UJT Relaxation Oscillator

Aim

       To design and simulate UJT Relaxation Oscillator circuit

 Theory

    UJT is an uni-junction device. This single pn junction device consists of a lightly doped n-type silicon bar. The p- type impurity is diffused into the

    base producing the pn junction. The above figure shows the equivalent circuit of UJT. The resistance of the silicon bar is called inter base

    resistance RBB represented by the two resistors in series viz. Rb1 and Rb2. The pn junction is represented in the emitter by a diode D. The

    operation of UJT may be explained in three different modes.

With no voltage applied to the UJT, the inter base resistance is given by

  Rbb=Rb1 + Rb2

If a voltage Vbb is applied between the bases with emitter open, the voltage will divide up across Rb1 and Rb2.

Voltage across Rb1,

Or  

The ratio          is called the intrinsic stand-off ratio represented by h . ThusThe value of lies between 0.51 and 0.82.

If a progressively rising positive voltage is applied to the emitter the diode will become forward biased when input voltage exceeds h Vbb by Vd,

 the forward voltage drop across the silicon diode. Now the emitter current increases regeneratively until it is limited by the emitter power supply.
Here we can define the peak point voltage of the UJT,

                          Thus when input positive voltage to the emitter is less then Vp, the pn-junction remains reverse biased and the emitter current is practically zero. 

    When the input voltage exceeds Vp, the diode is forward biased and the emitter current reaches a saturation value limited by Rb1and the forward

    resistance of pn-junction.

 UJT Relaxation Oscillator circuit, mainly used for triggering purposes is shown above. This circuit is ideally suited for triggering an SCR –

    since UJT is capable of generating sharp, high powered pulses of short duration whose peak and average power don’t exceed the

    power capabilities of the SCR gate for which they are intended. When power is applied to the given circuit, capacitor C starts charging

  exponentially through R to the applied voltage VCC. The voltage across C is the voltage-Ve applied to the emitter of UJT. When C is charged to

    Vp, then UJT turns ON. This greatly reduces the effective resistance between emitter and base1 of UJT. A sharp pulse of current flows from

    base1 to emitter, discharging C through Rb1. When the capacitor voltage drops below Vp, UJT is brought back to the previous state and

    the capacitor again begins to charge towards Vbb. This produces a sawtooth wave

  Intrinsic stand-off ratio h =0.4 to 0.6

h =0.5 (we take)substituting the value of h in (1)                                               









 Capacitor C is charged through R towards supply voltage VBB. As long as capacitor voltage VE is below a stand-off voltage VP set by the voltage

     across B1-B2 and the transistor stand-off ratio h

                                                                    Sweep Amplitude = VP-VV

                            

At peak point emitter voltage VE=VP and current through R is given by