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Unijunction Transistor

      The unijunction transistor(UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter . This trigger voltage is a fraction (n) of interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and characteristic curve are shown below.

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     The emitter terminal does not inject current into the base region until its voltage reaches Vp. Once Vp is reached the base circuit conducts and a postive pulse appears at the B1 terminal and a negative pulse at B2. The UJT incorporates a negative resistance region, a low emitter current, and a high output pulse current at terminals B1 and B2, making it an ideal pulse trigger. A simple RC timer circuit using a UJT is shown below.

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The very basic specifications of a UJT are:

(a) Vbb(max) - The maximum interbase voltage that can be applied to the UJT
(b) Rbb-the interbase resistance of the UJT
(c) n - The intrinsic standoff ratio which defines Vp.
(d) Ip - The peakpoint emitter current