Question

Define a unijunction transistor (UJT) and its relaxation oscillator circuits.

Answer 1

A Unijunction Transistor (UJT) is a three-terminal semiconductor device that operates as a diode with a unique characteristic known as the negative resistance region. It is primarily used in relaxation oscillator circuits to generate repetitive waveforms like square waves, sawtooth waves, or pulse trains.

Here's a brief explanation of the UJT and relaxation oscillator circuits:

Unijunction Transistor (UJT):
A UJT is constructed with a single junction between its emitter and base regions, hence the name "unijunction." It has three terminals: emitter (E), base 1 (B1), and base 2 (B2). The device is made from a lightly doped N-type silicon material. The UJT's most notable characteristic is the negative resistance region in its current-voltage characteristic curve.

When a voltage is applied to the emitter terminal, the UJT acts like a diode with a relatively high forward voltage drop. As the voltage increases, the UJT remains in the "off" state. However, once a critical voltage known as the "intrinsic standoff voltage" (Vp) is reached, the UJT enters the negative resistance region, and its current starts to rapidly increase. This negative resistance behavior is exploited in relaxation oscillator circuits.

Relaxation Oscillator Circuits:
A relaxation oscillator is a circuit that generates repetitive waveforms without requiring an external input signal. It relies on the charging and discharging of a capacitor through a negative resistance device like a UJT. When the capacitor charges to a certain threshold voltage, it discharges through the UJT, and the cycle repeats.

A popular relaxation oscillator circuit using a UJT is the "UJT relaxation oscillator." Here's a basic schematic:

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           R1
     +----/\/\----+
     |             |
     |             |
  V+ |             |
     |             |
     |            _|_
     |          /   \ C
     |          \   /
     |          _|_|
     |             |
     +-------------+
           |
           R2
           |
          ---
          /// (UJT)
           |
          GND


Explanation:

V+: Supply voltage
R1: A resistor connected between V+ and the emitter terminal of the UJT
R2: A resistor connected between the base 1 terminal of the UJT and ground (GND)
C: A capacitor connected between base 1 and ground (GND)

The UJT relaxation oscillator works as follows:

Initially, the capacitor (C) is discharged, and the UJT is in its off state.
The capacitor (C) starts to charge through R1. As the voltage across C increases, the voltage at base 1 (VB1) also rises.
Once VB1 reaches the intrinsic standoff voltage (Vp) of the UJT, it enters the negative resistance region, causing a rapid discharge of the capacitor through R2 and the UJT.
The capacitor discharges, and the process repeats, generating a repetitive waveform at the output.

The frequency of the generated waveform depends on the values of R1, R2, and C in the circuit. By adjusting these components, you can control the frequency and shape of the output waveform.