Module 5: Specialized Components and Timing Techniques

1. The Unijunction Transistor (UJT)
2. The Unijunction Transistor, or UJT, is a unique three-terminal semiconductor device that is fundamentally different from both conventional diodes and bipolar junction transistors. It has only a single P-N junction, but its most defining characteristic is a region of negative resistance in its operating curve. This property, where an increase in current is accompanied by a decrease in voltage, makes the UJT exceptionally useful as an electronic switch in a variety of oscillator, timing, and trigger circuits.
3. Construction
4. The UJT is constructed from a lightly doped bar of n-type silicon. Ohmic contacts are made at either end of this bar, which are designated as Base 1 (B1) and Base 2 (B2). A single, heavily doped p-type region, called the Emitter (E), is formed on the side of the silicon bar, located physically closer to B2 than to B1. This forms the single P-N junction from which the device gets its name. The resistance of the silicon bar between B1 and B2 is known as the interbase resistance, RBB.
5. Working Principle (Using the Equivalent Circuit)
6. The operation of a UJT is best understood by visualizing its equivalent circuit, which consists of a diode connected to the junction of two series resistors, RB1 and RB2. The sum of these two internal resistances is the total interbase resistance (RBB = RB1 + RB2).
   1. Intrinsic Standoff Ratio (η): The two internal resistors, RB1 and RB2, form a voltage divider. The physical placement of the emitter junction determines the ratio of these resistances. This fixed ratio is a key parameter of the UJT called the intrinsic standoff ratio, designated by the Greek letter eta (η). It is defined as: η = RB1 / RBB.
   2. Initial OFF State: When a voltage (VBB) is applied across the two bases (B2 positive, B1 grounded), the voltage divider action establishes a voltage at the junction point between RB1 and RB2. This voltage is η * VBB. Initially, with no voltage applied to the emitter (VE = 0), the emitter P-N junction is reverse-biased. The UJT is in its OFF state, and only a very small leakage current flows.
   3. Firing the UJT – The Peak Voltage (VP): To turn the UJT ON, the emitter voltage VE must be increased. The UJT will remain OFF until VE becomes sufficiently positive to forward bias the emitter diode. This requires the emitter voltage to overcome both the internal voltage at the junction (η * VBB) and the forward voltage drop of the diode (VD, typically ~0.7V). The critical emitter voltage at which the UJT “fires” or turns ON is called the Peak Voltage (VP): VP = η * VBB + VD
   4. Negative Resistance Phenomenon: As soon as the emitter voltage reaches VP, the emitter diode becomes forward-biased, and a current IE begins to flow from the emitter into the base 1 region. This current consists of charge carriers (holes from the p-type emitter) which are injected into the n-type silicon bar. These additional charge carriers drastically increase the conductivity of the silicon region between the emitter and base 1, which in turn dramatically lowers the resistance of RB1. As the emitter current IE continues to increase, the resistance of RB1 continues to decrease. According to Ohm’s law, this falling resistance causes the emitter voltage VE to drop. This is the defining characteristic of the negative resistance region: increasing current leads to decreasing voltage.
   5. Saturation – The Valley Voltage (VV): This negative resistance behavior continues until the device reaches a point of saturation. The point at which the emitter voltage is at its minimum is called the Valley Voltage (VV), and the corresponding emitter current is the Valley Current (IV). Beyond this point, any further increase in emitter current will cause the device to behave like a normal forward-biased diode, and the emitter voltage will begin to increase again.
7. V-I Characteristics Curve
8. The V-I characteristic curve of a UJT clearly illustrates its unique behavior and is divided into three distinct regions:
   1. Cut-off Region: This is the region to the left of the peak point (VP, IP). Here, the emitter voltage is less than the peak voltage, the emitter junction is reverse-biased, and the UJT is effectively OFF.
   2. Negative Resistance Region: This is the region between the Peak Point (VP, IP) and the Valley Point (VV, IV). In this region, as the emitter current increases, the emitter voltage decreases. This is the unstable region where the UJT is used for switching and oscillation.
   3. Saturation Region: This is the region to the right of the valley point. Here, the device is fully ON and behaves like a saturated semiconductor device, where voltage increases with current.
9. Applications
10. The UJT’s unique switching characteristic makes it ideal for use as a relaxation oscillator and in phase control circuits. It is also widely used to provide clock pulses for digital circuits, in timing control applications, for controlled firing of thyristors (like SCRs), and to generate synchronizing pulses for deflection circuits in CROs.
11. The UJT as a Relaxation Oscillator
12. The UJT’s unique properties, particularly its well-defined firing voltage (VP) and its ability to rapidly discharge a capacitor through its negative resistance region, make it perfectly suited for creating a simple yet highly effective relaxation oscillator. This type of oscillator produces a non-sinusoidal waveform whose timing is determined by the charging and discharging of a capacitor.
13. Construction and Working
14. The UJT relaxation oscillator circuit is elegantly simple. It consists of a UJT, a resistor (R), and a capacitor (C). The RC components are connected in series from the supply voltage (VBB) to ground, with the UJT’s emitter connected to the junction of R and C. The output can be taken as a sawtooth wave across the capacitor or as sharp pulses across a small resistor placed in the B1 or B2 path.
15. The oscillation cycle proceeds in a repeating, step-by-step manner:
    1. Capacitor Charging: When power (VBB) is first applied, the UJT is in its OFF state. The capacitor C begins to charge exponentially through the resistor R towards the supply voltage VBB.
    2. Reaching the Firing Point: The voltage across the capacitor, Vc, rises according to the RC time constant. This Vc is also the emitter voltage VE of the UJT. This charging continues until Vc reaches the UJT’s predetermined peak voltage, VP = ηVBB + VD.
    3. UJT Fires: The moment Vc equals VP, the UJT’s emitter-base1 junction becomes forward-biased, and the device rapidly switches ON, entering its negative resistance region.
    4. Capacitor Discharging: The ON-state UJT provides a very low-resistance path from the emitter to base 1. The energy stored in the capacitor is now rapidly discharged through this low-resistance path. This discharge is much faster than the charging phase.
    5. UJT Turns OFF: The capacitor continues to discharge until its voltage drops to the UJT’s valley voltage (VV). At this point, the emitter junction is no longer sufficiently forward-biased to maintain conduction, and the UJT switches back to its OFF state.
    6. Cycle Repeats: With the UJT now OFF, the capacitor begins to charge through resistor R once again, and the entire cycle repeats itself. This continuous charge-discharge cycle generates a periodic sawtooth waveform across the capacitor.
16. Applications of Relaxation Oscillators
17. Relaxation oscillators are versatile and find use in a wide array of applications, including function generators, electronic beepers, switch-mode power supplies (SMPS), inverters, flashing light circuits (blinkers), and voltage-controlled oscillators (VCOs).
18. Synchronization and Frequency Division
19. In any complex electronic system that contains multiple waveform generators, it is often essential that they all operate in unison. Synchronization is the process of ensuring that these multiple generators reach a specific reference point in their cycle at precisely the same time. This alignment is crucial for the stable and predictable operation of the overall system.
20. Synchronization can occur in two primary ways:

