FET Fundamentals Unit 6 – Unijunction Transistors
Exercise 2 – UJT Waveform Generation
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate the operation of a
relaxation oscillator by using a UJT circuit. You will verify your results with an oscilloscope.
DISCUSSION
- The UJT is configured as a relaxation oscillator.
- This relaxation oscillator produces a sawtooth waveform at E, a positive pulse at B1, and a
negative pulse at B2. - When C2 charges to the firing voltage (VP), the emitter current starts to flow. At this point,
C2 discharges through E, B1 and R3. Since R1 is much larger than R3 plus RB1, the
discharge time of C2 is shorter than the charge time. - The energy discharged from C2 produces the waveforms.
- The charge time (τ) controls the frequency of the waveforms.
This equation is used: f = 1 / τ = 1 / (R1 x C2)