UJT Relaxation Oscillator

Definition

A UJT relaxation oscillator is an electronic circuit that utilizes a Unijunction Transistor (UJT) and an RC (Resistor-Capacitor) network to generate periodic, non-sinusoidal waveforms, such as sawtooth waves and sharp trigger pulses, without requiring an external input signal.

Main Content

1. Unijunction Transistor Structure and Symbols

Basic Construction: The Unijunction Transistor (UJT) consists of a lightly doped N-type silicon bar with a heavily doped P-type material alloyed onto its side. This structure creates a single PN junction, which distinguishes it from standard Bipolar Junction Transistors (BJTs) that have two junctions.
Terminal Configuration: The UJT features three terminals: Base 1 ($B_1$) at the bottom, Base 2 ($B_2$) at the top, and the Emitter ($E$) connected to the P-type region along the side of the channel.

       UJT Schematic Symbol:

             Base 2 (B2)
                 |
                 |
                / \
         E ---->   \
       (Emitter)    \
                 |
                 |
             Base 1 (B1)

2. Intrinsic Standoff Ratio

Internal Resistance Equivalent: Internally, the N-type channel behaves as two series resistors, $R_{B1}$ (resistance between Emitter and Base 1) and $R_{B2}$ (resistance between Emitter and Base 2). The total inter-base resistance is $R_{BB} = R_{B1} + R_{B2}$.
The Standoff Ratio ($\eta$): The intrinsic standoff ratio, denoted by the Greek letter eta ($\eta$), represents the voltage divider ratio inside the device when the emitter is open-circuited. It is mathematically expressed as: $\eta = \frac{R_{B1}}{R_{B1} + R_{B2}}$ This ratio typically ranges between 0.5 and 0.8. It determines the peak trigger voltage ($V_P$) required to turn the UJT ON, calculated using the formula: $V_P = \eta V_{BB} + V_D$ (where $V_{BB}$ is the applied inter-base voltage and $V_D$ is the forward voltage drop of the emitter diode, roughly 0.7V).

3. Oscillator Circuit Setup

Timing Component Configuration: The timing network of the oscillator is formed by a charging resistor ($R$) and a capacitor ($C$). The resistor $R$ is connected between the positive supply voltage ($V_{BB}$) and the Emitter ($E$), while the capacitor $C$ is connected from the Emitter to the ground.
Base Resistors: Two external stabilization resistors, $R_1$ and $R_2$, are connected in series with Base 1 and Base 2 respectively. $R_1$ is used to extract the output pulse wave, while $R_2$ provides temperature compensation.

               Complete Circuit Diagram:

                     +V_BB (Supply Voltage)
                       |
                       +-----------+---------+
                       |           |         |
                      [R]         [R2]       |
                       |           |         |
                       |          (B2)       |
                       +---------( UJT )     |
                       |          (B1)       |
                      === [C]      |         |
                      / \         [R1]       |
                       |           |         |
                       +-----------+---------+
                       |           |
                      GND         GND

Working / Process

1. Capacitor Charging Phase

Initial State: When the power supply $V_{BB}$ is turned on, the capacitor $C$ begins to charge exponentially from $0\text{ V}$ through the charging resistor $R$. At this stage, the UJT's emitter-to-base diode is reverse-biased because the emitter voltage ($V_E$) is lower than the internal peak voltage ($V_P$).
High Impedance Condition: Since the UJT remains in its OFF state, it acts as an open switch with extremely high resistance. Virtually no current flows from the emitter to Base 1, allowing the capacitor to charge undisturbed with a time constant governed by $\tau = R \times C$.

2. Triggering and Rapid Discharge Phase

Reaching Peak Voltage: As charging continues, the voltage across the capacitor ($V_E$) eventually rises to reach the peak threshold voltage ($V_P$).
Conduction and Negative Resistance: Once $V_E \geq V_P$, the emitter-base PN junction becomes forward-biased. This injects holes into the N-channel region between the Emitter and Base 1, drastically reducing the resistance $R_{B1}$. This phenomenon is known as the negative resistance property. The UJT turns fully ON, presenting a very low resistance path.
Sudden Discharge: The capacitor $C$ rapidly discharges its stored energy through this low-resistance emitter-to-Base 1 ($B_1$) path. This sudden surge of discharge current passes through the external resistor $R_1$, generating a sharp positive-going voltage pulse at the $B_1$ terminal.

3. Reset Phase and Cycle Repetition

Reaching Valley Point: The capacitor continues to discharge until its voltage drops to a minimum level called the valley voltage ($V_V$).
UJT Turn-Off: At this point, the current flowing through the emitter drops below the holding current required to sustain conduction. The emitter junction becomes reverse-biased again, and the UJT returns to its highly resistive OFF state.
Initialization of Next Cycle: With the UJT turned off, the low-resistance path to ground is broken. The capacitor $C$ begins to charge again through resistor $R$ toward $V_{BB}$, restarting the entire cycle. This continuous cycle of slow charging and rapid discharging creates a stable, repeating oscillation.

                     Output Waveforms:

      Emitter Voltage V_E (Sawtooth Wave)
        ^
      Vp|    /|  /|  /|  /|
        |   / | / | / | / |
      Vv|  /  |/  |/  |/  |
        +-----------------------> Time (t)

      Base 1 Voltage V_B1 (Pulse Wave)
        ^
        |   _   _   _   _
        |  | | | | | | | |
      0 +--|_|-|_|-|_|-|_|------> Time (t)

Advantages / Applications

Simplicity and Low Cost: The oscillator requires very few components (one UJT, two resistors, and one capacitor) to generate highly stable waveforms, making it extremely cost-effective.
Excellent Temperature Stability: The frequency of the oscillations remains highly stable over a wide temperature range due to the built-in compensation properties of the UJT channel resistors.
Thyristor and SCR Triggering: The sharp, high-current pulses generated at Base 1 ($B_1$) are ideal for triggering Silicon Controlled Rectifiers (SCRs) and Triacs in power control circuits.
Time Delay and Sweep Generators: Due to its linear charging characteristic, it is frequently used to drive sweep circuits in analog oscilloscopes, television receivers, and industrial timers.

Summary

A UJT relaxation oscillator is a simple, highly efficient circuit that converts DC power into periodic non-sinusoidal waveforms using a single Unijunction Transistor. By utilizing an RC timing network and the unique negative resistance characteristic of the UJT, the circuit continuously cycles through a slow capacitor-charging phase and a rapid discharge phase. This process yields a linear sawtooth wave at the emitter terminal and sharp trigger pulses at the Base 1 terminal, making it a foundational tool for industrial timing and thyristor triggering systems.