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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Unijunction transistors. Reference data
Encyclopedia of radio electronics and electrical engineering / Reference materials The article describes the device, the principle of operation and the use of unijunction transistors. A unijunction transistor or, as it is also called, a two-base diode, is a three-electrode semiconductor device with one pn junction. Its structure is conventionally shown in Fig. 1, a, a conventional graphic designation in the diagrams - in fig. 1b.
The basis of a unijunction transistor is a semiconductor crystal (for example, with n-type conductivity), called the base. At the ends of the crystal there are ohmic contacts B1 and BZ, between which there is a region that has a rectifying contact with a p-type semiconductor acting as an emitter. It is convenient to consider the principle of operation of a single-pass transistor using the simplest equivalent circuit (Fig. 1, c), where RB1 and RB2 - resistance between the corresponding terminals of the base and the emitter, and D1 is the emitter p-p junction. The current flowing through the resistances RB1 and RB2, creates a voltage drop on the first of them, biasing the diode D1 in the opposite direction. If the voltage at the emitter Ue is less than the voltage drop across the resistance RB1, diode D1 is closed, and only leakage current flows through it. When is the voltage UЭ becomes higher than the voltage across the resistance RB1, the diode begins to pass current in the forward direction. In this case, the resistance RB1 decreases, which leads to an increase in current in the circuit D1 RB1, and this, in turn, causes a further decrease in the resistance RB1. This process proceeds like an avalanche. resistance RB1 decreases faster than the current through the pn junction increases, as a result, a region of negative resistance appears on the current-voltage characteristic of the unijunction transistor (Fig. 2) (curve 1). With a further increase in current, the dependence of the resistance RB1 decreases from the current through the pn junction, and for values greater than a certain value ( Ioff), it does not depend on the current (saturation region).
With a decrease in the bias voltage Ucm, the current-voltage characteristic shifts to the left (curve 2) and, in its absence, turns into a characteristic of an open pn junction (curve 3).
The main parameters of unijunction transistors that characterize them as circuit elements are:
The equivalent of a unijunction transistor can be built from two ordinary transistors with different types of conduction, as shown in Fig. 3.
Here, the current flowing through the divider, consisting of resistors R1 and R2, creates a voltage drop on the second of them, which closes the emitter junction of transistor T1. With an increase in the voltage at the emitter, transistor T1 begins to pass current to the base of transistor T2, as a result of which it also opens. This leads to a decrease in voltage at the base of transistor T1, which, in turn, causes it to open even more, etc. In other words, the process of opening transistors in such a device also proceeds like an avalanche and the current-voltage characteristic of the device has a form similar to that of a unijunction transistor. Devices on unijunction transistors Unijunction transistors (two-base diodes) are widely used in various automation devices, pulse and measuring equipment - generators, threshold devices, frequency dividers, time relays, etc. One of the main types of devices based on unijunction transistors is a relaxation oscillator, the circuit of which is shown in fig. one.
When the power is turned on, the capacitor C1 is charged through the resistor R1. As soon as the voltage across the capacitor becomes equal to the turn-on voltage of the unijunction transistor T1, its emitter junction opens and the capacitor quickly discharges. As the capacitor discharges, the emitter current decreases and when it reaches a value equal to the turn-off current, the transistor closes, after which the process repeats again. As a result, short bipolar pulses appear on the bases B1 and B2, which are the output signals of the generator. The oscillation frequency f of the generator can be calculated using the approximate formula:
where R is the resistance of the resistor R1, Ohm; C-capacitance of the capacitor C1, F; η is the transfer coefficient of a unijunction transistor. For a given oscillation frequency, the capacitance of the capacitor should be chosen as large as possible in order to obtain a signal with the desired amplitude on the load (R2 or R3). An important advantage of a unijunction transistor generator is that the frequency of its oscillations slightly depends on the magnitude of the supply voltage. In practice, a change in voltage from 10 to 20 V leads to a change in frequency of only 0,5%. If, instead of resistor R1, a photodiode, photoresistor, thermistor, or other element is included in the charging circuit that changes its resistance under the influence of external factors (light, temperature, pressure, etc.), then the generator turns into an analog converter of the corresponding physical parameter into a pulse repetition rate . Slightly changing the scheme, as shown in Fig. 2, the same generator can be turned into a voltage comparison device. In this case, the base circuits of the transistor are connected to a reference voltage source, and the charging circuit is connected to the source under study. When the voltage of the latter exceeds the turn-on voltage, the device will begin to generate pulses of positive polarity. In the device, the diagram of which is shown in Fig. 3, the capacitor is charged through the resistor R4 and the resistance of the emitter-collector section of the bipolar transistor T1. Otherwise, the operation of this generator does not differ from that described earlier. The charging current, and, consequently, the frequency of the sawtooth voltage, taken in this case from the emitter of the unijunction transistor T2, is regulated by changing the bias voltage at the base of the transistor T1 using the tuning resistor R2. The deviation of the linearity of the form of vibrations generated by such a device does not exceed 1%
The moment of switching on the unijunction transistor can be controlled by applying a pulse of positive polarity to the emitter circuit or negative polarity to the B2 base circuit. The operation of the waiting multivibrator is based on this principle, the circuit of which is shown in Fig. 4. To obtain the desired mode of operation, the maximum voltage across the capacitor C1, which depends on the ratio of the resistances of the resistors of the divider R1R2, is set lower than the turn-on voltage of the transistor. The difference between these voltages is chosen taking into account possible interference in the trigger circuit, which can lead to false alarms of the device. When a pulse of negative polarity is applied to the B2 base circuit, the interbase voltage UB1B2 decreases (modulates), as a result, the transistor T1 opens and a pulse of positive polarity appears on the base of B1.