• One-to-one basis: All generators are forced to operate at the exact same frequency, locked in phase with each other.
• Synchronization with frequency division: The generators operate at different frequencies that are integer multiples of each other (e.g., one generator runs at exactly half the frequency of another). They are still synchronized because they reach their reference points simultaneously, but one does so every cycle while the other does so every second (or nth) cycle.
• In relaxation devices like the UJT oscillator or other sweep circuits, synchronization is achieved by applying an external train of short pulses. These synchronizing pulses are typically applied to the base or emitter of the switching device (like the UJT). The purpose of the sync pulse is to prematurely trigger the discharge cycle of the generator. For instance, in a UJT oscillator, a sync pulse can add to the capacitor voltage, causing the total emitter voltage to reach the peak voltage VP before it would have naturally. This forces the UJT to fire and the capacitor to discharge in lock-step with the external pulse train.
• For synchronization to be successful, a necessary condition must be met: the period of the synchronizing pulse train (TP) must be less than the natural, free-running period of the generator (TO). This ensures that the pulse always arrives in time to terminate the charging cycle early.
• This principle directly leads to the concept of Frequency Division. If a sweep generator’s natural period TO is adjusted to be slightly longer than two times the sync pulse period TP, the first pulse that arrives after the UJT has fired will not be able to trigger it, because the capacitor voltage will still be too low. However, the second pulse will arrive when the capacitor has charged to a high enough voltage that the pulse can push it over the VP threshold. In this scenario, the sweep generator becomes synchronized to every second pulse of the input train. The output frequency of the sweep generator is now exactly half the frequency of the input pulse train. This technique can be extended to divide by any integer n, making the sweep circuit a simple and effective frequency divider.

1. Blocking Oscillators
2. A Blocking Oscillator is another type of relaxation oscillator, specifically designed to generate very narrow, high-power pulses. Its key distinguishing feature is the use of a pulse transformer to provide the regenerative feedback necessary for oscillation. The transformer’s properties are central to the circuit’s operation.
3. Monostable Blocking Oscillator (with Base Timing)

• Construction and Operation: This circuit is designed to have one stable state (transistor OFF) and requires an external trigger to produce a single output pulse. The transformer’s primary winding is in the collector circuit, and the secondary winding is in the base circuit. A negative trigger pulse applied to the collector initiates the process. Due to the transformer’s winding polarity, this causes a positive voltage to be induced in the secondary winding, which drives the transistor’s base positive. This begins a regenerative process that rapidly drives the transistor into saturation. The circuit remains in this state for a duration determined by the transformer’s characteristics and a timing resistor in the base circuit. Eventually, transformer core saturation or other effects cause the base current to decrease, which initiates a reverse regenerative process that cuts the transistor OFF, returning it to its stable state.
• Disadvantage: A major drawback of the base-timed configuration is that the output pulse width is highly unstable. It depends heavily on the transistor’s current gain (hFE), a parameter that is notoriously sensitive to changes in temperature and varies significantly between individual transistors.
• Astable Blocking Oscillator (Diode Controlled)
• Construction and Operation: This circuit is free-running and requires no external trigger (after an initial pulse to start the process). It includes a pulse transformer, a transistor, a capacitor, and a diode. The cycle begins when the transistor is turned ON. Transformer action provides feedback that reinforces this ON state. However, the transformer’s action also charges a capacitor, which eventually builds up a voltage that opposes the base current, cutting the transistor OFF. The transistor remains OFF while the capacitor discharges through a separate path. Once the capacitor has discharged sufficiently, the transistor is able to turn back ON, and the cycle repeats automatically. The diode plays a key role in steering the currents during the charge and discharge phases.
• Having covered various methods for generating signals, our final module will shift focus to the techniques for processing and controlling the transmission of these signals, introducing the concept of Sampling Gates.

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