Understanding UJT and Waveform Generation

A Unijunction Transistor (UJT) is a three-terminal semiconductor device with a unique characteristic that makes it ideal for certain applications, particularly in oscillator and timing circuits. It has one PN junction and behaves differently from a standard bipolar junction transistor (BJT) or a field-effect transistor (FET).

The UJT has three terminals: an emitter (E) and two bases (Base 1, B1, and Base 2, B2). The main application where the UJT excels is in generating non-sinusoidal waveforms.

UJT as a Relaxation Oscillator

The most common use of a UJT is in a relaxation oscillator circuit. This circuit typically consists of a UJT, a capacitor, and a resistor (RC circuit). The UJT acts as a voltage-controlled switch that rapidly discharges the capacitor when the voltage across it reaches a certain level.

Here's how a basic UJT relaxation oscillator works:

• A capacitor (C) is charged through a resistor (R) from a DC voltage source.
• The voltage across the capacitor increases exponentially.
• The UJT's emitter junction remains reverse-biased (or slightly forward-biased but with very high resistance) while the capacitor voltage is below the UJT's peak voltage ($V_P$).
• When the capacitor voltage reaches the peak voltage ($V_P$), the UJT's emitter junction becomes forward-biased, and the resistance between the emitter and Base 1 ($R_{E-B1}$) drops dramatically.
• This low resistance path allows the capacitor to discharge rapidly through the emitter and Base 1.
• As the capacitor voltage drops below a certain level (the valley voltage, $V_V$), the UJT turns off again, and its emitter-Base 1 resistance returns to a high value.
• The capacitor then begins to charge again, and the cycle repeats.

Waveform Produced by UJT Relaxation Oscillator

Let's look at the voltage waveform across the capacitor in this circuit:

• During the charging phase, the capacitor voltage rises relatively slowly (exponentially) as it charges through the resistor R.
• During the discharging phase, the capacitor voltage drops very quickly as it discharges through the low resistance of the UJT's emitter-Base 1 path when the UJT is ON.

This pattern of a relatively slow charge followed by a rapid discharge creates a voltage waveform across the capacitor that resembles a sawtooth. The voltage rises linearly (approximately, especially if the charging time is much shorter than the RC time constant or if R is large) and then drops sharply.

Therefore, the primary waveform generated by a UJT in a relaxation oscillator circuit is a sawtooth wave.

Analyzing the Options

Let's consider the other options provided:

• Sine wave: Sine waves are typically generated by linear oscillators like Wien bridge oscillators or phase-shift oscillators, or by using resonant LC circuits. A UJT relaxation oscillator produces a non-sinusoidal, sharp rise and fall waveform.
• Square wave: Square waves have abrupt transitions between two distinct voltage levels and spend equal time at each level (for a perfect square wave). While UJTs can be part of circuits that produce pulse-like waveforms, the direct voltage across the capacitor in a standard UJT relaxation oscillator is not a square wave. Multivibrators (like those using transistors or op-amps) are common square wave generators.
• Triangle wave: Triangle waves rise and fall linearly and symmetrically. While similar to a sawtooth, the discharge phase in a standard UJT relaxation oscillator is much faster than the charge phase, making it distinctly a sawtooth, not a triangle wave. Triangle waves are often generated by integrating a square wave.
• Sawtooth wave: As explained above, the charging and rapid discharging action in a UJT relaxation oscillator directly generates a sawtooth voltage waveform across the capacitor.

Based on the operation of the standard UJT relaxation oscillator circuit, the output waveform generated across the capacitor is a sawtooth wave.

Revision Table: UJT Waveform Generation

Additional Information on UJT Relaxation Oscillators

The frequency of the sawtooth wave generated by the UJT relaxation oscillator can be controlled by changing the values of R and C. The approximate frequency (f) can be calculated using the formula:

where $\eta$ (eta) is the intrinsic stand-off ratio of the UJT, a parameter determined by the internal construction of the specific UJT. The intrinsic stand-off ratio ($\eta$) dictates the peak voltage ($V_P$) relative to the supply voltage ($V_{BB}$). $V_P = \eta V_{BB} + V_D$, where $V_D$ is the voltage drop across the emitter junction (typically around 0.7V).

The UJT's characteristic curve exhibits a negative resistance region between the peak point and the valley point, which is crucial for the oscillation to occur. This negative resistance allows the device to switch rapidly from a high-resistance state (OFF) to a low-resistance state (ON).

Besides relaxation oscillators for generating sawtooth waves, UJTs are also used in timing circuits, triggering silicon-controlled rectifiers (SCRs) and triacs, and in pulse generation.