PROGRAMMABLE UNIJUNCTION TRANSISTOR

The programmable unijunction transistor (PUT) is an improved version of UJT.But  PUT  is  a  four  layer  PNPN  device  it  also  known  as  small  version  of thyristors as shown in figure 12.

Its operation is similar to the UJT hence it is always considered with UJT, itstrigger voltage VP can be programmed or decided by the designer via external potential divider [ but in UJT, it is fixed for the given device] thus it is known   as programmable unijunction transistor.

The  symbol  of  PUT  is  shown  in  figure  12.  In  this  case  the  gate  terminal  isconnected to the N region [but in thyristor or SCR gate is connected with P     region], thus the anode and cathode constitute the PN junction which controls the ON and OFF states of PUT.

Usually positive potential is applied to gate with respect to cathode. When theanode voltage is less than gate voltage VG the anode gate junction becomes reverse biased, the PUT is in OFF state.

When the anode voltage exceeds the gate voltage V anode cathode junctionbecomes  forward  biased,  thus  PUT  is  turned  ON.  In  the  ON  state,  the  PUT behaves like other four layer PNPN devices [SCR]. The PUT is also known as complementary SCR [because the gate is connected with ‘N’ layer instead of ‘P’ layer].

Advantages of PUT over UJT

1.  The  switching  voltage  is  easily  yarned  by  changing  Vg  through  the  potential divider.

2. PUT can operate at lower voltages then IC’s. 3. Peak current is lower UJT.

PUT as relaxation oscillator

A PUT can be used as relaxation oscillator it is shown in figure 12. The gate    voltage  VG  is  maintained  from  the  supply  by  the  potential  divider  R1  and  R2  and determines the peak point voltage Vp In the case of the UJT, VP is fixed for a device by  the  dc  supply  voltage.  But  VP  of  a  PUT  can  be  varied  by  varying  the  potential divider it and it Tithe anode voltage VA is less than the gate voltage V the device will remains in its off state.