Saturday, January 1, 2011

【 Weak current College 】 unijunction transistor structure and equivalent circuit 】

Unijunction transistor outline shown in Figure 1. In a low doped N-type Silicon rods on the use of diffusion process of forming a highly doped P, P and N district contacts form a pn junction, constituted a unijunction transistor (UJT). Its structure is shown in Figure 2 (a), the p-type semiconductor leads to emitter electrode; E N-type semiconductor leads at the ends of two electrodes, respectively base B1 and B2 base, B1 and B2 of the N-type region can be equivalent to a pure resistance, i.e. the RBB base resistance. The resistance of resistance as the emitter current. Unijunction transistor because there are two base, also known as double base transistor. The symbol shown in Figure 2 (b) below. Figure 1 unijunction transistors unijunction transistor outline of equivalent circuit shown in Figure 2 (c) below, the emitter are P and N-type Silicon rods forming of pn junction diode is equivalent to D; N-type Silicon rods for doping concentration is low and rendering high resistance, diodes and base B2 between equivalent resistance to rB2, diodes and base B1 between equivalent resistance to rB1; the resistance from rB1 E-B1 voltage control, so is equivalent to a variable resistor. Figure 2 structure of unijunction transistor schematics and equivalent circuit working principle and characteristic curve when b1-b2-VBB, between power supply and transmission drain, A base point voltage and current of b2:-η called unijunction transistor divider ratio, its numeric key and the structure of the pipe, usually in the 0.5 ~ 0.9. Figure 3 unijunction transistor characteristics curve of the test when e a b1 voltage Ueb1 zero or (Ueb1 < UA), diode under reverse voltage, the current Ie emitter to diode reverse current, the IEO. When Ueb1 increases, the PN forward voltage is greater than the opening of the voltage, the IE into forward current, e from the emitter streamwise base b1, at this point, the hole concentration in very high concentration to e P zone is low Silicon rods ofA-b1 district into non-equilibrium least child; semiconductor materials resistance and its carrier concentrations are closely related, the injected carrier to rb1 decrease; and rb1 's reduce, reduce the pressure drop, causing the PN forward voltage increases, IE with increases, the injected carrier will more, thus further reducing the rb1; when IE grows to a certain extent, the breakover voltage diode will change little, UEB1. Will the decrease and decrease the rb1, demonstrated negative resistance characteristics. Negative resistance characteristics: refers to the input voltage to a numeric value, the greater the input current, input equivalent resistance less features. Once a unijunction transistor enters the negative resistance of the working area, enter the current IE's increased only by the input circuit of external resistors limit, unless the input loop open or IE reduced to a small value, otherwise the tubes will always maintain the on-State. Unijunction transistor characteristics curve as in Figure 3 that when UEB1 increases to UP (peak-point voltage), PN began as a wizard, UP = UA + Uon; increased again, UEB1 pipes entered negative resistance area, with the IE increase, decrease, reduce the rb1 UEB1 until UEB1 = Uv (Valley point voltage). IE = IV Valley point current), IE then increases, the tubes into the saturated zone. Unijunction transistor has three work areas unijunction transistor negative resistance characteristics widely timing circuit and oscillation circuit. In addition, unijunction transistor with negative resistance characteristics of devices and tunnel diode, A bipolar transistor, negative resistance MOSFET, etc. Unijunction transistors each pin of unijunction transistor method to determine the emitter E method is: the multimeter in R * R * 100 retaining or 1K, black table amount received hypothetical emitter, red table amount received also the Poles, two low-resistance, the black table amount received is unijunction transistor emitter. Unijunction transistor B1 and B2 of the judgment by the multimeter in R * R * 100 retaining or 1K retaining, using the black table amount received emitter, red table pens respectively other poles, two measurements, the resistance of a red table amount received is B1. It should be noted that the aforementioned identification method B1, B2, not necessarily all of unijunction transistor is applicable, the individual pipes E--B1 forward between the resistance value is smaller. But accurately decide which polar B1 B2, which is great in the actual use is not particularly important. Even if B1, B2 used upside down, will not damage the pipe, affect only the amplitude of the output impulse (single junction transistor more pulse generator), when it finds the output pulse amplitude offset hours, as long as the original B2 B1, assumed the reversed over it. Unijunction transistor performance to what double base diodes or bad can measure its extreme resistance value between judged correctly. Multimeter R×1k stalls, the black table amount received emitter E, red table pen, in turn, received two base (B1 and B2), normally should have several thousand euro to 10 thousands of European resistance value. Then the red table amount received emitter E, black table pen loop 2 base, the normal resistance to infinity. Double base diode two base (B1 and B2), reactive resistance values are 2 to 10 k Ω, if measured a resistance value between the two poles of the described above is a large difference between the normal value, the diode is damaged. Application of unijunction transistors unijunction transistor with a large capacity of pulse and circuit is simple, so the application in a variety of switches, the timing circuit or trigger the SCR and so gained wide application. Its switching characteristics with high temperature stability, basically do not vary with temperature. Figure 4 shows the composition of unijunction transistor oscillator circuit. The so-called oscillation, means that there will be no input signal, the output of a certain frequency, the circuit must increase value of voltage or current signals. When the switch is powered on, the voltage of the capacitor C is zero on the tube to the globe, through a power resistor R to VBB C charge, grow over time voltage capacitor grows on uC; once UEB1 increases to peak-point voltage UP after the pipe into the negative resistance area that input equivalent resistance sharply reduced, making C input through the tube circuit quickly discharge, iE with rapidly decreased, when reduced to Valley point UEB1 voltage Uv tube cut-off after; capacitors began charging. The above process cycle trip, only when the power outage will stop, the resulting oscillations. ByThe charging time constants in lofty to discharge time constant, stable oscillation, the capacitor voltage waveform on fig. 4 (b) below. Figure 4 unijunction transistor composition of the oscillation circuit and waveform in order to improve reliability, in the use of the following issues: (1) in the second base B2 series 1 current limiting resistor R2, limit the UJT of peak power (2) a circuit of CT or VP (peak voltage) is large, CT should a protection resistor in series to protect the emitter B1 from electrical damage. For example: capacitance greater than 10 μ F CT or VP is greater than 30V series resistance should be appropriate, the additional resistance of resistance at the very least, should take each Micro-CT series 1 Ω resistor. Otherwise, the larger capacitor discharge current gradual damage UJT EB1 knot of the oscillator frequency or MONOSTABLE circuit timing width as time increases gradually change. (3) in some applications, using a diode and single-tube base B2 or emitter E-series, which can improve temperature stability and reduce the effects of changes in the power supply voltage (4) single-tube and Silicon controlled rectifier anti radiation characteristics of the poor, should not be used in radiation environments.

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