Describe the operation of a relaxation oscillator using a unijunction transistor.
1 Answer
A relaxation oscillator is an electronic circuit that generates a periodic output waveform without the need for an external clock signal. One type of relaxation oscillator can be constructed using a unijunction transistor (UJT). The UJT relaxation oscillator is based on the charging and discharging of a capacitor through the emitter and base terminals of the UJT.
Here's a step-by-step description of how a relaxation oscillator using a unijunction transistor operates:
Components: The basic components of a UJT relaxation oscillator include a unijunction transistor (UJT), a resistor (R), a capacitor (C), and a power supply (Vcc).
Unijunction Transistor (UJT): A unijunction transistor is a three-terminal semiconductor device with two base terminals (B1 and B2) and one emitter terminal (E). It is a type of thyristor and has a unique characteristic that makes it suitable for relaxation oscillator circuits.
Charging Phase:
Initially, the capacitor (C) is discharged and has zero voltage across it.
The UJT is biased by a positive supply voltage (Vcc) connected between the emitter terminal (E) and one of the base terminals (B1).
As Vcc is applied, the UJT remains in its off-state, acting as an open switch between its emitter and B2 base terminal.
Triggering:
The voltage across the capacitor (C) starts to rise gradually through the resistor (R) connected between the B2 terminal and the capacitor.
As the voltage across the capacitor increases, it eventually reaches a point where it forward-biases the B2 terminal of the UJT.
This triggers the UJT to enter its negative resistance region, effectively turning it on and allowing current to flow from the emitter (E) to the B1 base terminal.
Discharging Phase:
When the UJT turns on, it provides a low-resistance path for the discharge of the capacitor (C) through the emitter terminal (E) and B1 base terminal.
The voltage across the capacitor rapidly decreases as it discharges through the UJT, causing the UJT to exit its negative resistance region and turn off.
Repetition:
With the UJT turned off, the capacitor continues to charge again, and the process repeats.
The cycle of charging, triggering, discharging, and turning off repeats at a rate determined by the values of the resistor (R), capacitor (C), and the UJT's inherent characteristics.
Output Signal:
The voltage across the capacitor (C) serves as the output signal of the relaxation oscillator.
This voltage waveform is typically a sawtooth-like shape, characterized by a gradual rise during the charging phase and a rapid drop during the discharging phase.
In summary, a relaxation oscillator using a unijunction transistor generates a periodic output waveform by alternately charging and discharging a capacitor through the emitter and base terminals of the UJT. The UJT's unique negative resistance characteristic and the timing components (resistor and capacitor) determine the frequency of oscillation.
Here's a step-by-step description of how a relaxation oscillator using a unijunction transistor operates:
Components: The basic components of a UJT relaxation oscillator include a unijunction transistor (UJT), a resistor (R), a capacitor (C), and a power supply (Vcc).
Unijunction Transistor (UJT): A unijunction transistor is a three-terminal semiconductor device with two base terminals (B1 and B2) and one emitter terminal (E). It is a type of thyristor and has a unique characteristic that makes it suitable for relaxation oscillator circuits.
Charging Phase:
Initially, the capacitor (C) is discharged and has zero voltage across it.
The UJT is biased by a positive supply voltage (Vcc) connected between the emitter terminal (E) and one of the base terminals (B1).
As Vcc is applied, the UJT remains in its off-state, acting as an open switch between its emitter and B2 base terminal.
Triggering:
The voltage across the capacitor (C) starts to rise gradually through the resistor (R) connected between the B2 terminal and the capacitor.
As the voltage across the capacitor increases, it eventually reaches a point where it forward-biases the B2 terminal of the UJT.
This triggers the UJT to enter its negative resistance region, effectively turning it on and allowing current to flow from the emitter (E) to the B1 base terminal.
Discharging Phase:
When the UJT turns on, it provides a low-resistance path for the discharge of the capacitor (C) through the emitter terminal (E) and B1 base terminal.
The voltage across the capacitor rapidly decreases as it discharges through the UJT, causing the UJT to exit its negative resistance region and turn off.
Repetition:
With the UJT turned off, the capacitor continues to charge again, and the process repeats.
The cycle of charging, triggering, discharging, and turning off repeats at a rate determined by the values of the resistor (R), capacitor (C), and the UJT's inherent characteristics.
Output Signal:
The voltage across the capacitor (C) serves as the output signal of the relaxation oscillator.
This voltage waveform is typically a sawtooth-like shape, characterized by a gradual rise during the charging phase and a rapid drop during the discharging phase.
In summary, a relaxation oscillator using a unijunction transistor generates a periodic output waveform by alternately charging and discharging a capacitor through the emitter and base terminals of the UJT. The UJT's unique negative resistance characteristic and the timing components (resistor and capacitor) determine the frequency of oscillation.