6/8/2023 0 Comments Solve elec osclillateur![]() The emitter current corresponding to valley point is denoted as Iv and the corresponding emitter voltage is denoted as Vv. This negative resistance region in the UJT characteristics is employed in relaxation oscillators.Īt last the emitter current Ie will be increased to a point that no more increase in conductivity is possible. In the graph (Fig:2) you can see that the regions between Vp (peak voltage point) and Vv (valley voltage) have a negative slope. This situation is similar to a negative resistance scenario. This results in a condition where emitter current Ie increases and the emitter voltage Ve decreases. This increased conductivity reduces RB1 and η. This phenomenon of increasing conductivity by the insertion of holes is called conductivity modulation. As a result, the region between emitter(E) and B1 terminal starts saturating by holes and the conductivity of this region starts to increase. These holes are repelled by B2 and attracted by B1. ![]() At this point a considerable amount of emitter current (Ie) flows and a significant number of holes are injected into the junction. But the Ve can be only increased up to a particular point called Vp (peak voltage). This minimum value of the emitter voltage Ve for which the emitter current starts to flow is called the firing voltage of UJT.Īs the Ve is increased the emitter current Ie is also increased and the junction behaves like a typical P-N junction. When this factor is considered, the equation can be re written as This condition can be expressed using the following equation.Ĭonsidering the intrinsic stand off ratio η= RB1/(RB1+RB2), the equation becomesĪ typical silicon diode has a forward voltage drop of 0.7V. Current starts flowing into the emitter only when the bias voltage Ve has exceeded the forward drop of the internal diode (Vd) plus the voltage drop across RB1 (Vrb1). Emitter terminal E of the UJT is forward biased using the voltage source Ve. B2 terminal of the UJT is made positive with respect to B1 terminal using the voltage source Vbb. UJT characteristics and circuit arrangementįor ease of understanding, the internal model of the UJT is used in the circuit (Fig 1). The typical range of intrinsic standoff ratio is from 0.4 to 0.8. It can be expressed as η = RB1/(RB1+RB2) or η = RB1/R BBO. Intrinsic standoff ratio (η) : It is the ratio of RB1 to the sum of RB1 and RB2. ![]() If RB1 is the resistance of the bar from E to B1 and RB2 is the resistance of the bar from E to B2, then R BBO can be expressed as R BBO= RB1 +RB2. The typical range of R BBO is from 4KΩ to 10KΩ. In simple words, it is the resistance of the N-Type bar when measured lengthwise. R BBO : It is the resistance between the terminals B1 and B2. The diode symbol shown in the internal circuit model represents the P-N junction formed between the heavily doped P-region (E) and the lightly doped N-Type bar. The internal block diagram, simplified internal circuit model and circuit symbol of a UJT is given in the figure below. Resistance between E & B1 is higher than the resistance between E & B2 because E is constructed close to B2. This heavily doped P region is called emitter and it is designated as E. A heavily doped P-type region is constructed on one side of the bar close to the B2 region. Since the silicon bar is lightly doped, the resistance between B1 and B2 is very high (typically 5 to 10 KΩ). These end terminals are called B1 and B2. The base material for a UJT is a lightly doped N-Type Silicon bar with ohmic contacts given at the lengthwise ends. The UJT has three terminals designated B1, B2 and E. UJT (uni junction transistor)įrom the name itself, the UJT or uni junction transistor is a semiconductor device that has only one junction. Before going into the details of UJT relaxation oscillator let’s have a look at the uni junction transistor (UJT). The UJT relaxation oscillator is called so because the timing interval is set up by the charging of a capacitor and the timing interval is ceased by the the rapid discharge of the same capacitor. It has a negative resistance region in the characteristics and can be easily employed in relaxation oscillators. UJT is an excellent switch with switching times in the order of nano seconds. UJT relaxation oscillator is a type of RC ( resistor-capacitor) oscillator where the active element is a UJT (uni-junction transistor).
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