator oscillates without the need for external commands.
The astable is a real generator of square and rectangular waves.
- Monostable : also called one-shot, this multivibrator has a stable state, in which it can remain indefinitely, and a quasi-stable state.
By means of an input command signal (trigger signal) it is possible to switch the monostable from the stable state to the quasi-stable state.
It can be used as a timer or as a delayer. - Bistable : this circuit, also called flip-flop, has two stable states in which it can remain indefinitely. The circuit passes from one state to another only following an external command.
The flip-flop is widely used as a memory cell and frequency divider. - Schmitt trigger : it is a particular bistable that switches from one state to another when the input voltage Vi exceeds the so-called upper threshold voltage V+.
It is used to square signals of various waveforms and as a threshold detector; it also forms the basis of many snap circuits.
ASTABLE MULTIVIBRATOR
This multivibrator has no stable state and is capable of providing an output signal (typically a square wave) without the aid of external commands. It is known as a square wave or clock generator . To trigger the oscillation, an RC (Resistor-Capacitor) network is needed which implement a known delay, so that the multivibrator can generate an oscillation. An astable multivibrator consists of two amplifier stages connected in a positive feedback loop by two capacitive-resistive coupling networks. The amplifying elements can be junction or field effect transistors, vacuum tubes, operational amplifiers, or other types of amplifiers. The circuit is usually drawn symmetrically as a crossed pair. The two output terminals can be defined on active devices and have complementary states. One has high voltage while the other has low voltage, except during brief transitions from one state to another.
SCHMITT TRIGGER
In order for an astable circuit to change its output logic state autonomously, it is necessary to supply two reference voltages before starting the “self-oscillation” process. The circuit par excellence capable of generating oscillations is the Schmitt trigger. The thresholds are defined by two voltages VT+ and VT– .
When the input signal reaches the threshold VT+ the output switches to the high logic level (+5V), vice versa when the input reaches the voltage VT– the output switches to the low logic level 0V.
In the figure above you can see a Schmitt trigger made with NAND gates configured as an inverter. By applying a time-varying signal to the input, we get a square waveform at the output. Before understanding how an astable multivibrator is made it is necessary to understand how a capacitor is charged through an RC circuit.
ASTABLE-CIRCUIT RC MULTIVIBRATOR
If the capacitor were connected directly to the 5V battery, the charge on it would be instantaneous, being connected via a resistance of a certain ohmic value, the charge will take place in the time te formulated with the expression T=RC (seconds) defined as a time constant of the RC circuit.
THE CHARGE OF THE CAPACITOR
Suppose that initially the capacitor is discharged, as soon as we connect the generator it will start to charge through the resistance R. At each successive instant the capacitor’s plates store more and more charges until it reaches the battery voltage. At this point there are two voltages that are equal and opposite in sign, so the current in the circuit will be I=0.
Two points are highlighted in the graph.
- when the charge on the capacitor has reached 63% which is equal to the time constant of the circuit T=RxC or we have:
VRC = 0.63 X VCC = 0.63×5 = 3.15V
- When the charge is worth 5RC i.e. it has reached 100%.
If the time constant of the circuit were 20ms, the capacitor would reach 63% of the charge after 20ms and 100% after 100ms.
CAPACITOR DISCHARGE
Suppose we remove the 5V generator and short-circuit the poles, in which case the capacitor discharge would occur through the resistance R. To change its state, a logic gate needs to reach the thresholds already described above. So using a triggered circuit and using, as we have seen, the charge and discharge on a capacitor, it is possible to make the circuit oscillate at a certain frequency without the aid of voltages of 0V and 5V. The essential thing is that the voltage of the capacitor can reach the two logical reference thresholds. This allows the circuit, suitably primed, to self-oscillate.
ASTABLE MULTIVIBRATOR-SQUARE WAVE GENERATOR
CIRCUIT OPERATION
Referring to the figure and the graph let’s consider the following: let’s assume that initially the LED is off, this means that the output of NAND logic gate 2 is at logic level 0. Its input will be at level 1 as well as the output of the NAND 1. It can be deduced that the capacitor begins to charge through the resistance R1 as it finds the opposite terminal connected to ground (0V) from the output of NAND 2 as level 0. Reached the recognition threshold (logic level 1) at the input of NAND 1, the led is turned on as the input of NAND 2 is at logic level 0, however in this new state the capacitor starts to discharge through R1 since the output of NAND 1 is at 0V potential. As soon as the voltage across the capacitor reaches the threshold recognized as the 0 VT- level, the states of the gates are changed again and the cycle repeats itself. Resistor R2 is part of the positive feedback circuit (Feedback means that the output is fed back into the input) necessary to trigger the spontaneous oscillation of the triggered astable multivibrator. The VCC voltage is used only and exclusively to put the led diode into conduction, allowing it to turn on.
DUTY CYCLE
The percentage value expresses the permanence at logic level 1 with respect to logic 0. In our case the Duty Cycle will be 50%.
