FET Fundamentals Unit 6 – Unijunction Transistors
Exercise 1 – UJT Operating Characteristics
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate the operating
characteristics of a unijunction transistor by using a UJT test circuit. You will verify your results
with a multimeter and an oscilloscope.
DISCUSSION
- The diode in the equivalent circuit represents the PN junction.
- The interbase resistance ranges from 4.7 kΩ to 9.1 kΩ for this UJT.
- The emitter voltage (VE) must be able to overcome the junction voltage (VJ) and the diode
drop (VD) for emitter current to flow. - Emitter current flows from B1 to the emitter providing a regenerative (positive feedback)
effect. - The regenerative effect gives a UJT its unique negative resistance characteristic.
- No emitter current flows in the cutoff region.
- Once firing voltage is reached at the emitter terminal, emitter current starts to increase
(milliampere range). - Emitter voltage decreases because of the negative resistance effect between the emitter and
B1. - Beyond the saturation region, emitter current increases with emitter voltage and the
resistance effect becomes positive again.