Unijunction transistors are also used in step-shaped voltage generators. A symmetrical (sinusoidal, rectangular, etc.) signal is fed to the input of such a device (see Fig. 5). With a positive half-wave of the signal, the capacitor C1 is charged through the resistor R2 and the resistance of the emitter-collector section of the transistor T1 to a certain voltage, much lower than the turn-on voltage of the unijunction transistor T2. During the action of the next positive half-wave, the voltage across the capacitor increases in steps by the same amount and so on until it becomes equal to the turn-on voltage of transistor T2. A stepped voltage is removed from its emitter. The operation of frequency dividers is based on the use of this principle. One stage on a unijunction transistor is capable of providing a division factor of up to 5. By combining several such devices into a single whole, you can get a divider with a much larger division factor. For an example in fig. 6 shows a diagram of a frequency divider by 100. The first stage of the device divides the frequency of the positive polarity pulses arriving at its input by 4, the other two by 5.
As can be seen from the diagram, the frequency divider stages differ from each other only in the resistances of the resistors in the charge circuits of capacitors C1-C3. The charge time constant of the capacitor C1 is determined by the resistors Rl, R2. R4 and R6; C2 - resistors R3. R4 and R6; C3-R5 and R6. When the power is turned on, capacitors C1-C3 begin to charge. The voltage pulses of positive polarity entering the input of the device are added to the voltage on the capacitor C1, and as soon as their sum reaches a value equal to the turn-on voltage, the unijunction transistor opens and the capacitor is discharged through its emitter junction. As a result, the voltage drop across resistors R4 and R6 increases abruptly, and this leads to a decrease in the interbase voltages of transistors T2 and T2. However, transistor T2 will open only when the voltage across capacitor CXNUMX becomes sufficient to turn it on at a reduced interbase voltage. The third stage of the divider works similarly.
The scheme of the time relay, which is characterized by very high efficiency, is shown in fig. 7. In the initial state, the DZ thyristor is closed, so the device practically does not consume energy (leakage currents are small and can be neglected). When a trigger pulse of positive polarity is applied to the control electrode, the thyristor opens. As a result, relay P1 is activated and with its contacts (conditionally not shown in the diagram) turns on the actuator. At the same time, capacitors C1 and C2 begin to charge through resistors R1 and R2. Since the resistance of the first of these resistors is many times greater than the second, capacitor C2 will be charged first, and when the voltage across capacitor C1 reaches the turn-on voltage, the unijunction transistor will open and capacitor C1 will discharge through its emitter junction. The pulse of positive polarity that has arisen at the same time on the resistor R2 will add up with the voltage on the capacitor C2, as a result of which the DZ thyristor will close and de-energize the relay R1 until the next trigger pulse arrives.
The device, the circuit of which is shown in Fig. 8, is designed for analog conversion of voltage into frequency. Here, transistor T2 is used in a relaxation oscillator, T1, together with resistors R1 and R2, is included in the charging circuit of capacitor C1. When the voltage at the base of the transistor T1 changes, the resistance of its emitter-collector section changes, and therefore, depending on the input voltage, the unijunction transistor T2 opens with a greater or lesser frequency. By the frequency of the pulses taken from the load resistor R3 in the base circuit B1, one can judge the voltage at the input of the device. Publication: cxem.net
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