What is a unijunction transistor (UJT) and its use in relaxation oscillators?
1 Answer
A Unijunction Transistor (UJT) is a type of semiconductor device that has three terminals: an emitter (E), a base 1 (B1), and a base 2 (B2). It is a two-layered, three-terminal device that works on the principle of negative resistance characteristics. UJTs are mainly used in pulse and timing circuits, particularly in relaxation oscillators.
Here's how a UJT relaxation oscillator works:
Basic UJT Structure: The UJT has a unique structure with a lightly doped n-type region (n-region) between two heavily doped p-type regions (p-regions). The n-region behaves like a variable resistor, and its resistance decreases as the voltage applied across the device increases.
Emitter Terminal (E): The emitter terminal is one of the p-regions. It emits minority carriers (holes) into the n-region.
Base 1 Terminal (B1): The base 1 terminal is the other p-region. It is connected to one end of a resistor (RB1).
Base 2 Terminal (B2): The base 2 terminal is connected to the other end of the resistor (RB1) and one end of a capacitor (C). The other end of the capacitor is connected to the emitter terminal.
Operation: When a voltage is applied between the emitter terminal (E) and the base 2 terminal (B2), the UJT remains in a high-resistance state. As the voltage increases, the UJT reaches a certain point (valley point) where the emitter current (IE) increases sharply. This is due to the voltage drop across the n-region reaching a critical level and causing the n-region to conduct heavily.
Relaxation Oscillation: The capacitor (C) and resistor (RB1) form a charging circuit, and when the UJT enters the low-resistance state, the capacitor starts charging through the resistor. As the capacitor charges, the voltage across it increases until it reaches a certain threshold, at which point the UJT switches back to the high-resistance state, discharging the capacitor. This cycle repeats, resulting in a sawtooth waveform at the base 2 terminal (B2).
Frequency Control: The frequency of the relaxation oscillator can be controlled by adjusting the values of the resistor (RB1) and the capacitor (C).
The sawtooth waveform produced by the UJT relaxation oscillator can be used for various applications, including triggering other circuits, generating time delays, or creating audio tones in electronic musical instruments.
Please note that UJTs have become less common in modern electronics due to the development of other semiconductor devices, such as operational amplifiers and programmable timers, which offer more precise and versatile timing functions.
Here's how a UJT relaxation oscillator works:
Basic UJT Structure: The UJT has a unique structure with a lightly doped n-type region (n-region) between two heavily doped p-type regions (p-regions). The n-region behaves like a variable resistor, and its resistance decreases as the voltage applied across the device increases.
Emitter Terminal (E): The emitter terminal is one of the p-regions. It emits minority carriers (holes) into the n-region.
Base 1 Terminal (B1): The base 1 terminal is the other p-region. It is connected to one end of a resistor (RB1).
Base 2 Terminal (B2): The base 2 terminal is connected to the other end of the resistor (RB1) and one end of a capacitor (C). The other end of the capacitor is connected to the emitter terminal.
Operation: When a voltage is applied between the emitter terminal (E) and the base 2 terminal (B2), the UJT remains in a high-resistance state. As the voltage increases, the UJT reaches a certain point (valley point) where the emitter current (IE) increases sharply. This is due to the voltage drop across the n-region reaching a critical level and causing the n-region to conduct heavily.
Relaxation Oscillation: The capacitor (C) and resistor (RB1) form a charging circuit, and when the UJT enters the low-resistance state, the capacitor starts charging through the resistor. As the capacitor charges, the voltage across it increases until it reaches a certain threshold, at which point the UJT switches back to the high-resistance state, discharging the capacitor. This cycle repeats, resulting in a sawtooth waveform at the base 2 terminal (B2).
Frequency Control: The frequency of the relaxation oscillator can be controlled by adjusting the values of the resistor (RB1) and the capacitor (C).
The sawtooth waveform produced by the UJT relaxation oscillator can be used for various applications, including triggering other circuits, generating time delays, or creating audio tones in electronic musical instruments.
Please note that UJTs have become less common in modern electronics due to the development of other semiconductor devices, such as operational amplifiers and programmable timers, which offer more precise and versatile timing functions.