BJT TRANSISTOR ASTABLE MULTIVIBRATOR
OPERATION
Neither state is stable, and the circuit continuously transitions from one state to another. The circuit therefore behaves like a particular relaxation oscillator, capable of producing square waves. Assume that in the circuit in the figure initially the transistor T1 conducts. The collector voltage is practically zero and C1 is charged across R2. When the potential between C1, R2, and the base of T2 reaches 0.6 V, T2 conducts, bringing the potential at its collector to zero. C2 starts charging causing interdiction of T1 and C1 discharges via R1-R2. In the new state C2 charges through R3 until the voltage reaches 0.6 V, whereupon T1 becomes conducting again, charging C1 and causing T2 to be cut off. C2 downloads via R3-R4. The cycle repeats indefinitely, with a period determined by the values of the resistors and capacitors. If the values of R2/C1 and R3/C2 differ, the on/off times of the two transistors are not symmetrical and it is thus possible to vary the duty cycle of the signal. The circuit can also be seen to be composed of two positive-feedback common-emitter amplifier stages.
Resistors can have, for example, the following values:
R2 = R3 = 22 kΩ
R1 = R4 = 470 ohms
The capacitors, depending on the required frequency, can have capacities from hundreds of pF to hundreds of uF.
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SCHMITT TRIGGER
The Schmitt trigger is used to convert an analog signal to a digital signal with specific voltage thresholds. The circuit includes two comparators or logic gates, resistors R1 and R2, and ground connections (GND1 and GND3). Here is a detailed explanation of operation:
Circuit Components:
-R1 and R2: These resistors form a voltage divider that sets the Schmitt trigger thresholds.
-GND1 and GND3: Ground or earth points of the circuit.
-INPUT (Input): Point where the analog signal is introduced into the circuit.
-OUTPUT (Output): Point where the digital signal is taken.
Operation:
1. INPUT SIGNAL: An analog signal is applied to the input. This signal may vary gradually or have noise.
2. First Comparator: The first comparator receives the signal through R1. This component has two inputs; one for the signal and one connected to a voltage reference (established by the voltage divider with R1 and R2).
3. Hysteresis: The Schmitt trigger uses hysteresis to prevent unwanted responses to noise or minor variations in the input signal. This means that the circuit has two voltage thresholds: one for on (when the signal goes from low to high) and one for off (when it goes from high to low). The thresholds are set so that the turn-on voltage is greater than the turn-off voltage.
4. Second Comparator: After the signal has been processed by the first comparator, it can be further conditioned by a second comparator, which ensures that the output signal is stable and clean (without noise or oscillation).
5. Output: The output signal is then a digital signal that changes state only when the input signal exceeds specific thresholds. The output can be connected to additional processing circuits or devices.
Applications:
The Schmitt trigger is commonly used in digital systems to:
-Clean up analog signals that may be affected by noise.
-Generate crisp digital signals from inaccurate analog inputs.
-Function as a basis for timers and oscillator circuits.
This type of circuit is very useful in applications where it is necessary to ensure that the transition from one signal level to another is sharp and accurate, eliminating ambiguities caused by small fluctuations in the input signal.
ASTABLE MULTIVIBRATOR
The circuit shows an implementation of an oscillator circuit using two NAND gates. Let’s go through step by step how this circuit works:
Circuit Components
1. Two NAND ports – Labeled as “1” and “2”.
2. Resistors – R1, R2, R3.
3. One capacitor – C1.
4. Power supply – 5V.
Circuit Operation.
The operation of this circuit is based on the feedback property and delay times created by the capacitor and resistors.
1. Oscillation start:
-When the circuit is energized, capacitor C1 starts discharged.
-Initially, we assume that both NAND gates have their inputs low (0), so their outputs will be high (1) due to the nature of the NAND gates (if all the inputs of a NAND gate are low, the output is high).
2. Capacitor Charging:
-Capacitor C1 begins to charge through R1. The charging time of the capacitor (time constant τ) depends on the value of R1 and C1.
-While C1 is charging, the input to NAND port 1 slowly changes from low to high.
3. State Change of NAND Port 1:
-When the input to NAND port 1 becomes high (due to capacitor C1 reaching sufficient voltage), the output of NAND port 1 becomes low (0).
4. Feedback and Oscillation:
-The low output of NAND port 1 is sent to the input of NAND port 2. This changes the output of NAND port 2 from high to low, since now both its inputs are low.
-The low output of NAND port 2 is returned to NAND port 1, keeping its input low.
5. Capacitor Discharge:
-With the output of NAND port 1 now low, capacitor C1 begins to discharge through R2 and the input of NAND port 1 slowly returns to low.
-This process reverses the NAND port outputs again when C1 fully discharges.
6. Repetition:
-This charge and discharge process repeats, generating a continuous oscillation.
Conclusion
The circuit thus functions as an astable oscillator, where capacitor C1 periodically charges and discharges, and this cycle causes the output states of the NAND gates to continuously change, generating an oscillating waveform at the output. This waveform can be used for various purposes, such as a clock signal in a digital circuit or to generate audio tones.
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