FET Fundamentals Unit 6 – Unijunction Transistors
This is a UJT relaxation oscillatorcircuit. A UJT is a nonlinear device that typically switches
between on and off states as the voltage at E increases and decreases with the charging and
discharging of the timing capacitor (C2).
The UJT is used for timing, triggering, sensing, and wave-form generation circuits such as a
relaxation oscillator.
NEW TERMS AND WORDS
emitter current - the base 1 to emitter current that flows after the UJT is fired (conducting).
negative resistance - the unique characteristic of a UJT in which the base 1 resistance and
voltage decrease as the base 1 current increases.
intrinsic standoff ratio - a ratio used to determine the firing voltage (Vp) of a UJT; represented
by eta.
relaxation oscillator - an oscillator that has positive feedback and that switches between off and
on states. The off time is called its relaxation time.
graduated voltage drop - the voltage distributed along a bar of semiconductor material (similar
to the voltage drops along a string of series resistors); also called voltage gradient.
firing voltage (Vp) - the emitter terminal voltage, with respect to base 1, required for the UJT to
conduct emitter current.
valley voltage (Vv) - the emitter terminal voltage at which the UJT moves to its off state
EQUIPMENT REQUIRED
F.A.C.E.T. base unit
FET FUNDAMENTALS circuit board
Multimeter
Oscilloscope, dual trace