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:
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:
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
Here are a few of our most requested UJTs
Programmable Unijunction Transistors (PUT) supplied by American Microsemiconductor
American Microsemiconductor produces a very broad line of standard unijunction transistors.
Many of the basic applications include:
oscillators, timers, sawtooth generators, SCR triggers, frequency dividers, stable voltage sensing,